ORCID Profile
0000-0002-2968-582X
Current Organisations
University of Southampton
,
University of Western Australia
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Research Topics
In Research Link Australia (RLA), "Research Topics" refer to ANZSRC FOR and SEO codes. These topics are either sourced from ANZSRC FOR and SEO codes listed in researchers' related grants or generated by a large language model (LLM) based on their publications.
Top 5 Research Topics
ANZSRC Field of Research (FoR)
Civil Engineering | Civil Geotechnical Engineering | Geotechnical Engineering | Civil Engineering not elsewhere classified | Maritime Engineering | Ocean Engineering | Construction Engineering | Civil Engineering Not Elsewhere Classified | Construction Materials | Marine Engineering | Physical Oceanography | Stochastic Analysis and Modelling | Geomechanics | Resources Engineering and Extractive Metallurgy |
ANZSRC Socio-Economic Objective (SEO)
Oil and Gas Extraction | Civil Construction Design | Oil and gas | Rail Infrastructure and Networks | Road Infrastructure and Networks | Commercial Construction Design | Industrial Construction Design | Oil and gas | Oil and Gas Exploration | Industrial Energy Conservation and Efficiency | Industrial | Coal Mining and Extraction | Civil | Application Tools and System Utilities
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Publications
Characterization of the solid-fluid transition of fine-grained sediments
Publisher: ASMEDC
Date: 2009
Abstract: Characterization of the strength of fine-grained sediments as they evolve from an intact seabed material to a remolded debris flow is essential to adequately model submarine landslides and their impact on pipelines and other seabed infrastructure. In the current literature, two distinct approaches for modelling this material behavior have been considered. In the soil mechanics approach, fine-grained soils are characterized by the undrained shear strength, su. The critical state framework proposes a relation between su and the water content, or void ratio of the soil. In addition, rate effects and strain softening effects are described by multiplying a reference value of su by a function of the shear strain rate or the accumulated shear strain respectively. In the fluid mechanics approach, slurries of fine-grained material are characterized by a yield strength and a viscosity parameter, which describes the change in shear stress with shear strain rate. Empirical relationships have been proposed, which relate the yield strength and the viscosity to the sediment concentration. This paper demonstrates that the two modelling approaches are essentially similar, with only some formal differences. It is proposed that the strength of fine-grained sediments can be modelled in a unified way over the solid and liquid ranges. To support this unified approach, an experimental c aign has been conducted to obtain strength measurements on various clays prepared at different water content. The testing program includes fall cone tests, vane shear tests, miniature penetrometers (T-bar and ball) and viscometer tests. Rate effects and remolding effects are investigated over a wide range of water contents spanning the domains of behavior that are usually defined separately as soil and fluid. The present paper focuses on analyzing the results of fall cone, vane shear and viscometer tests. Analysis of the results shows that the variation in shear strength over the solid and liquid ranges can be described by a unique function of water content — suitably normalized — for a given soil. Furthermore, the effect of strain rate and degree of remolding can be accounted for by multiplying the basic strength parameter by appropriate functions, which are independent of the current water content.
Reliability of mooring lines and piles for a permanently manned vessel in a tropical cyclone environment
Publisher: Elsevier BV
Date: 2019
Modelling changes to submarine pipeline embedment and stability due to pipeline scour
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: In this paper a beam bending model is combined with existing predictive formulas for pipeline scour to study changes to pipeline stability during scour and lowering. The model is introduced and demonstrated for a range of simplified conditions, including scour-induced lowering of a pipeline resulting from multiple uniformly spaced scour initiation points. The model is then used with a synthetic seabed generated with a variety of length scales. In this simulation the pipeline is ‘laid’ onto the seabed, leading to the formation of ‘natural’ initiation points for scour. The distribution and spacing of the initiation points (which are a function of the pipeline bending stiffness, tension and seabed roughness) lead to different rates of pipeline lowering and stability. The resulting model may be used within a probabilistic framework to estimate changes to pipeline stability resulting from sediment mobility and scour.
Interpretation of T-bar penetrometer tests at shallow embedment and in very soft soils
Publisher: Canadian Science Publishing
Date: 02-2010
DOI: 10.1139/T09-096
Abstract: The cylindrical T-bar penetrometer was developed for profiling the undrained strength of soft soils in the centrifuge and is now a widely-used offshore site investigation tool. The conventional interpretation of the T-bar test is to convert the measured penetration resistance to soil strength using a single bearing factor associated with steady flow of soil around the bar. This paper describes a new analysis for the interpretation of T-bar penetrometer tests at shallow embedment and in soft soils, which is an increasingly significant consideration in the design of seabed infrastructure, including pipelines. The analysis captures two mechanisms that are usually neglected: (i) soil buoyancy and (ii) the reduced bearing factor arising from the shallow failure mechanism mobilized prior to the full flow of soil around the bar. The framework derives from theoretical considerations and is calibrated using large deformation finite element analyses. The depth at which the steady deep penetration condition is reached is shown to depend on the normalized soil strength, s u /γ′D, and may be up to several diameters deep. The effect of this new procedure on the inferred soil strength compared with the conventional approach is illustrated through T-bar tests in three different centrifuge s les, spanning a range of strength ratios.
Lateral Boundary Effect in Centrifuge Tests for Spudcan Penetration in Uniform Clay
Publisher: Trans Tech Publications, Ltd.
Date: 05-2014
DOI: 10.4028/WWW.SCIENTIFIC.NET/AMM.553.458
Abstract: The effect of the centrifuge strongbox boundary on the penetration resistance of a spudcan foundation in uniform clay has been studied using Large Deformation FE analysis. Both smooth and rough strongbox boundaries were considered with various strongbox sizes. The spudcan penetration resistance and soil flow mechanisms were analysed. It was observed that, when the strongbox size was reduced, the spudcan penetration resistance was decreased for a smooth boundary and increased for a rough boundary. The depth of cavity formed above the spudcan during its penetration, in most cases, was determined by the soil flow around mechanism without cavity wall failure. However, cavity wall failure could be initiated when a smooth strongbox boundary was very close to the spudcan. The strongbox boundary effect on the spudcan penetration resistance can be avoided when the distance of the strongbox boundary to the spudcan centre is larger than 1.5 times of spudcan diameter for a rough boundary or 2 times of spudcan diameter for a smooth boundary.
The effect of radial fins on the uplift resistance of buried pipelines
Publisher: Thomas Telford Ltd.
Date: 03-2017
Abstract: This paper investigates the potential for increasing the uplift resistance of buried pipelines through the addition of radial fins on the pipe circumference. Experiments conducted in loose sand showed that fins extending by 20% of the pipe diameter increase the vertical peak uplift resistance by up to 25%, depending on embedment depth and fin configuration. A limit equilibrium solution – based on known values of peak friction and dilation angles – predicts the uplift resistance within 13% of the measurements. The trends of peak uplift resistance with embedment and fin configuration were also replicated in numerical analyses conducted using a non-associated Mohr–Coulomb soil model. The numerically predicted peak uplift resistances were within 10 and 21% of the experimental values for rough and smooth interfaces, respectively. Soil failure mechanisms from the numerical analyses were broadly consistent with that assumed in the limit equilibrium solution. However, the experimentally observed mechanisms differed subtly, with a limited extent of lifted soil above the pipe and circulatory flow occurring from above to beneath the pipe. This mechanism was approached in the numerical analyses for a smooth interface by specifying a small negative dilation angle, which had minimal effect on the predicted peak uplift resistance.
Modelling the soil resistance on seabed pipelines during large cycles of lateral movement
Publisher: Elsevier BV
Date: 2008
Finite-Element Modeling of Offshore Pipeline Lateral Buckling
Publisher: American Society of Civil Engineers (ASCE)
Date: 11-2019
Enhancement of bearing capacity from consolidation: due to changing strength or failure mechanism?
Publisher: Thomas Telford Ltd.
Date: 02-2019
Abstract: Bearing capacity of shallow foundations is higher following preload (or self-weight)-induced consolidation because the soil strength changes, and perhaps because the failure mechanism changes. Previous studies have illustrated this effect by plotting or predicting changes in either bearing capacity factor or strength. In this study, the relative contribution of these two effects is explored. This is achieved by formalising a definition of bearing capacity factor, which is described in terms of the average strength mobilised in the deformation mechanism at failure. Using the alternative definition of bearing capacity factor, the gain in foundation capacity is shown to be almost entirely due to changes in soil strength, rather than bearing capacity factor, which remains largely unaffected by the strength gains. This observation should encourage future studies into consolidated bearing capacity to present gains in capacity in terms of changes in mobilised strength rather than changes in bearing capacity factors, and supports the use of prediction methods that focus on defining the change in soil strength.
Reply to the discussion by McCarron on Large-scale modelling of soil-pipe interaction during large amplitude cyclic movements of partially embedded pipelines
Publisher: Canadian Science Publishing
Date: 05-2008
DOI: 10.1139/T08-041
Centrifuge modeling of the cyclic lateral response of a rigid pile in soft clay
Publisher: American Society of Civil Engineers (ASCE)
Date: 07-2011
In Situ Measurement of the Dynamic Penetration of Free-Fall Projectiles in Soft Soils Using a Low-Cost Inertial Measurement Unit
Publisher: ASTM International
Date: 27-01-2016
DOI: 10.1520/GTJ20140135
Geotechnical centrifuge modelling techniques for submarine slides
Publisher: ASMEDC
Date: 2009
Abstract: The gradual shift of hydrocarbon developments into deeper waters has presented fresh challenges for offshore geotechnical engineering. Many installations in deep water require export pipelines to shore which can be many hundreds of kilometers in length. These pipelines must negotiate unstable regions of soft seabed around the steep continental shelf and variable terrain including canyons with depths ranging from tens of meters to a several hundred meters. These challenging conditions present potential geohazards for which little understanding has been developed so far. One of the major geohazards is the impact of a submarine landslide on nearby pipelines which could potentially damage the pipeline. Consequently, a Joint Industry Project (JIP) has been initiated at the Centre for Offshore Foundation Systems to improve methods for assessing the potential damage to pipelines and to provide information to assist re-routing of a pipeline to a safer alignment if necessary. This paper presents an overview of the development of specific modeling techniques to (i) trigger a submarine landslide in a geotechnical centrifuge, (ii) measure the strength properties of the slide material before, during, and after the slide failure and (iii) measure the interaction between the runout material and the seabed.
Softening and consolidation around seabed pipelines: centrifuge modelling
Publisher: Thomas Telford Ltd.
Date: 02-2018
Abstract: Solutions for lateral breakout and axial response of submarine pipelines are well established if the undrained shear strength conditions of the soil are known and defined simply (such as uniform or increasing proportionally with depth). In reality, the geometry of the free surface and the distribution of undrained shear strength around a submarine pipeline post-lay are affected by the lay process. This is because of soil berms that form adjacent to the pipe, and remoulding and subsequent reconsolidation of the seabed. The effect of post-lay consolidation on the subsequent lateral and axial response of submarine pipelines has not been previously investigated through physical model testing. This paper presents results from centrifuge model tests describing lateral breakout behaviour of a pipe on soft clay as a function of (a) pipe installation conditions, (b) post-lay pipe weight and (c) consolidation prior to breakout. In addition, the effect of post-lay consolidation on axial pipe response is studied. The experimental results are compared with available numerical and analytical predictions. The results quantify the influence of the installation process, pipe weight and post-installation consolidation on the lateral breakout resistance and trajectory of the pipe and also the axial pipe response, and show how existing prediction methods can capture these effects.
Subsea pipeline walking with velocity dependent seabed friction
Publisher: Elsevier BV
Date: 2019
Best Practice Geotechnical Characterization and Pipe-soil Interaction Analysis for HPHT Pipeline Design
Publisher: OTC
Date: 04-05-2015
DOI: 10.4043/26026-MS
Abstract: This paper summarizes best practice cost-effective geotechnical design of high pressure high temperature (HPHT) pipelines, with an emphasis on HPHT lines in deep water, that are susceptible to lateral buckling and axial walking. It presents guidance on (i) tools for optimal geotechnical characterization of shallow seabed conditions along flowline routes and (ii) techniques to assess pipe-soil interaction parameters for input to pipeline structural modeling. Recent research into pipe-soil interaction measurement tools and analysis techniques has resulted in rapid advancements that are being quickly utilized in practice. Advanced pipe-soil interaction techniques, firmly rooted in soil mechanics theory and linked to the operational sequence of the pipeline through cycles of start-up and shut-down, are presented. They are used to demonstrate benefits that can be realized on typical projects to reduce pipeline walking predictions and the associated mitigation measures such as hold-back anchors. Recommendations cover the appropriate selection of characterization tools and analysis techniques based on project-specific requirements such as seabed geotechnical complexity, pipeline operating conditions, project timing, mitigation strategies for global stability and the desired approach to risk management and use of the observational method. The results, observations and conclusions presented in this paper represent the culmination of several years of research and development in pipe-soil interaction, as well as our experience on recent projects around the world where these techniques have been employed. The paper offers recommendations to guide both geotechnical studies and pipeline engineering work, and unlock benefits from maintaining close integration of these two disciplines through the design and operation of a pipeline.
Tolerably mobile subsea foundations – observations of performance
Publisher: Thomas Telford Ltd.
Date: 11-2014
Abstract: Increasing demands for oil and gas exploration in deep water with soft seabed conditions are resulting in the size and weight of subsea shallow foundations stretching the capabilities of installation technologies. One innovation to reduce foundation footprints involves designing foundations to move in a tolerable manner to absorb applied loads rather than being engineered to resist these loads and remain stationary. Critical design considerations are the evolution of foundation capacity and the mode of foundation displacement. The foundation should be designed to slide with acceptable settlement and rotation to prevent overstressing the joints with connected pipelines. This paper presents observations from centrifuge model tests of a mat foundation designed to slide under applied loading. The foundation is subjected to a simulated lifetime of operation, with many cycles of sliding and intervening periods of consolidation. The results provide insights to assist design, including a remarkable rise in the lateral foundation resistance over the sliding events, through repeated episodes of shearing and reconsolidation, and quantification of the accumulated settlements and rotations. The foundation is shown to translate with minimal rotation. The settlement between sliding events is more significant. This is due to the tendency for soft clay to contract on shearing, as excess pore pressures generated during sliding subsequently dissipate. The sliding-induced consolidation settlements control the tolerability of the performance of the mobile foundation.
ISSPEA 2019: 1st International Symposium on Screw Piles for Energy Applications
Publisher: University of Dundee
Date: 27-05-2019
DOI: 10.20933/100001123
Sedimentation-induced burial of subsea pipelines: Observations from field data and laboratory experiments
Publisher: Elsevier BV
Date: 08-2016
The influence of permeability on the erosion rate of fine-grained marine sediments
Publisher: Elsevier BV
Date: 10-2018
The parkable piezoprobe for determining cv and strength – modelling and interpretation methods
Publisher: Thomas Telford Ltd.
Date: 05-2019
Abstract: The parkable piezoprobe is a site investigation tool for measuring the coefficient of consolidation, in situ offshore, at shallow embedment depths. The device applies a similar bearing pressure to subsea infrastructure so it reaches a comparable self-weight penetration when ‘parked’ at the seabed, representative of an unburied pipeline. Instrumentation on the device allows the dissipation of penetration-induced excess pore pressure to be recorded at various locations on the surface. From these dissipation responses the coefficient of consolidation can be inferred, which is a key parameter in the design of many offshore structures founded in surficial soil, such as pipelines or shallow foundations. The intent is that this device is deployed from a seabed frame, while other activities such as penetrometer testing or s ling take place in parallel. This paper presents robust interpretation methods for the parkable piezoprobe by using a combination of centrifuge experiments and large deformation finite-element analyses. The centrifuge tests demonstrate that the penetration response of the parkable piezoprobe is adequately captured by existing bearing capacity models, allowing the optimum device weight to be identified. A comprehensive interpretation method is then developed for the dissipation stage. This yields accurate estimates of the coefficient of consolidation, even for cases where there is no prior knowledge of the soil parameters or the depth to which the device embeds under its own self-weight.
Response of Piles with Wings to Monotonic and Cyclic Lateral Loading in Sand
Publisher: American Society of Civil Engineers (ASCE)
Date: 03-2012
Novel Anchoring Solutions for FLNG - Opportunities Driven by Scale
Publisher: OTC
Date: 04-05-2015
DOI: 10.4043/26032-MS
Abstract: FLNG facilities present a more onerous anchoring requirement than existing floating structures. Optimisation of the anchoring technology through improved design or through novel anchor types offers potential cost and risk benefits. These benefits may also be applicable to smaller moorings for MODUs and FPSOs. This paper uses concept–level design calculations of anchor capacity to compare different anchor technologies in the context of FLNG and MODU/FPSO applications. Also, new observations from physical modelling of chain–soil interaction are presented. Opportunities are identified for significant cost and schedule savings by adopting the alternative plate anchor technologies that are either suction or dynamically installed. Considering fabrication alone, the estimated costs are reduced by 70% for FLNG and 80% for MODUs relative to the conventional suction caisson option. When installation vessel costs are considered, the absolute cost saving could be far higher than from fabrication alone because installation could be from an anchor–handling vessel rather than a construction barge with a heavy lift crane. Torpedo anchors have also been considered, but are less attractive. Centrifuge model data and calculations of the shape and capacity of the embedded anchor chain suggest that there may be over–looked capacity from the mooring chain both on and within the seabed. At the same time, upscaling of embedded plates to the scale required for FLNG applications increases the amount of chain slack that would be released into the mooring during in service loading, and this effect requires consideration in the overall mooring system design. Research and development activities aligned with the opportunities for reduced cost and risk in anchoring design are set out.
Changes in Pipeline Embedment due to Sediment Mobility: Observations and Implications for Design
Publisher: American Society of Mechanical Engineers
Date: 09-06-2013
Abstract: This paper describes temporal variations in embedment of several existing pipelines on the North-West Shelf (NWS) of Australia, and the sediment mobility processes that cause them. Distinct and explainable patterns in the extent, distribution and rate of the development of pipeline embedment have been revealed through systematic detailed examination of repeated annual integrity surveys by ROV. This represents a unique data-set that has been used to optimize the reliability of a newly designed pipeline. This paper explains why these clear findings should not be overlooked in both the buckling and stability design of initially unburied pipelines, which is in contrast to currently established industry practice. This new information supports the presumption that conventional approaches for calculating the hydrodynamic stability of unburied pipelines may be more conservative than necessary. Conversely, and arguably more importantly, it is shown that conventionally accepted methods for calculating pipe-seabed resistance forces when planning buckling schemes should be considered unsafe if embedment due to sediment mobility is possible. Consequently, this paper proposes an innovative calculation methodology that statistically captures these sediment mobility effects, and which facilitates a more justifiable geotechnical input to pipeline engineering than what is conventionally adopted. This methodology is currently being used by the authors as a state-of-the-art design practice for unburied offshore pipelines in regions of sediment mobility.
Modelling the dynamic embedment of seabed pipelines
Publisher: Thomas Telford Ltd.
Date: 2011
DOI: 10.1680/GEOT.8.P.148
Abstract: The as-laid embedment of a seabed pipeline is an important design parameter. As a pipe is laid on the seabed it oscillates, owing to vessel motion and hydrodynamic loading of the hanging pipe. This movement significantly increases the pipe embedment beyond the theoretical value related to the static pipe weight, even when corrected for any stress concentration caused by the hanging catenary. Dynamic lay effects are either ignored in practice, or are accounted for by scaling up the static embedment by an empirical factor, leading to significant uncertainty in this important design parameter. A series of centrifuge model tests has been conducted using two clays – kaolin and a high-plasticity natural clay – to simulate the dynamic embedment process. The results indicate that only a few cycles of small- litude oscillation (±0·05D) are required to double or triple the pipe embedment, owing to the combined effect of lateral ploughing and soil softening. In these experiments the pipe embedment increased to up to eight times the static embedment after 100 cycles of motion, which represents a typical lay process. A model is proposed for the cycle-by-cycle embedment of a pipeline under a given sequence of small- litude oscillations at a given applied vertical force. The trajectory of the pipe movement is assessed using a flow rule derived from plasticity-based yield envelopes. The effect of soil remoulding is explicitly captured by linking the accumulated disturbance to the decay in soil strength. Using input parameters derived from theoretical considerations and T-bar penetrometer tests, the model captures the essential features of the dynamic embedment process. With modest optimisation of the model parameters, the mean discrepancy between the calculated and measured embedment is only 12% for both clays. The ultimate states predicted by this cycle-by-cycle model also provide a rough estimate of the maximum pipe embedment for fully remoulded conditions, which include some degree of water entrainment caused by the lay process, evident in the optimised parameters. This ultimate embedment is governed by the remoulded soil strength and the pipe weight (augmented by any stress concentration). The litude of the cyclic motion affects the rate of softening, and hence the rate of settlement. This model provides a framework for assessing the as-laid embedment of seabed pipelines on a more rigorous basis than current practice.
Lifelong embedment and spanning of a pipeline on a mobile seabed
Publisher: CRC Press
Date: 06-11-2015
DOI: 10.1201/B17703-106
DEM study of particle scale effect on plain and rotary jacked pile behaviour in granular materials
Publisher: Elsevier BV
Date: 09-2023
Using the 'step zero' approach to design a centrifuge modelling program
Publisher: CRC Press
Date: 18-12-2014
DOI: 10.1201/B16200-51
Friction fatigue on displacement piles in sand
Publisher: Thomas Telford Ltd.
Date: 12-2004
The design of Subsea foundations subject to general cyclic loading using a massively scalable web based application
Publisher: OTC
Date: 30-04-2018
DOI: 10.4043/28911-MS
Abstract: Subsea developments require the design of large numbers of shallow skirted foundations to support structures such as manifolds, pipeline and umbilical terminations and in-line tees. Safe and economic design relies on the accurate assessment of foundation capacity against thousands of load-combinations. Performing these design calculations is a significant computational task. The objective of this paper is to demonstrate how new developments in cloud computing can be utilized to optimize foundation design. Engineering design is no longer limited by computing power thanks to the introduction of low-cost on-demand cloud computing platforms. This paper describes a massively scalable cloud based application for rapidly assessing the vertical-horizontal-moment-torsional capacity of shallow skirted foundations against thousands of cyclic load case combinations that arise from numerous environmental and service conditions. The detrimental effect of cyclic loading and the beneficial effect of consolidation on soil strength are incorporated within a single workflow. It is shown that cloud technologies can radically improve traditional engineering design procedures, allowing engineers to focus on the innovative and creative aspects of their work, while the tasks of preparing, executing and documenting calculations become near instantaneous and more easily assessed for quality assurance. More critically, the technology allows rapid and rigorous optimization of the foundation dimensions to achieve the most cost-effective solution that satisfies all load cases. The scalability of the application allows multiple users to run large numbers of calculations simultaneously across a virtually unlimited number of computer nodes. The system can be accessed through a standard web browser and can run simulations on any internet-connected device. Results are saved in the cloud and can be accessed anywhere and shared among colleagues, enhancing collaboration and quality assurance. The approach results in demonstrably superior design outcomes, achieved more quickly. This paper presents what is believed to be the world's first web based application for shallow foundation design that exploits the availability of low cost on-demand cloud computing services. The paper will explain some of the challenges in implementing such a system and provide ex les. We believe this type of technology represents the future for geotechnical design work, providing better design in a more efficient manner.
Changing soil response during episodic cyclic loading in direct simple shear tests
Publisher: International Society of Offshore and Polar Engineers
Date: 03-2023
Pipe-Soil Structure Interaction: Best Practices for Spools, Buckle Initiators, End Connections and Sand Waves
Publisher: OTC
Date: 05-2017
DOI: 10.4043/27689-MS
Abstract: Pipe-soil interaction behavior forms a key input into the design of pipelines. The global response of the pipeline can be critically affected by the resistance provided by the seabed. Accurate modeling of pipe-soil interaction behavior leads to improved reliability and can lead to design optimizations that offer considerable cost benefits. Extensive recent research and advances in design approaches, particularly through the development of high pressure high temperature lines, has led to significant revisions and improvements to pipe-soil interaction modeling over the past decade. There are now well-established techniques for estimating the as-laid embedment and axial and lateral pipe-soil interaction responses – the non-linear ‘springs’ used in pipeline structural modelling – for pipelines laid on the seabed. However, these methods, as applied in practice, generally involve the key assumption that the pipe exerts a vertical force on the seabed, V equal to its submerged self-weight, W – the ‘V=W assumption’. However, this assumption is not appropriate for pipe elements that are close to points of fixity, constraint or vertical upsets – e.g. in-line tees, buckle initiators and sand waves – or which form spools. This paper illustrates ex les of all of these situations, showing pipe-soil interaction ‘springs’ generated with and without the ‘V=W assumption’, to show the contrasting behavior. Recommendations are provided for practical modeling of this soil-structure interaction behavior. In some design scenarios, the pipeline may be sufficiently robust that adoption of the ‘V=W assumption’ may not affect the integrity of the design. In other scenarios, it may be necessary to eliminate the ‘V=W assumption’ in order for a satisfactory design to be demonstrated. This may be achieved by performing integrated soil-structure interaction analyses, or through iterative refinement of the pipe-soil ‘springs’. In all cases, an awareness of the ‘V=W assumption’ is required across the pipeline engineering and geotechnical teams, who should interact to determine the appropriate basis to demonstrate an acceptable design.
The uplift resistance of pipes and plate anchors buried in sand
Publisher: Thomas Telford Ltd.
Date: 12-2008
Abstract: The design of buried anchors and pipelines requires assessment of the peak uplift resistance. This paper describes a limit equilibrium solution for the uplift resistance of pipes and plate anchors buried in sand. The geometry of this solution reflects observations from model tests. Peak angles of friction and dilation are found using established correlations that capture the influence of stress level and density. These angles govern the geometry of the failure mechanism and the mobilised resistance. The solution is validated using a database of 115 model tests on pipes and strip anchors assembled from the published literature. Good agreement with the overall database is shown, without optimisation of any input parameters. The method overpredicts the uplift resistance of smooth model pipes by ∼10%, highlighting the influence of pipe roughness. In contrast, it is shown that the solution for uplift resistance based on the limit theorems of plasticity is generally unconservative. The assumption of normality, which is required by the limit theorems, leads to an unrealistic failure mechanism involving uplift of a far wider zone of soil than is seen in model tests. Plasticity theory, with normality, is inappropriate for modelling this class of kinematically restrained problem in drained conditions, as normality is not observed. As finite element analysis is not routinely used in practice—partly owing to the difficulty in selecting appropriate input parameters to describe dilatancy and plastic flow—the simple analytical idealisation described in this paper provides a useful tool for uplift resistance prediction. Simple charts for the prediction of peak uplift resistance from critical state friction angle, relative density and normalised burial depth are presented, to aid the design of buried pipes and anchors.
A novel approach for time-dependent axial soil resistance in the analysis of subsea pipelines
Publisher: Elsevier BV
Date: 09-2015
Discussion of “Accuracy of Digital Image Correlation for Measuring Deformations in Transparent Media” by Samer Sadek, Magued G. Iskander, and Jinyuan Liu
Publisher: American Society of Civil Engineers (ASCE)
Date: 04-2005
Limit analysis of the undrained bearing capacity of offshore pipelines
Publisher: Thomas Telford Ltd.
Date: 2015
Abstract: The ultimate bearing capacity of a rigid, plane-strain pipe segment embedded in undrained clay is studied using numerical limit analysis. The pipe is considered to be ‘wished in place' at invert penetrations ranging from zero to five pipe diameters, thus providing coverage of both on-bottom (partially embedded) and trenched (fully embedded) offshore pipelines. The soil is modelled as a rigid-plastic Tresca material with either uniform strength or strength proportional to depth. The effects of soil weight, interface roughness and interface tensile capacity are investigated in a systematic manner. All calculations are performed using the finite-element limit analysis code OxLim, which uses adaptive mesh refinement to compute tightly bracketed lower- and upper-bound plasticity solutions. The velocity fields from the upper-bound analyses provide the corresponding failure mechanisms. The paper initially focuses on purely vertical loading (penetration and uplift), and then addresses combined vertical and horizontal loading. A comprehensive set of design curves and failure envelopes is presented, with the results explained in terms of the changing failure mechanisms. These results are immediately applicable in practice. In particular, current industry-standard procedures for design against pipeline upheaval are critically reviewed, and are shown to have potentially unconservative shortcomings.
Hydrodynamic forces on subsea cables immersed in wave boundary layers
Publisher: Elsevier BV
Date: 06-2022
Apparatus for measuring pipe-soil interaction behavior using shallow ‘pipe-like’ penetrometers
Publisher: ASTM International
Date: 10-07-2020
DOI: 10.1520/GTJ20180293
UK perspective research landscape for offshore renewable energy and its role in delivering Net Zero
Publisher: IOP Publishing
Date: 12-09-2022
Abstract: This paper sets out the role of offshore renewable energy (ORE) in UK targets for Net Zero greenhouse gas emissions by 2050 and provides a review of the research challenges that face the sector as it grows to meet these targets. The research challenges are set out in a Research Landscape that was established by the ORE Supergen Hub following extensive consultation with the ORE community. The challenges are ided into eight themes, each challenge is described, and current progress is summarised. The progress of the ORE sector in recent years has seen huge cost reductions, which have encouraged the great ambition for the sector seen in UK Government targets. However, in order to meet these critical targets and achieve Net Zero, further innovations and novel technologies will be needed and at pace, driven forward by new research and innovation. The strategy of the Supergen ORE Hub in framing the research and innovation activities within a community-developed research landscape and working together across disciplines and with close collaboration between academia and industry is a necessary component in achieving the ambition of sustainable energy generation.
Numerical simulations of dynamic embedment during pipe laying on soft clay
Publisher: ASMEDC
Date: 2009
Abstract: Prediction of the as-laid embedment of a pipeline, which affects many aspects of pipeline design, is complicated by the dynamic motions that occur during the lay process. These motions cause pipelines to embed deeper than predicted based on static penetration models, as the seabed soils are both softened and physically displaced by the pipeline motion. This paper describes the results of 2D numerical analyses using a large displacement finite element approach aimed at quantifying pipeline embedment due to cyclic lateral motion at various fixed vertical load levels. The validity of the numerical results is first assessed by comparison with published data from centrifuge model tests in two different types of clay. A parametric study varying the normalized vertical load is then presented, which suggests a simple approach for estimating an upper limit to the dynamic embedment.
The changing strength of carbonate silt: parallel penetrometer and foundation tests with cyclic loading and reconsolidation periods
Publisher: Canadian Science Publishing
Date: 11-2020
Abstract: This paper describes a centrifuge study using novel penetrometer tests (T-bar and piezoball) and model foundation tests to explore through-life changes in the strength of a reconstituted natural carbonate silt. The test procedures include episodic cyclic loading, which involves intervals of pore pressure dissipation between cyclic packets. These loads and the associated remoulding and reconsolidation cause significant changes in the soil strength and foundation capacity. Soil strength changes from penetrometer tests differed by a factor of 15 from the fully remoulded strength to a limiting upper value after long-term cyclic loading and reconsolidation. For the model foundation tests, the foundation capacity of a surface foundation and a deep-embedded plate were studied. The soil strength interpreted from the measured foundation capacity varied by a factor of up to three due to episodes of loading and consolidation, with an associated order of magnitude increase in the coefficient of consolidation. The results show a remarkable rise in soil strength over the loading events and provide a potential link between changes in soil strength observed in penetrometer tests and the capacity of foundations, allowing the effects of cyclic loading and consolidation to be predicted.
Cyclic consolidation and axial friction for seabed pipelines
Publisher: Thomas Telford Ltd.
Date: 14-07-2014
Abstract: Finite-element analyses of the axial sliding resistance of a seabed pipeline embedded in soft normally consolidated modified Cam clay are presented. The study demonstrates the rise in axial resistance associated with episodes of movement with intervening periods of consolidation. It is shown that the excess pore pressures generated during undrained axial movements lead to significant consolidation and strength gain in the surrounding soil. For modified Cam clay, using properties representative of kaolin, the resistance rises exponentially with cycles and reaches the drained limit within approximately 10–20 episodes of movement and consolidation. The rate of gain in resistance is shown to be controlled by the volumetric stiffness ratio, κ/λ, for soil that is initially normally consolidated. Simple relationships are proposed that will assist in utilising this beneficial phenomenon in design practice. The increase in axial sliding resistance due to consolidation may mean that mitigation requirements against cyclic axial pipe walking, caused by thermal expansion, may be reduced or eliminated.
Coupled consolidation analysis of pipe-soil interactions
Publisher: Canadian Science Publishing
Date: 06-2013
Abstract: Current design practice for pipe–seabed interaction in soft soils is generally based on the assumption of undrained behaviour throughout laying and subsequent operation. In reality, drainage and consolidation around a partially embedded pipe can have a marked effect on the vertical penetration and horizontal breakout resistance. In this paper, a large-deformation finite element methodology coupled with the “modified Cam clay” plasticity soil model has been developed to study the coupled consolidation behaviour of soil around partially embedded seabed pipelines. Simulations of penetration show that after laying, subsequent consolidation leads to further embedment by an amount dependent on the level of drainage that occurred during laying. Also, if the pipe is embedded under undrained conditions, the waiting period between laying and operation allows the soil around the pipe to consolidate under the pipe self-weight. The consolidation process results in an increase in the strength of the soil. The lateral breakout resistance and the direction of pipe movement on breakout thus depend on the consolidated strength of the soil around the pipe, as well as the applied loading. The envelopes of vertical–lateral combined loading bearing capacity differ markedly from those predicted assuming undrained behaviour throughout.
Effects of variability in lateral pipe-soil interaction and pipe initial out-of-straightness on controlled lateral buckling of pre-deformed pipeline
Publisher: Elsevier BV
Date: 06-2019
Anchor geotechnics for floating offshore wind: Current technologies and future innovations
Publisher: Elsevier BV
Date: 07-2023
Uplift mechanisms of pipes buried in sand
Publisher: American Society of Civil Engineers (ASCE)
Date: 02-2008
Solutions for downslope pipeline walking on a seabed with a peaky trilinear soil resistance model
Publisher: ASME International
Date: 05-10-2021
DOI: 10.1115/1.4048331
Abstract: Offshore pipelines used for transporting hydrocarbons are cyclically loaded by great variations of pressure and temperature. These variations can induce axial instability in such pipelines. This instability may cause the pipelines to migrate globally along their length an effect known as pipeline walking. Traditional models of pipeline walking have considered the axial soil response as rigid-plastic (RP) however, such behavior does not match observations from physical soil tests. It leads to inaccurate estimates of walking rate (WR) per cycle and over design. In this paper, a trilinear (3L) soil resistance model is used to represent seabed resistance to investigate the behavior of pipeline walking. Different parameters, i.e., shapes and properties of trilinearity (within the peaky soil model type), have been considered leading to a closed-form solution. This solution improves the understanding of the main properties involved in the peaky trilinear soil behavior by providing a set of analytical expressions for pipe walking, which were benchmarked and validated against a set of finite element analyses.
Lateral resistance of pipes on rocky seabeds: comparison between measurements and models based on synthetic seabeds
Publisher: American Society of Mechanical Engineers
Date: 25-06-2017
Abstract: The behavior of pipelines, cables and umbilicals on rocky seabeds has to date received little research attention. This is despite the marine renewable energy and oil and gas industries relying on these ‘pipes’ to cross a variety of rocky seabed types in the presence of extreme metocean conditions. Present design solutions are challenging and costly, yet there remains a track record of in-service failures. This paper forms part of a wider research effort being undertaken by the University of Western Australia (UWA) into pipe behavior on rocky seabeds. This work includes the effects intermittent gaps have on hydrodynamic forces, the effect of seabed roughness on enhanced boundary layer thickness and the validity of existing hydrodynamic force models for small diameter cables. In this paper, the lateral resistance of pipes on rocky seabeds is investigated using both physical and numerical testing of model pipes over artificially-created rocky seabeds. Four model pipes of varying diameter have been displaced laterally over 1 m square model rocky seabeds, with a range of pipe to rock diameters. The lateral resistance of the physical pipe tests were recorded using load cells and a digital data-logger. Analysis of the physical test results has enabled comparison to (and refinement of) numerical models as well as improved understanding of the importance of different parameters. Our results show peak frictions above 6 arising under conditions where interface friction is only about 0.3, which contrasts dramatically with the friction value of 0.6 nominated in F109. This work contributes towards generation of new design methods suitable for application to field conditions.
Theoretical framework for predicting the response of tolerably mobile subsea installations
Publisher: Thomas Telford Ltd.
Date: 07-2017
Abstract: Tolerable mobility of subsea foundations and pipelines supporting offshore oil and gas developments has recently become an accepted design concept. It enables a smaller foundation footprint and so is a potential cost-saving alternative to conventionally engineered ‘fixed’ seabed foundations. Dominant sources of loading on subsea infrastructure arise from connection misalignment or thermal and pressure-induced expansion, and these are reduced if the structure is permitted to displace while ensuring that additional loading is not induced by excessive settlements. A sound prediction of the resulting sliding response will provide a robust design basis for mobile subsea infrastructure. This paper presents a theoretical model based on critical state soil mechanics to predict the performance of a subsea installation that is founded on soft, normally consolidated or lightly overconsolidated soil, and subjected to intermittent horizontal sliding movements. The framework is validated against centrifuge test results and is shown to capture the essential elements of the soil–structure interaction, which include: (a) the changing soil strength from cycles of sliding and pore pressure generation (b) the regain in strength due to dissipation of excess pore pressure (consolidation) and (c) the soil contraction and consequent settlement of the foundation caused by the consolidation process.
Field observations of as-laid pipeline embedment in carbonate sediment
Publisher: Thomas Telford Ltd.
Date: 09-2012
Abstract: Reliable prediction of the embedment of untrenched subsea pipelines is of increasing importance as hydrocarbon developments progress into deeper waters, located further from shore. Pipeline design issues such as hydrodynamic stability, lateral buckling and axial walking require accurate assessment of the pipe embedment, in order to assess correctly the pipe–soil resistance forces and the thermal insulation provided by the soil. This study presents a detailed back-analysis of the laying process and the as-laid condition of a pipeline on carbonate sediments. The pipe embedment is linked to the relevant soil properties, metocean conditions, vessel motions, and lay geometry along the route. A cycle-by-cycle framework is proposed for the development of embedment as the pipe is subjected to oscillations during laying. The calculations use parameters obtained from standard in situ tests, and are applied across a range of soil and lay conditions along this particular pipeline route. The proposed calculation framework incorporates the effect of the lay rate and the pipeline catenary on the embedment process. It offers a significant improvement on the current practice of applying empirical multiplicative factors to the calculated static embedment in order to account for dynamic lay effects.
The variability of marine sediment erodibility with depth: Centimetric scale effects detected from portable erosion flume tests
Publisher: Elsevier BV
Date: 08-2021
Concept design of a new CPT module for direct in situ measurement of P-Y soil responses
Publisher: CRC Press
Date: 23-06-2022
Modelling the axial soil resistance on deep-water pipelines
Publisher: Thomas Telford Ltd.
Date: 09-2012
Abstract: Axial pipe–soil resistance is an important aspect of deep-water pipeline design, since it influences the longitudinal and lateral buckling responses under thermally induced expansion and contraction of the pipeline. Experimental evidence has shown that the axial resistance, expressed as a proportion of the submerged pipeline weight, can vary by an order of magnitude, depending on the rate of axial movement and cumulative time. This paper provides a theoretical framework for assessing the magnitude of axial friction. The framework is developed within a critical-state context using effective stresses, applicable to any degree of drainage in the soil, quantifying the magnitude and duration of excess pore pressures generated near the pipe/soil interface. Two other aspects of behaviour are added to match the observed velocity dependence of axial resistance: (a) a damage term, leading to contractive volumetric strain at the interface and (b) strain-rate dependence of the mobilised soil strength. Analytical expressions are derived that capture the above features of the response. The resulting variations of normalised frictional resistance with time and velocity are then shown to match experimental data from interface shear-box tests, representing a planar idealisation of the same behaviour, and from model pipe tests.
Load capacity of caisson anchors exposed to seabed trenching
Publisher: Elsevier BV
Date: 2019
A parkable piezoprobe for measuring c(v) at shallow depths for offshore design
Publisher: Thomas Telford Ltd.
Date: 2014
Abstract: The coefficient of consolidation (c v ) at shallow depth is an important parameter for the design of many offshore facilities, particularly pipelines. This paper sets out the concept, operation and interpretation of a new simple tool for measuring c v in situ at shallow depths. This tool is a large cylindrical penetrometer with a hemispherical tip, which is embedded statically under self-weight, for ex le from a winch. After embedment, the tool is ‘parked' – not requiring support from a drill-string – and the pore pressure dissipation is monitored. This ‘parkable' feature allows the tool to be added to conventional drilling or penetrometer systems and used in parallel. The operation of the tool has been simulated using large-deformation finite-element analysis with the Cam Clay plasticity soil model. Results in the form of the normalised undrained penetration response and pore water pressure dissipation–time history are presented. The penetration response indicates the weight requirement of the device. The dissipation curves allow c v to be determined from field measurements of pore pressure.
Estimation of soil strength in fine-grained soils by instrumented free-fall sphere tests
Publisher: Thomas Telford Ltd.
Date: 12-2016
Abstract: The dynamic response of a sphere in soft clay is considered through field tests in which a 0·25 m dia. steel sphere was allowed to free-fall in water and dynamically penetrate the underlying soft soil. The test data, collected in a lake and a sea environment, relate to sphere velocities of up to 8 m/s, reaching sphere invert embedments close to ten diameters. An inertial measurement unit located within the sphere measured the motion response of the sphere during free-fall and penetration in soil. The resulting acceleration data were used within a simple framework that accounts for both geotechnical shearing resistance and fluid mechanics drag resistance, but cast in terms of a single capacity factor that can be expressed in terms of the non-Newtonian Reynolds number. The merit of the framework is demonstrated by using it as a forward model in a series of inverse analyses that calculate the undrained shear strength profile from acceleration data measured in free-fall sphere tests. The good match between these profiles and those obtained from ‘push-in’ piezoball penetrometer tests points to the potential for an instrumented free-fall sphere to be used as a tool for characterising the near-surface strength of soft seabeds.
Partially Mobile Shallow Subsea Foundations: A Practical Analysis Framework
Publisher: American Society of Civil Engineers (ASCE)
Date: 08-2021
Mechanisms of pipe embedment and lateral breakout on soft clay
Publisher: Canadian Science Publishing
Date: 05-2008
DOI: 10.1139/T08-009
Abstract: Pipelines laid on the seabed expand and contract during operating cycles as a result of thermal loading, which can lead to lateral buckling. Analysis of this behaviour requires assessment of the vertical penetration and lateral breakout responses. This paper reports centrifuge modelling of these processes, using advanced image analysis techniques to observe the soil deformation. Simple mechanisms are fitted to the observed deformation patterns, allowing the mobilized soil strength to be back-calculated. The vertical embedment mechanisms closely match plasticity solutions. Even if heave is accounted for, the penetration resistance is slightly higher than calculations based on the undrained strength inferred from a T-bar penetrometer. This discrepancy can be attributed to the additional remoulding and softening during steady flow around a T-bar compared to shallow pipe penetration. The lateral breakout response is brittle, and the peak resistance is governed by the available tensile resistance behind the pipe. During steady lateral sweeping the pipe rises close to the original soil surface. At this stage the resistance is governed by the growth of a soil berm ahead of the pipe. Accurate assessment of the near-surface soil strength is difficult, h ering the use of theoretical solutions.
Calibration of UWA's O-tube flume facility
Publisher: American Society of Mechanical Engineers
Date: 07-2012
Abstract: The O-tube facility, designed and established at the University of Western Australia, is an innovative closed loop flume in which a random storm sequence can be reproduced via control of a large pump system. The O-tube facility is capable of simulating hydrodynamic conditions near the seabed and the interaction with seabed sediment and any infrastructure that is resting on it. The purpose of carrying out the O-tube calibration described in this paper is to obtain the relationship between the motor rotation movement and the flow velocity generated in the O-tube, such that any required storm history within the performance envelope of the O-tube can be reproduced. A range of flow velocities and the corresponding pump speeds were measured under steady current, oscillatory flow and combined flow conditions. It was found that the relationship between the pump speed and the flow velocity varies with the oscillatory flow period. Based on the pump characteristic curves and O-tube system curves, the correlation between the motor speed and the flow velocity was derived by applying hydraulic theory and the principle of energy conservation. The derived correlation is validated by reproducing a wide range of target storm series, including a (1:5.8) scaled 100-year return period storm from the North West Shelf of Western Australia in 40 m water depth.
Editorial: embedded foundations under complex loading
Publisher: Thomas Telford Ltd.
Date: 08-2020
An analytical study of the effect of penetration rate on piezocone tests in clay
Publisher: Wiley
Date: 2006
DOI: 10.1002/NAG.490
The effects of penetration rate and strain softening on the vertical penetration resistance of seabed pipelines
Publisher: Thomas Telford Ltd.
Date: 07-2012
Abstract: Offshore pipelines in deep water are generally laid directly on the seabed, without any additional stabilisation measures. Design parameters that determine the soil resistance to lateral and axial motion of the pipeline are a function of the amount of vertical embedment. However, this latter quantity is difficult to estimate, partly because of the effects of soil heave around the pipeline as it penetrates, and partly because the soil shear strength depends on the strain rate and the degree of softening as the soil is sheared and remoulded. In this paper, a large deformation finite-element approach was adopted to study pipe–soil interaction during vertical embedment of pipelines on the seabed. The simple Tresca soil model was modified to incorporate the combined effects of strain rate and softening. The present large deformation finite-element method was validated by comparing the results with data from centrifuge model tests. A parametric study was then performed, varying the strain rate and softening parameters to explore their effects on penetration resistance. Simple expressions for penetration resistance, incorporating the effects of strain rate and softening, have been developed. The effects of soil strength vertical heterogeneity and buoyancy have also been explored.
Lateral resistance of rigid pipelines and cables on rocky seabeds
Publisher: Canadian Science Publishing
Date: 06-2019
Abstract: Accurate assessment of lateral resistance is critical to ensure the on-bottom stability and integrity of subsea pipelines and cables in the oil–gas and marine renewable energy industries. However, on rocky seabeds recommended practices provide limited recommendations on pipe–seabed interaction, suggesting only a single value for the friction coefficient of 0.6. This paper reports on a programme of physical experiments and theoretical modelling investigating the lateral resistance of pipes on rocky seabeds. It is shown that the peak and mean effective friction can significantly exceed the interface (or Coulomb) friction coefficient when the pipe diameter (D) is similar to the median rock diameter (d n50 ). Only when the pipe diameter becomes large compared to the rock size does the mean effective friction approach the interface friction. The effective friction coefficient was found to vary with variability in rock size and shape, as well as the length of pipe relative to median rock diameter. Each of these findings is reproduced well using the theoretical model. Collectively, the results demonstrate that the effective lateral friction coefficient may be higher than 0.6 for mean friction, and significantly higher for peak friction. This implies that inaccuracy may exist in current design, which may be rectified using the theoretical model.
Analysis of Axial Response of Submarine Pipeline to Debris-Flow Loading
Publisher: American Society of Civil Engineers (ASCE)
Date: 02-2021
Strength assessment during shallow penetration of a sphere in clay
Publisher: Thomas Telford Ltd.
Date: 29-09-2014
Abstract: Strength interpretation from the measured penetration resistance of full-flow penetrometers, such as the T-bar and ball, is generally based on a constant bearing capacity factor associated with a deep flow-round mechanism. This approach may underestimate the strength of near-surface sediments, which is becoming increasingly important for the design of offshore infrastructure such as pipelines, steel catenary risers and mudmats. This paper describes a series of centrifuge experiments designed to capture the change in the capacity factor of a ball penetrometer during shallow penetration. A rigorous consideration of soil buoyancy is provided. This is an important consideration in soils with a higher strength to self-weight ratio because a cavity is formed by the passage of the ball and remains open to greater depths. The depth at which a full-flow mechanism develops is related to the dimensionless strength ratio, expressed as the ratio of the undrained shear strength to the effective unit weight and penetrometer diameter. This observation forms the basis for proposed formulations that describe the evolution of the bearing capacity factor with depth for different dimensionless strength ratios. These formulations can be used to determine more accurately the undrained shear strength of near-surface soil over the range of dimensionless strength ratios that is of interest to offshore applications.
A wireless high-speed data acquisition system for geotechnical centrifuge model testing
Publisher: IOP Publishing
Date: 11-08-2009
Centrifuge Modelling of Riser-Soil Stiffness Degradation in the Touchdown Zone of a Steel Catenary Riser
Publisher: ASMEDC
Date: 2008
Abstract: Steel catenary risers (SCRs) are economical to assemble and install compared to conventional vertical risers. However, accurate evaluation of the fatigue life of an SCR remains a major challenge due to uncertainty surrounding the interaction forces at the seabed within the touchdown zone (TDZ). Fatigue life predictions are heavily dependant on the assumed stiffness between the riser and the seabed and therefore an accurate assessment of seabed stiffness — or more specifically the nonlinear pipe-soil resistance — is required. During the lifespan of an SCR, vessel motions due to environmental loading cause repeated penetration of the riser into the seabed within the TDZ. This behaviour makes assessment of seabed stiffness difficult due to the gross deformations of the seabed and the resulting soil remoulding and water entrainment. This paper describes a model test in which the movement of a length of riser pipe was simulated within the geotechnical beam centrifuge at the University of Western Australia. The model soil was soft, lightly over-consolidated kaolin clay with a linearly increasing shear strength profile with depth, typical of deepwater conditions. The pipe was cycled over a fixed vertical distance from an invert embedment of 0.5 diameters to above the soil surface. This range represents a typical vertical oscillation range of a section of riser within the TDZ during storm loading. The results indicate a significant degradation in the vertical pipe-soil resistance during cyclic vertical movements. Due to the cyclic degradation in soil strength, the component of the vertical resistance created by buoyancy was significant, particularly due to the influence of heave. A new approach to the interpretation of heave-enhanced buoyancy was used to extract the separate influences of soil strength and buoyancy, allowing the cyclic degradation in strength to be quantified. During cycling, the soil strength reduced by a factor of 7.5 relative to the initial penetration stage. This degradation was more significant than the reduction in soil strength during a cyclic T-bar penetration test. This contrast can be attributed to the breakaway of the pipe from the soil surface which allowed water entrainment. This dramatic loss of strength and therefore secant stiffness, and the significance of the buoyancy term in the total vertical pipe-soil resistance, has implications for the fatigue assessment of SCRs.
Effects of drainage on the response of a sliding subsea foundation
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-108
Finding space for offshore wind to support net zero: A methodology to assess spatial constraints and future scenarios, illustrated by a UK case study
Publisher: Elsevier BV
Date: 08-2023
Elastoplastic consolidation solutions for scaling from shallow penetrometers to pipelines
Publisher: Canadian Science Publishing
Date: 06-2017
Abstract: The build-up of friction on seabed pipelines is an important design consideration, affecting their stability and the resulting in-service strain and fatigue. The consolidation beneath a partially embedded pipeline has been investigated in the past and linked to the build-up of axial pipe–soil resistance. This paper extends previous work by providing solutions for consolidation around a new class of shallow penetrometer, to provide a basis to scale from site investigation results directly to the build-up of pipeline friction. Small-strain finite element analyses, using the Modified Cam Clay soil model, are presented for the novel toroid and ball penetrometers. The effects of initial penetrometer embedment, device roughness, strength gradient, and overload ratio have been explored in a comprehensive manner, and are compared with pipe results. The toroid penetrometer shows excellent agreement with an element of an infinitely long pipe, simplifying the scaling process. The ball penetrometer shows a faster consolidation response, typically by a factor of three, reflecting the more effective drainage mechanisms of a three-dimensional device compared to a plane strain device. The dissipation responses are fitted by simple equations to aid application in design.
Pipe Clamping Mattresses to Mitigate Flowline Walking; Physical Modelling Trials on Three Offshore Soils
Publisher: OTC
Date: 09-08-2021
DOI: 10.4043/31064-MS
Abstract: Pipe cl ing mattresses (PCMs) are a relatively new system for providing anchoring force to pipelines, to mitigate offshore flowline ‘walking’. They represent a cost-effective and highly efficient alternative to anchor piles, rock dump and conventional concrete mattresses. The system comprises a hinged concrete structure that cl s onto a section of laid pipeline, with concrete ballast logs securing the cl ing action – with the benefit that 100% of the submerged weight of the PCM contributes to axial friction. PCMs have been applied successfully to one deepwater project, but performance data showing the influence of soil type, and allowing a general design framework to be established, has not yet been available. This paper addresses this gap by investigating the performance of PCMs through three series of centrifuge tests, supported by three Operators. Each series comprises tests on a different reconstituted deepwater soil as follows: (a) West African clay (b) Gulf of Mexico clay and (c) carbonate silty sand. In each test, a scaled pipeline is installed in-flight and cycled axially to represent its prior operating life. Scaled PCM models and ballast units are then installed onto the pipe in-flight, mimicking the use of PCMs to mitigate pipeline walking during operation. After installation of the PCMs, further axial cycles are applied, with the system settlement and changes in axial resistance and excess pore pressure measured. The paper shows the performance and applicability of PCMs for a range of soil types, highlighting variations in axial resistance and settlement. The suite of results will help to calibrate design tools for industry, removing unnecessary conservatism and enabling an optimised pipeline anchoring solution to be designed. Key results are equivalent friction factors for the combined pipe-PCM system and PCM settlement, which both show behaviour dependent on soil type. In the clay soils, friction increases significantly over time due to ‘consolidation hardening’. This provides validation of an important effect that has only recently been recognised in pipeline design. In contrast, hardening behavior is not evident in silty sand – although the study suggests there is potential for increasing resistance associated with settlement, which appears to mobilize additional (wedging) stress around the pipeline. Upon PCM installation, the pipelines embed further due to the added weight. Additional settlement occurs during cycling of the system, due to immediate soil deformation and consolidation-related compression. The magnitude of embedment is greater for the clay soils, but in all cases does not cause the cl ing action to release. Overall, the efficiency of the PCM system in providing a high level of anchoring force per unit weight placed on the seabed is confirmed. Long term anchoring forces in the range 50-100% of the submerged weight of the PCM are demonstrated. This is several times more efficient than the commonly used alternative of a rock berm.
New design methods for subsea power cables are helping the global marine renewable energy industry lower costs and improve reliability
Publisher: American Society of Mechanical Engineers
Date: 05-06-2022
Abstract: University of Western Australia’s Oceans Graduate School to transform the design methods used to analyse the on-bottom stability of subsea power cables. These cables form the vital connection enabling renewable energy to be reliably and cost-effectively transported from the source to the consumer — whether the collection device at the end of the line is wave, tidal, fixed or floating wind. These novel design methods are equally applicable to umbilicals and other small diameter pipelines. Existing oil and gas pipeline codes overlook much of the physics that is relevant to small diameter pipes and cables. The new research has unlocked significant improvements through new laboratory, numerical and field observational analysis and modelling. The results of this research are presently being incorporated into new design guidance, including the draft British Standards Institute BS 10009 being developed under PEL 114 technical committee guidance. This paper provides a summary of the research together with observations and lessons learnt in the application of these new design methods to over 7.4 GW of new offshore wind and other renewable energy cables. Given that as at 2020 the global grid-connected total offshore wind capacity was 35 GW, this contribution demonstrates strong evidence of field validation of these research outcomes, as well as their relative importance and transformative potential to contribute to global decarbonisation.
Dynamic seabed-anchor capacity enhancements for taut-moored floating offshore wind
Publisher: TU Delft OPEN Publishing
Date: 03-10-2023
Effect of prior loading cycles on vertical bearing capacity of clay
Publisher: Thomas Telford Ltd.
Date: 12-2014
Abstract: The strength of a foundation on soft clay rises over time due to consolidation under the applied load. This effect can be advantageous in design. Previous studies have identified relationships between the level of monotonic vertical preload, as a proportion of the initial bearing capacity, and the subsequent consolidated bearing capacity. This study explores the influence of cycles of preloading by way of centrifuge model tests, and shows that repeated preloading leads to a further gain in bearing capacity. The gains are more significant if the preloading involves two-way cycles. For the cases explored in this study, the cyclic effect typically adds 50% to the gain in bearing capacity caused by the same preload applied monotonically.
A re-examination of the hydrodynamic forces acting on partially-buried submarine pipelines
Publisher: American Society of Mechanical Engineers
Date: 07-2012
Abstract: The hydrodynamic forces exerted on a pipeline partially buried in a permeable seabed subjected to combined random oscillatory flow are investigated experimentally. The tests were carried out in a re-circulating flume that can generate steady currents, oscillatory flow and combined flow, known as the O-tube. The performance of the testing facility was validated against published experimental data under regular oscillatory flow conditions. Four tests with different embedment depths, under random oscillatory flow conditions were then carried out. The flow velocity, hydrodynamic pressure around the model pipe and pore pressure in the soil were monitored. The hydrodynamic forces were calculated through pressure integration around the model pipe. The hydrodynamic force reduction due to embedment was compared with the model given in the most widely-used pipeline stability design guideline, DNV-RPF109. A horizontal load reduction of 55% was found for the fully buried pipe, which was lower than the 70% reduction suggested by DNV-RP-F109. The variation of the vertical hydrodynamic load with embedment ratio (embedment depth to pipe diameter) was also found to be different from the model suggested by DNV-RP-F109. It was found the fully buried pipe experienced a large vertical force, even higher than that of the fully exposed pipe, under low KC number conditions.
The evolution of seabed stiffness during cyclic movement in a riser touchdown zone on soft clay
Publisher: Thomas Telford Ltd.
Date: 02-2017
Abstract: Steel catenary risers are pipelines that convey fluids from the seabed to floating structures. The stiffness of the pipe–seabed response, which is the ratio between soil resistance and pipe embedment, in the touchdown zone strongly affects the fatigue accumulation rate, so is an important design parameter. This paper reports a centrifuge modelling study into the long-term pipe–seabed interaction forces on soft clay seabeds, with tests representing many months of behaviour at prototype scale. The results show that the penetration and extraction resistance during large- litude cycles degrades during the initial few tens of cycles, in the same way that cyclic penetrometer tests capture the fall in soil strength from the intact to the remoulded state. Calculations using bearing capacity factors for a cylinder provide good predictions of this response, although if the cycles of movement involve the pipe breaking away from the soil then the resistance reduces by more than the ratio of intact to remoulded strength, and this is attributed to entrainment of water in the soil around the pipe. However, with further cycles, as pore pressure dissipation occurs, the seabed stiffness recovers due to the gain in soil strength from consolidation. Eventually, the remoulding and water entrainment effects are wholly erased, and the stiffness exceeds the initial state. These observations suggest that current design practice – which factors down the soil stiffness to represent the influence of the cyclic degradation and remoulding process – may overlook a significant effect that raises the seabed stiffness, and potentially also reduces the fatigue life.
Laboratory development of a vertically oriented penetrometer for shallow seabed characterization
Publisher: Canadian Science Publishing
Date: 2015
Abstract: Current site investigation practice for offshore pipeline design relies on soil parameters gathered from boreholes or in situ test soundings to depths of 1–2 m below the mudline. At these depths, the fine-grained seabed is very soft and possesses low undrained strength, which can be difficult to measure. This paper describes a centrifuge test programme undertaken to evaluate the feasibility and performance of a novel penetrometer designed to assess the shallow strength of soft seabed over continuous horizontal profiles. This device is termed the vertically oriented penetrometer (VOP). Tests were performed on a normally consolidated kaolin s le, with the VOP translated horizontally at velocities ranging from 1 to 30 mm/s, after embedding the VOP at 30 and 45 mm depths. All tests involved many cycles of VOP forward and backward movement to assess its potential to derive the ratio of intact to fully remoulded strength. Strength determination is achieved by dragging the VOP at a specified embedment depth along the soil surface, and deriving the soil strength from the measured resistance as if the VOP were a laterally loaded pile. The VOP is shown to yield comparable strength measurements to that of a T-bar penetrometer. The VOP is a potentially valuable addition to the range of tools used to characterize soil strength, both in small-scale centrifuge models and, following practical development, potentially also in the field.
Analysis of large diameter pipe pile drivability in tokyo bay using piezocone data
Publisher: American Society of Civil Engineers
Date: 15-02-2010
Evaluation of elastic stiffness parameters for pipeline-soil interaction
Publisher: American Society of Civil Engineers (ASCE)
Date: 06-2016
Anchor loads in taut moorings: The impact of inverse catenary shakedown
Publisher: Elsevier BV
Date: 09-2017
Shear strength of soil berm during lateral buckling of subsea pipelines
Publisher: Elsevier BV
Date: 09-2019
Microscale observation and modeling of soil-structure interface behavior using particle image velocimetry
Publisher: Elsevier BV
Date: 02-2006
DOI: 10.3208/SANDF.46.15
An image-based deformation measurement system for the geotechnical centrifuge
Publisher: Thomas Telford Ltd.
Date: 09-2005
DOI: 10.1680/IJPMG.2005.050301
Abstract: New techniques of image capture and analysis have been recently applied to geotechnical centrifuge modelling. These techniques improve the utility of the geotechnical centrifuge by increasing the detail and precision of deformation measurements. This paper describes the use of a simple image-based deformation measurement system on a drum centrifuge. Precise deformation fields comprising many thousands of measurement points can be evaluated from images of plane strain tests using small soil models typically comprising 2 litres of soil. Digital still photography is used to capture high resolution images. A weighted shutter allows a continuous sequence of images to be captured without the need for an onboard PC or communication with the camera via sliprings. Particle Image Velocimetry (PIV) and close range photogrammetry are used to process the resulting images. Target markers are not needed since PIV operates on the spatial variation in image brightness to measure displacement. Close range photogrammetry allows image distortion to be corrected. Distortion leads to a 5% variation in image scale (or pixel size) for this test geometry. An ex le test of a skirted strip foundation under eccentric vertical load is analysed. The instantaneous velocity field at failure is manipulated to allow the realism of simple upper bound failure mechanisms to be examined. It is interesting to note that although the measured failure load matches almost exactly the optimal upper bound solution, the observed deformation mechanism differs significantly.
Recent advances in offshore geotechnics for deep water oil and gas developments
Publisher: Elsevier BV
Date: 05-2011
Analysis of Soil Strength Degradation during Episodes of Cyclic Loading, Illustrated by the T-Bar Penetration Test
Publisher: American Society of Civil Engineers (ASCE)
Date: 06-2010
Continuous wavelet analysis of pavement profiles
Publisher: Elsevier BV
Date: 03-2016
Comparing CPT and pile base resistance in sand
Publisher: Thomas Telford Ltd.
Date: 2005
Characterisation of near-surface sediments using a blend of vertical and shallow rotational penetrometers
Publisher: CRC Press
Date: 23-06-2022
A fluid-pipe-soil approach for stability design of seabed pipelines
Publisher: OTC
Date: 02-05-2011
DOI: 10.4043/21368-MS
Abstract: The conventional approach to submarine pipeline stability design considers interactions between water and pipeline (fluid-pipe) and pipeline and seabed (pipe-soil). The seabed is typically assumed hydrodynamically stable in this approach. Interactions between the water and the seabed (fluid-soil) are generally considered only as an afterthought. A new approach for assessing the stability of submarine pipelines is under development and is aimed at including seabed stability (or mobility) as a key aspect of the design analysis. An overview of this approach is presented in this paper. A practical method for utilising this design approach has also been developed, and is based on a combination of numerical analysis and physical model testing. On-bottom stability design of submarine pipelines is based on assessing the effects of the environment, namely the ocean and the seabed, on the pipeline. In short, a 'stable' pipeline does not displace (or displaces only by a small and allowable distance) when subjected to any environmental loading that may occur - in particular steady-state and oscillatory (wave-induced) on-bottom currents. This approach is known as 'absolute stability' design. As this method has evolved, the criteria for defining pipeline stability have loosened, and now extend to allowing the pipeline to displace a significant predefined distance laterally - up to tens of pipe diameters - under a given loading condition. Whether the distance is arbitrary, based on the operational constraints or the mechanical strength of the pipeline depends on the design approach and the code of practice utilised. This approach is known as 'dynamic stability' design, reflecting that a full dynamic analysis of the structural response is required to predict the displacement of the pipeline during a design storm event. There are three main interactions that affect the stability of a submarine pipeline. They are the interactions between the water and the pipeline (fluid-pipe) the interactions between the pipeline and the seabed (pipe-soil) and the interactions between the water and the seabed (fluid-soil). Fluid-pipe interactions result in hydrodynamic loading of the pipeline. Pipe-soil interactions result in the mobilization of soil resistance - which is often treated as two independent components, arising respectively from ‘friction’ between the pipeline and the seabed, and passive resistance to pipeline movement provided by the soil that is ahead of the embedded part of the pipe. Strictly these two components are not separate mechanisms, but it is common practice, and a reasonable simplification, to consider them in this way. Fluid-soil interactions result in seabed instabilities such as scour, fluctuations and potential build-up of excess soil pore pressure, and potentially liquefaction of the seabed soil. Pipe-soil interactions such as pipeline displacement may also lead to excess pore pressure generation. Each of the interactions outlined above are dependent on the other interactions and their effect on certain parameters. For ex le, the degree of pipeline embedment is affected by scour and liquefaction (fluid-soil), and in turn affects the hydrodynamic loads acting on the pipeline (fluid-pipe), as well as the passive resistance provided by the soil (pipe-soil). Figure 1 summarises these interactions that affect subsea pipeline stability.
Improved stability design of subsea pipelines on mobile seabeds
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: The on-bottom stability design of subsea pipelines is important to ensure safety and reliability but is challenging to achieve, particularly in Australia due to onerous metocean and seabed conditions, and the prevalence of light gas pipelines. This challenge has been lified by the fact that industry design guidelines have given no guidance on how to incorporate the potential benefits of seabed mobility, which can lead to lowering and self-burial of the pipeline on a sandy seabed. In this paper, we review the learnings of the STABLEpipe Joint Industry Project (JIP), which was initiated with the aim of developing new design guidelines to assess the on-bottom stability of pipelines on mobile seabeds. The paper summarises the new research undertaken within the STABLEpipe JIP to better predict sedimentation and scour, pipe-fluid interaction and pipe-soil interaction. New design methods to assess the on-bottom stability are also outlined, which have been developed based on the new research. These methods have been adopted in a DNVGL guideline authored by the JIP researchers in collaboration with DNVGL and presently available for use by the JIP participants. Finally, applications of the STABLEpipe JIP outcomes and focus areas for further work are discussed.
Interface load transfer degradation during cyclic loading
Publisher: Elsevier BV
Date: 08-2003
Free field sediment mobility on Australia's North West Shelf
Publisher: American Society of Mechanical Engineers
Date: 09-06-2013
Abstract: Under cyclonic conditions, sediment on the North West Shelf (NWS) of Australia may become mobile in shallow water due to classical sediment transport or local liquefaction, and this can affect, for ex le, the on-bottom stability of subsea pipelines. In this paper, three calcareous sediments s led from the NWS are analysed, together with realistic metocean data, to illustrate this potential for sediment mobility on the NWS. Specifically, experiments are performed in a recirculating flume (known as an O-Tube) to measure the erosional behaviour and an additional series of experiments are performed using a shaking table, on which each of the sediments have been liquefied and excess pore pressure measurements recorded to back calculate the consolidation coefficient. Soil characterisation data, threshold velocity measurements and shaking table results have then combined to illustrate the potential for sediment mobility for each of the NWS sediments. Best practice models are used to calculate wave and current combined shear stress at the seabed and excess pore pressure accumulation. We find that for these sediments, freshly deposited in laboratory s les, mobility due to sediment transport or liquefaction is very likely in cyclonic conditions on the NWS. Liquefaction is most likely for loosely packed silt, whilst sediment transport is most likely for sand. However, we also show that in more extreme cyclonic conditions there are a subset of sediments that can become mobile due to both sediment transport and liquefaction.
Episodic simple shear tests to measure strength changes for whole-life geotechnical design
Publisher: Thomas Telford Ltd.
Date: 03-2021
Quantifying spatial variability of as-laid embedment for subsea pipeline design
Publisher: American Society of Mechanical Engineers
Date: 31-05-2015
Abstract: In deep water, the pre-service embedment of on-bottom pipelines is mainly controlled by the lay process and its effects on the soil strength. Predicted ranges in local pipeline embedment are often very wide due to uncertainties associated with the surface soil properties, the influence of the sea state and vessel motions on soil remolding, and the complexity of the large-deformation process of pipe-seabed penetration. Pipeline embedment has a significant impact on pipe-soil interaction resistance — commonly described by the ‘friction factors’ used for pipeline design. Most design checks, such as for controlled lateral buckling, need to be satisfied under both low and high extremes of pipe-soil resistance and therefore pipe embedment. A wide range in pipe embedment can create significant design challenges for pipeline engineers, for ex le where engineered buckle mitigation structures are required to alleviate potential high strains in the pipeline. Data obtained from as-laid field surveys of subsea pipeline embedment have been analyzed to illustrate quantitatively how embedment varies along a pipeline route. This variation is linked to both anthropogenic influences, for ex le as a result of the stop-start pipeline construction process, as well as natural variability in (i) soil conditions along the route and (ii) sea state conditions during laying. Accounting for this variability through statistical analysis of as-laid embedment, and incorporation of appropriate length scales relevant to each design check, can have significant beneficial impacts on pipeline design through reductions in the ranges of pipe-soil friction factors. In some cases these may reduce or negate the need for expensive mitigation. The approaches illustrated in this paper have already found beneficial use on real projects.
The effect of permeability on the erosion threshold of fine-grained sediments
Publisher: Elsevier BV
Date: 2021
A dynamic large deformation finite element method based on mesh regeneration
Publisher: Elsevier BV
Date: 10-2013
Unlocking the benefits of long-term pipeline-embedment processes: Image analysis-based processing of historic survey data
Publisher: American Society of Civil Engineers (ASCE)
Date: 11-2016
The Use of Centrifuge Model Testing to Provide Geotechnical Input Parameters for Pipeline Engineering
Publisher: American Society of Mechanical Engineers
Date: 09-06-2201
Abstract: Pipe-soil interaction testing in geotechnical centrifuges is used as a means of providing project-specific information to support the assessment of ‘friction factors’ and other geotechnical inputs to pipeline engineering. The centrifuge testing method allows moderate-sized soil s les (−0.01–1 m3) taken from the field to be used directly to determine site-specific behavior. The tests might involve simple uplift resistance testing for buried, backfilled pipelines or complicated installation and loading sequences designed to mimic the complex laying and loading histories relevant to laterally-buckling or storm loading of unburied pipelines. The paper explains briefly the principles behind centrifuge modeling and describes more fully how such testing should be used to gain benefit for a project.
An efficient and locking‐free material point method for three‐dimensional analysis with simplex elements
Publisher: Wiley
Date: 05-05-2021
DOI: 10.1002/NME.6685
Abstract: The Material Point Method is a relative newcomer to the world of solid mechanics modelling. Its key advantage is the ability to model problems having large deformations while being relatively close to standard finite element methods, however its use for realistic engineering applications will happen only if the material point can be shown to be both efficient and accurate (compared to standard finite element methods), when modelling complex geometries with a range of material models. In this paper we present developments of the standard material point method aimed at realizing these goals. The key contribution provided here is the development of a material point method that avoids volumetric locking (arising from elastic or elasto‐plastic material behavior) while using low‐order tetrahedral finite elements for the background computational mesh, hence allowing unstructured background grids to be used for complex geometries. We also show that these developments can be effectively parallelized to improve computational efficiency.
Centrifuge modeling to support the design of subsea pipelines
Publisher: American Society of Civil Engineers
Date: 29-03-2012
A comparative field study of free-fall cone and sphere penetrometers in soft sediment
Publisher: Elsevier BV
Date: 07-2023
Stability of subsea pipelines during large storms
Publisher: The Royal Society
Date: 28-01-2015
Abstract: On-bottom stability design of subsea pipelines transporting hydrocarbons is important to ensure safety and reliability but is challenging to achieve in the onerous metocean (meteorological and oceanographic) conditions typical of large storms (such as tropical cyclones, hurricanes or typhoons). This challenge is increased by the fact that industry design guidelines presently give no guidance on how to incorporate the potential benefits of seabed mobility, which can lead to lowering and self-burial of the pipeline on a sandy seabed. In this paper, we demonstrate recent advances in experimental modelling of pipeline scour and present results investigating how pipeline stability can change in a large storm. An emphasis is placed on the initial development of the storm, where scour is inevitable on an erodible bed as the storm velocities build up to peak conditions. During this initial development, we compare the rate at which peak near-bed velocities increase in a large storm (typically less than 10 −3 m s −2 ) to the rate at which a pipeline scours and subsequently lowers (which is dependent not only on the storm velocities, but also on the mechanism of lowering and the pipeline properties). We show that the relative magnitude of these rates influences pipeline embedment during a storm and the stability of the pipeline.
Predicting the changing soil response for vertical pipe-seabed interaction accounting for remoulding, reconsolidation and maintained load
Publisher: American Society of Mechanical Engineers
Date: 25-06-2017
Abstract: Steel catenary risers (SCRs) are subjected to fatigue in the touchdown zone (TDZ) where the pipe interacts with the seabed. In this zone the seabed is subjected to intermittent episodes of cyclic loading and reconsolidation during long-term operation. Cyclic loading, reconsolidation and maintained load can cause variations in the soil strength and stiffness, which has a significant influence on the fatigue life of the riser in the TDZ. The weakening effect of cyclic loading on soil strength is well recognized throughout design practice, and methodologies for determining the cyclic ‘fatigue’ of clay during undrained cyclic loading are well established (e.g. Andersen et al. 1988 Andersen 2015). However, traditional undrained assessments neglect the effects of drainage and consolidation that inevitably occur in pipe-seabed interaction during long-term operational stages, and can lead to changes in stiffness by a factor of up to 5 or 10. This overlooked effect of consolidation on soil resistance and stiffness can be very important for SCR fatigue analysis. In this paper, a new analytical framework considering these effects has been used to analyze vertical pipe-seabed interaction. This framework is developed using a critical-state concept with effective stresses, and by discretizing the soil domain as a one-dimensional column of soil elements. The model can accurately capture the changing soil resistance and stiffness to account for the effects of remoulding, reconsolidation and maintained load. The framework is used to back-analysis the pipe-soil interaction response during small and large litude vertical cycles. The simulation prediction compares well with the measured results from the laboratory (Aubeny et al., 2008), and can accurately capture the observed changes in stiffness of up to a factor of 5.
Contributions to Geotechnique 1948-2008: Physical modelling
Publisher: Thomas Telford Ltd.
Date: 06-2008
DOI: 10.1680/GEOT.2008.58.5.413
Abstract: This paper reviews the major contributions to Géotechnique that relate to physical modelling, including developments in modelling technology, important experimental observations, and the resulting advances in geotechnical engineering. An increasing proportion of the papers published by this journal involve physical modelling, conducted either at 1g or in a geotechnical centrifuge. Over the 60 years since Géotechnique was first published, experimental techniques have advanced significantly, improving the realism of small-scale simulations, and raising the quality and detail of the measurements that can be made. These techniques are reviewed, and some of the consequent advances in relation to foundations, tunnels, retaining walls and slopes are highlighted, as reported in the pages of Géotechnique.
A simple model for the effect on soil strength of episodes of remoulding and reconsolidation
Publisher: Canadian Science Publishing
Date: 07-2010
DOI: 10.1139/T09-137
Abstract: Many geotechnical processes related to offshore foundations and pipelines on clay soils involve intermittent episodes of remoulding and reconsolidation. To assess the resulting response, it is necessary to predict the operative soil strength, which decreases due to remoulding but can increase due to reconsolidation. A simple framework that allows these processes to be linked, via concepts from critical state soil mechanics, is described. The framework is illustrated by back-analysing a T-bar penetrometer test that involved episodes of both remoulding and reconsolidation, which was conducted in a geotechnical centrifuge. It is shown that the model can capture the surprising increases in both the intact and the remoulded strength that are observed between episodes of remoulding.
Interpreting T-bar tests in ultra-soft clay
Publisher: Thomas Telford Ltd.
Date: 03-2014
Abstract: This technical note describes a simple methodology for reliably measuring the undrained strength of ultra-soft consolidating clay in the geotechnical centrifuge, using the T-bar penetrometer. This methodology relies on the T-bar resistance force owing to soil strength being equal, albeit in the opposite direction, during penetration and extraction when the soil is fully remoulded. The other components of resistance from soil buoyancy, bar self-weight and soil lateral pressure on the T-bar axial strain gauge act in the same direction regardless of the direction of bar movement. The method uses cycles of penetration and extraction to determine the correction required to eliminate these effects. The methodology was validated by comparing the strength inferred from 81 T-bar penetrometer tests with a large strain numerical simulation of consolidation and the resulting gain in soil strength.
Parametric solution of lateral buckling of submarine pipelines
Publisher: Elsevier BV
Date: 05-2020
The offshore renewables industry may be better served by new bespoke design guidelines than by automatic adoption of recommended practices developed for oil and gas infrastructure
Publisher: Frontiers Media SA
Date: 12-05-2023
DOI: 10.3389/FMARS.2023.1030665
Abstract: There is an emerging need for the offshore renewable industry to have their own bespoke design guidelines because the associated projects and offshore facilities differ in fundamental ways to oil and gas facilities. Offshore renewable energy (ORE) facilities have already surpassed the numbers of installed facilities in the oil and gas industry by an order of magnitude and demand is forecast to continue growing exponentially. In addition ORE facilities often have different response characteristics and limit states or failure modes as well as profoundly different risk and consequence profiles given they are generally uncrewed and do not contain explosive hydrocarbon fluids which might be released into the environment. Therefore, the purpose of this paper is to advocate for licensing bodies and regulators (such as the various national PEL 114 committees) to challenge the process of automatic adoption of oil and gas design processes, while pushing for offshore renewables to be treated differently, when appropriate, with more relevant and applicable guidance. To support this argument we present new bespoke design guidance developed for subsea cables based on specific modes of cable behaviour, which often differ from pipelines. We also show worked ex les from recent project experience. The results from on-bottom stability analyses of a set of cables are compared between conventional oil and gas guidance following DNV-RP-F109 versus the stability using cable-optimised approaches. The outcomes from the ‘conventional’ oil and gas results are not simply biased compared to cable-optimised design methods, with a trend of being either conservative or unconservative. Instead, the results of the two methods are very poorly correlated. This shows that the oil and gas approach isn't simply biased when applied to cables, but is instead unreliable because it doesn't capture the underlying failure conditions. These analytical comparisons are supported by field observation - the ocean doesn't lie, and makes short work of any anthropogenic structures which are designed with inadequate appreciation of the real world conditions. To support the rapid growth of ORE, we should therefore actively pursue opportunities to rewrite the design rules and standards, so that they better support the specific requirements of ORE infrastructure, rather than legacy oil and gas structures. With more appropriate design practices, we can accelerate the roll out of ORE to meet net zero, and mitigate the climate crisis.
UWA’s O-tube facilities: physical modelling of fluid-structure-seabed interaction
Publisher: CRC Press
Date: 18-12-2014
DOI: 10.1201/B16200-3
Simple prediction of the undrained displacement of a circular surface foundation on non-linear soil
Publisher: Thomas Telford Ltd.
Date: 11-2007
DOI: 10.1680/GEOT.2007.57.9.729
Abstract: Textbooks and university courses teach elasticity and plasticity as separate methods for analysing the stiffness and strength of a shallow foundation. The behaviour of real soil is neither linear elastic nor perfectly plastic. In this paper, two simple techniques for incorporating non-linearity in routine design have been validated using finite element (FE) analysis. These two techniques—Atkinson's method and the mobilisable strength design (MSD) approach—assume that the responses of an in idual soil element and the boundary value problem being considered are self-similar. Using this assumption, the soil element response can be scaled to predict the response of the boundary value problem. Atkinson's method is based on elasticity whereas MSD uses plasticity. Non-linearity has been captured in the FE analysis using a power law soil model. This approach uses minimal parameters, but is shown to capture accurately the undrained stress–strain response of typical clays under monotonic loading. Comparison with elastic and plastic solutions showed that the FE analysis was accurate to within 5% at the elastic and plastic extremes of the loading range. The responses of the soil and the boundary value problem are shown to be sufficiently self-similar up to two-thirds of the failure load for the foundation response to be predicted simply by a linear scaling of the soil response. Previously reported scaling factors for vertical loading are confirmed, and new factors for horizontal and moment loading are derived. These results show that, for this particular boundary value problem, soil non-linearity can be captured with sufficient accuracy for routine design, without recourse to sophisticated numerical analysis. These self-similarity methods are simple enough to be taught to undergraduates, and could be incorporated in textbooks alongside the core sections on elasticity and plasticity, providing guidance on the application of these techniques to real non-linear soil.
Effects of screw pile installation on installation requirements and in-service performance using the discrete element method
Publisher: Canadian Science Publishing
Date: 09-2021
Abstract: Existing guidance on the installation of screw piles suggest that they should be installed in a pitch-matched manner to avoid disturbance to the soil that may have a detrimental effect on the in-service performance of the pile. Recent insights from centrifuge modelling have shown that installing screw piles in this way requires large vertical compressive (or crowd) forces, which is inconsistent with the common assumption that screw piles pull themselves into the ground requiring minimal vertical compressive force. In this paper, through the use of the discrete element method (DEM), the effects of advancement ratio, i.e., the ratio between the vertical displacement per rotation to the geometric pitch of the helix of the screw pile helix, on the installation resistance and in-service capacity of a screw pile is investigated. The findings are further used to assess the applicability of empirical torque capacity correlation factors for large diameter screw piles. The results of the investigation show that it is possible to reduce the required vertical compressive installation force by 96% by reducing the advancement ratio and that although over-flighting a screw pile can decrease the subsequent compressive capacity, it appears to increase the tensile capacity significantly.
Interaction forces between pipelines and submarine slides — A geotechnical viewpoint
Publisher: Elsevier BV
Date: 07-2012
Chain-Soil Interaction in Carbonate Sand
Publisher: OTC
Date: 02-05-2016
DOI: 10.4043/27102-MS
Abstract: The design of mooring anchors of Floating LNG systems on Australia's North West Shelf (NWS) is challenging due to the high mooring loads and the seabed which comprises carbonate soils. Anchors with an embedded padeye require engineering of the chain inverse catenary and management of interfaces between the anchor and mooring system design teams. While considerable investigation of the anchor line performance in clay has been conducted, there has been limited investigation into the anchor line behavior in sand, and none in carbonate sand. The limited relevant information in the codes and standards and public domain may therefore result in over-conservative design for the ground chain, the anchors and the overall mooring system. In order to optimize the design of the mooring systems in relation to a developing Floating LNG project, a program of chain-soil interaction tests has been performed in a geotechnical centrifuge at the Centre of Offshore Foundation Systems in the University of Western Australia. The purpose of the testing was to provide data to allow better assessment of the shape and load distribution on the chain inverse catenary of a large mooring chain at high loads in carbonate sand. The testing achieved scaled loads equivalent to the 10,000 year return period storm experienced by an FLNG facility on the NWS, and included detailed profiling of the inverse catenary at different loading stages. The program spanned a range of chain sizes and soil densities as characterized by miniature cone penetrometer tests. This paper outlines the design considerations for chain-soil interaction and provides guidance for interface management, which sets the centrifuge modelling program in context. The results of the centrifuge program are presented and interpreted against the backdrop of conventional design assumptions and existing theories for chain-soil interaction. Using the interpreted results, a new method for the analysis of chain-soil interaction in carbonate sands is proposed, which includes the use of cone tip resistance profiles. The method and input parameters are calibrated via the centrifuge test results. The insights gained have resulted in improved design assumptions for the conditions modelled, and further refinements of the analysis approach are foreshadowed. These outcomes have led to improved estimates of the inverse catenary configurations and mooring anchor loads, and future work is anticipated to allow more general improvements to design practice.
Discussion on Application of Particle Image Velocimetry (PIV) in Centrifuge Testing of Uniform Clay
Publisher: Thomas Telford Ltd.
Date: 12-2005
Upper bound yield envelopes for pipelines at shallow embedment in clay
Publisher: Thomas Telford Ltd.
Date: 04-2008
Introduction to O-tube
Publisher: WORLD SCIENTIFIC
Date: 11-2011
Geotechnical design and construction aspects of a pipeline-escarpment crossing
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-60
Strength of a carbonate silt at the solid-fluid transition and submarine landslide run-out
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-154
A review of the UK and British Channel Islands practical tidal stream energy resource
Publisher: The Royal Society
Date: 11-2021
Abstract: This review provides a critical, multi-faceted assessment of the practical contribution tidal stream energy can make to the UK and British Channel Islands future energy mix. Evidence is presented that broadly supports the latest national-scale practical resource estimate, of 34 TWh/year, equivalent to 11% of the UK’s current annual electricity demand. The size of the practical resource depends in part on the economic competitiveness of projects. In the UK, 124 MW of prospective tidal stream capacity is currently eligible to bid for subsidy support (MeyGen 1C, 80 MW PTEC, 30 MW and Morlais, 14 MW). It is estimated that the installation of this 124 MW would serve to drive down the levelized cost of energy (LCoE), through learning, from its current level of around 240 £ / MWh to below 150 £ / MWh , based on a mid-range technology learning rate of 17%. Doing so would make tidal stream cost competitive with technologies such as combined cycle gas turbines, biomass and anaerobic digestion. Installing this 124 MW by 2031 would put tidal stream on a trajectory to install the estimated 11.5 GW needed to generate 34 TWh/year by 2050. The cyclic, predictable nature of tidal stream power shows potential to provide additional, whole-system cost benefits. These include reductions in balancing expenditure that are not considered in conventional LCoE estimates. The practical resource is also dependent on environmental constraints. To date, no collisions between animals and turbines have been detected, and only small changes in habitat have been measured. The impacts of large arrays on stratification and predator–prey interaction are projected to be an order of magnitude less than those from climate change, highlighting opportunities for risk retirement. Ongoing field measurements will be important as arrays scale up, given the uncertainty in some environmental and ecological impact models. Based on the findings presented in this review, we recommend that an updated national-scale practical resource study is undertaken that implements high-fidelity, site-specific modelling, with improved model validation from the wide range of field measurements that are now available from the major sites. Quantifying the sensitivity of the practical resource to constraints will be important to establish opportunities for constraint retirement. Quantification of whole-system benefits is necessary to fully understand the value of tidal stream in the energy system.
Predicting the rate of scour beneath subsea pipelines in marine sediments under steady flow conditions
Publisher: Elsevier BV
Date: 04-2016
Interface shear box tests for assessing axial pipe-soil resistance
Publisher: Thomas Telford Ltd.
Date: 2017
Abstract: The clay-interface shear resistance is an important parameter for the design of offshore pipelines, which slide on the seabed as a result of thermally induced expansion, contraction and lateral buckling. This paper presents a methodology for characterising the clay-interface resistance and quantifying the effect of drainage and consolidation during or in-between shearing episodes. Models for describing the clay-interface resistance during planar shearing are presented and compared to test data for a range of drainage conditions from drained to undrained and including the case of episodic consolidation. The test data are from two series of interface shear box tests carried out on marine clays. The effects of normal stress level (in the low stress range), overconsolidation and interface roughness are also examined.
A cyclic p-y model for the whole-life response of piles in soft clay
Publisher: Elsevier BV
Date: 2022
Risk-Based Assessment of Scour Around Subsea Infrastructure
Publisher: OTC
Date: 02-05-2016
DOI: 10.4043/27131-MS
Abstract: Scour poses a significant risk to infrastructure placed on mobile seabeds. Seabed mobility is common on the North West Shelf of Australia, in parts of the North Sea, and also occurs in the deepwater Gulf of Mexico, due to loop currents. Scour can undermine structures and, for shallow-skirted mudmat foundations, there can be significant consequences including excessive settlements, tilt and loss of bearing and sliding capacity. However, scour mitigation via engineered protection is costly, and to be avoided if possible. This paper describes a new quantitative risk-based approach for assessing the susceptibility of subsea infrastructure to scour processes. This probabilistic scour assessment accommodates measurable uncertainties in metocean and seabed conditions, using new characterization techniques. The approach allows operators and owners to better assess the optimum strategy to address scour risk, selecting from mitigation during installation or in-service monitoring, prediction and remediation. The paper describes (i) best practice approaches for assessing scour susceptibility and propagation rates with and without engineered protection, (ii) new methods for determining the applicable seabed and metocean inputs, (iii) a probabilistic framework for encompassing uncertainties, and (iv) how this approach can be applied in project applications. Our probabilistic method of assessing and presenting scour risk produces a distribution of estimates of scour depth and time rate. By capturing and quantifying the full range of uncertainties, this method facilitates decision-making by showing the range of potential outcomes and allowing the associated costs and consequences to be evaluated. This approach is superior to deterministic ‘worst case’ calculations, which are often used to assess scour susceptibility. In summary, this paper provides operators and owners with an improved methodology to unlock Capex and Opex savings through more accurate and informed scour assessments.
The influence of sea state on as-laid pipeline embedment: A case study
Publisher: Elsevier BV
Date: 07-2010
Continuous Characterisation of Near-Surface Soil Strength
Publisher: American Society of Mechanical Engineers
Date: 08-06-2014
Abstract: A sound understanding of near-surface soil strength is essential for the accurate prediction of the response of structures laid on or shallowly embedded in the seabed. However, characterisation of the uppermost region of the seabed, which is typically very soft and at a low-stress state, is extremely challenging. This paper demonstrates a novel technique for characterising the in situ undrained shear strength of near-surface soils using a newly-developed pile penetrometer. The pile penetrometer is vertically embedded into the near-surface soil and is driven laterally. A simple calculation of the resistance mobilised over the embedded depth of the pile penetrometer is presented along with its application to the continuous measurement of spatial variation in near-surface strength in virgin and disturbed regions of soil.
Scour and sedimentation of submarine pipelines: Closing the gap between laboratory experiments and field conditions
Publisher: CRC Press
Date: 29-08-2016
Modelling the degradation of penetration resistance during cyclic T-bar tests in a Gulf of Mexico clay
Publisher: Elsevier BV
Date: 12-2019
Large-deformation finite element analysis of pipe penetration and large-amplitude lateral displacement
Publisher: Canadian Science Publishing
Date: 08-2010
DOI: 10.1139/T09-147
Abstract: Seabed pipelines must be designed to accommodate thermal expansion — which is commonly achieved through controlled lateral buckling — and to resist damage from submarine slides. In both cases, the pipe moves laterally by a significant distance and the overall pipeline response is strongly influenced by the lateral pipe–soil resistance. Here, the process of pipe penetration and lateral displacement is investigated using a large-deformation finite element method, with a softening rate–dependent soil model being incorporated. The calculated soil flow mechanisms, pipe resistances, and trajectories agree well with plasticity solutions and centrifuge test data. It was found that the lateral resistance is strongly influenced by soil heave during penetration and the berm formed ahead of the pipe during lateral displacement. For “light” pipes, the pipe rises to the soil surface and the soil failure mechanism involves sliding at the base of the berm. In contrast, “heavy” pipes e downwards and a deep shearing zone is mobilized, expanding with continuing lateral movement. The different responses are reconciled by defining an “effective embedment” that includes the effect of the soil berm or wall ahead of the pipe. The relationship between normalized lateral resistance and effective embedment is well fitted using a power law.
Mems accelerometers for measuring dynamic penetration events in geotechnical centrifuge tests
Publisher: Thomas Telford Ltd.
Date: 06-2014
Abstract: Micro-electro mechanical system (MEMS) accelerometers are small, inexpensive sensors that have only recently been used in geotechnical centrifuge tests. This is unlike piezoelectric accelerometers, which are by comparison large and expensive but have been used extensively in geotechnical centrifuge tests over the past couple of decades. This paper examines the response of a single-axis ±500g MEMS accelerometer under both static and dynamic conditions in a centrifuge environment. The potential for MEMS accelerometers to be used to measure the depth of objects buried in soil is examined and the achievable resolution is discussed. Unlike piezoelectric accelerometers, which only measure changes in acceleration, MEMS accelerometers can measure both constant and changing accelerations. The merit of this feature is demonstrated through tests in which MEMS and piezoelectric accelerometers are embedded within a dynamically installed model anchor. The MEMS accelerometer is capable of measuring accelerations during both the free-fall phase and the soil embedment phase, whereas the piezoelectric accelerometer is only able to measure the changing accelerations that dominate during the soil embedment phase. Velocity profiles derived from numerical integration of the MEMS accelerations give mudline anchor velocities that agree with independent measurements and anchor embedment depths that agree with direct measurements.
Centrifuge modelling of active slide-pipeline loading in soft clay
Publisher: Thomas Telford Ltd.
Date: 2014
Abstract: Submarine slides are a significant hazard to the safe operation of pipelines in the proximity of continental slopes. This paper describes the results of a centrifuge testing programme aimed at studying the impact forces exerted by a submarine slide on an offshore pipeline. This was achieved by dragging a model pipe at varying velocities through fine-grained soil at various degrees of consolidation, hence exhibiting properties spanning from the fluid to the geotechnical domains, relevant to the state of submarine slide material. To simulate the high strain rates experienced by the soil while flowing around a pipe in the path of a submarine slide, tests were conducted at pipe–soil velocities of up to 4·2 m/s. The changing density and shear strength of the s les were back-calculated from T-bar penetrometer test results. A hybrid approach combining geotechnical and fluid-mechanics-based components of horizontal drag resistance was developed. This approach provides an improved method to link the density and strength of the slide material to the force applied on the pipe. Besides fitting the present observations, the method provides an improved reinterpretation of similar data from the literature.
Lifelong embedment and spanning of a pipeline on a mobile seabed
Publisher: Elsevier BV
Date: 2015
Lateral movement of pipelines on a soft clay seabed
Publisher: ASMEDC
Date: 2011
Abstract: Large deformation finite element analysis using the commercial software ABAQUS has been performed to study the lateral response of pipelines on a soft seabed. Initially, pipe soil interaction simulations are presented for the case of ideal soil, with non-softening strength. Lateral resistance profiles and trajectories of the pipe during lateral motion are investigated for different initial embedment of the pipe. A more realistic soil model incorporating the effects of strain rate and strain softening is then explored. Lateral resistance profiles and trajectories of the pipes from this realistic model are compared with the ideal soil case. Finally, the concept of effective pipe embedment — which accounts for the geometric changes caused by the soil berm ahead of the pipe — is applied to both the ideal and realistic soil model responses. The normalized horizontal resistance response is shown to be linked to the effective embedment in a simple manner, regardless of the other soil and pipeline parameters. This provides a useful contribution towards the development of a general model for describing large- litude lateral pipe-soil interaction, taking due account of the effects of changing geometry and soil strength.
The State of Knowledge of Pipe-Soil Interaction for On-Bottom Pipeline Design
Publisher: OTC
Date: 05-2017
DOI: 10.4043/27623-MS
Abstract: The paper reviews recent advances in the understanding of pipe-soil interaction, and provides a updated knowledge on best practices for on-bottom pipeline design. Since the late 1990s, major programs of research work have been undertaken to develop appropriate models for pipe-soil interaction for seabed pipelines in challenging environments and operating at high temperature and pressure, to mitigate design issues associated with geohazards, hydrodynamic stability and thermal expansion management. Project-specific programs of work have been extended into industry-wide Joint Industry Projects, and operating pipelines are now providing field observations to validate and refine the design analyses. Much of this new knowledge is now maturing into best practices that can be presented in codes and standards. This paper synthesises that work, and provides recommendations of methodologies suited to codification that will guide future projects. The paper has been authored by a team of practitioners and researchers that comprise a Technical Panel working under the API/ISO geotechnical committee, and the paper sets out some of our views on future additions to the API/ISO codes. Recent advances in the treatment of pipe-soil interaction in pipeline design cover a range of aspects, including (i) quantifying subaqeous flow (submarine slide) geohazards - slide runout behaviour, pipeline impact loads and pipe deformation, (ii) predicting pipeline embedment, including the effects of the lay process, and through-life changes due to sediment transport, (iii) modelling axial pipe-soil interaction, including the strong influence of drainage and consolidation on soft soils, (iv) modelling lateral pipe-soil interaction, including cyclic effects such as the growth of soil berms beside the pipe, (v) modelling scour and self-burial, in regions of hydrodynamic activity, and the resulting changes in pipeline stability. Many of these effects are complex, involving temporal changes in seabed bathymetry and soil strength. However, they can also offer significant design efficiencies, providing a motivation to capture them accurately. For ex le, self-burial of a pipeline through seabed mobility may lead to an improvement in stability that reduces the requirement for weight coating or secondary stabilization works. Also, long-term changes in seabed friction due to consolidation following each cycle of expansion and contraction may lead to a progressive stabilization, reducing the need for anchoring. This paper includes ex les where it has been possible for methods emerging from research to be applied in practical design, validated by observations from the laboratory or from operating pipelines. Many aspects of modern methods for pipe-soil interaction analysis are reaching a level of maturity that allows a consensus to be reached on best practices for design. This will unlock consistent and efficient approaches for future pipeline systems, and for management of existing systems.
Quantifying fishing activity targeting subsea pipelines by commercial trap fishers
Publisher: Springer Science and Business Media LLC
Date: 05-10-2021
Mooring system reliability in tropical cyclone and North Sea winter storm environments
Publisher: Elsevier BV
Date: 07-2019
Whole-life modelling of anchor capacity for floating systems: The RSN CSI approach
Publisher: Elsevier BV
Date: 09-2023
Physical and numerical simulation of shallow penetration of a cylindrical object into soft clay
Publisher: American Society of Civil Engineers
Date: 07-03-2008
DOI: 10.1061/40972(311)14
Effect of Surface Heave on Response of Partially Embedded Pipelines on Clay
Publisher: American Society of Civil Engineers (ASCE)
Date: 06-2009
Hydrodynamic forces on near-bed small diameter cables and pipelines in currents, waves and combined flow
Publisher: American Society of Mechanical Engineers
Date: 09-06-2019
Abstract: The on-bottom stability design of subsea pipelines and cables is important to ensure safety and reliability but can be challenging to achieve, particularly for renewable energy projects which are preferentially located in high energy metocean environments. Often these conditions lead to the seabed being stripped of all loose sediment, leaving the cables to rest on exposed bedrock where roughness features can be similar in size to the cables. As offshore renewable energy projects progress from concept demonstration to commercial-scale developments, new approaches are needed to capture the relevant physics for small diameter cables on rocky seabeds to reduce the costs and risks of export power transmission and increase operational reliability. These same considerations also apply to the cables and small diameter pipes — such as umbilicals — required by oil and gas projects located on rocky seabeds. Recent experimental testing using the University of Western Australia’s unique Large O-tube has enabled the experimental measurement of hydrodynamic forces on small diameter cables and pipes in proximity to smooth and rough beds. The tested conditions extend well beyond the existing published parameter range including much higher KC conditions together with seabed roughness which is comparable in size to the diameter. The results provide design data of great relevance to the ongoing development of marine renewable and conventional oil and gas projects, especially on rocky seabeds. This paper presents a summary of the existing knowledge on the subject as a preface to preliminary test results and gives tentative conclusions on the likely outcomes from this work.
Physical modelling of pipe embedment and equalisation in clay
Publisher: Thomas Telford Ltd.
Date: 07-2016
Abstract: Pipelines laid on the seabed are subjected to loads that may cause unacceptable displacements. On fine-grained soils, the capacity of a pipeline to resist these loads is affected by the pipe embedment and any excess pore pressures remaining in the surrounding soil from the laying process. This paper presents results from model tests, performed at near to full scale, investigating the embedment response and the subsequent pore pressure equalisation of a pipeline on a high plasticity marine clay. Existing models for the penetration and dissipation processes are compared with the experimental data. Conventional undrained bearing capacity theory, making minor allowances for strain rate and softening effects, shows good agreement with the observed penetration response. Dissipation solutions based on elastic and elasto-plastic soil models capture the general shape of the pore pressure response. The operative coefficient of consolidation varies between tests, spanning the range between the compression and recompression values observed in oedometer tests. The observations validate the theoretical solutions for penetration resistance, and highlight the uncertainty that must be considered in estimating equalisation times.
Centrifuge modelling of the pushover failure of an electricity transmission tower
Publisher: Canadian Science Publishing
Date: 04-2010
DOI: 10.1139/T09-112
Abstract: Centrifuge model tests were conducted to examine foundation failure mechanisms during rapid horizontal pushover of an electricity transmission line support tower, simulating a broken transmission line response or wind gust loading. A model transmission tower supported on four pad foundations in clay and backfilled with sand was loaded horizontally and the loads at each foundation were measured during fast and slow pushover. The tests examined the influence of tensile resistance mobilized at the underside of the footings, which is difficult to reliably incorporate within design practice due to a lack of accepted quantitative design methods. The measured performance of the tower footings was compared with results from a series of tests where a single footing is subjected to purely vertical loading in compression and tension and was found to be in good agreement. The tower response was back-analysed as a simple push–pull model and the calculated uplift capacity of the footing backfill provided a close match to the observed response of the tower footings subjected to slow pushover. During fast pushover, the additional capacity mobilized due to tensile resistance (suction) created by the reverse bearing capacity beneath the base of the footings subjected to uplift was quantified using a suction capacity factor.
Centrifuge modelling of an instrumented free-fall sphere for measurement of undrained strength in fine-grained soils
Publisher: Canadian Science Publishing
Date: 06-2016
Abstract: This paper describes centrifuge tests in which a model free-fall sphere was allowed to free fall in water before dynamically embedding within reconstituted s les of kaolin clay and two offshore natural clays. Instrumentation within the sphere measured accelerations along three orthogonal axes. The resultant acceleration was used to calculate sphere velocities and displacements. This allowed the penetration resistance acting on the sphere to be expressed in terms of a single capacity factor that captures soil resistance from both shearing and drag, and varies uniquely with the non-Newtonian Reynolds number. Undrained shear strength profiles obtained from a simple inverse analysis of the acceleration data show good agreement with those obtained using conventional push-in penetrometer tests.
Simple solutions for downslope pipeline walking on elastic-perfectly-plastic soils
Publisher: Elsevier BV
Date: 2019
Autonomous Identification of Suitable Geotechnical Measurement Locations using Underwater Vehicles
Publisher: IEEE
Date: 20-09-2021
LDFE study of bottom boundary effect in foundation model tests
Publisher: Thomas Telford Ltd.
Date: 09-2014
Abstract: Centrifuge model tests of deeply penetrating foundations have been widely used to assess the vertical bearing response, particularly in relation to the installation of spudcan foundations that support offshore drilling rigs. The potential influence of boundary effects owing to the proximity of these large foundations to the rigid base of the model container has not been previously addressed. In this study, large deformation finite-element (LDFE) analyses were conducted to assess the extent of the bottom boundary influence zone. Various foundation diameters were considered, with soil s les of sand overlying clay and uniform clay. The sand plug developed beneath the foundation is a major contributary factor to the boundary effect problem. The boundary effect is increased for sand over clay conditions, where a sand plug is entrapped beneath the foundation. The LDFE results were utilised to predict the thickness of the entrapped sand plug for different geometry and soil strength conditions. The results are distilled into a simple relationship that can be used to ascertain the bottom boundary influence zone when planning physical model tests and reinterpreting previous studies. The boundary influence zone predicted by the LDFE analysis agreed well with a corresponding centrifuge test.
Field measurements of the stiffness of jacked piles and pile groups
Publisher: Thomas Telford Ltd.
Date: 06-2006
Strength properties of ultra-soft kaolin
Publisher: Canadian Science Publishing
Date: 04-2014
Abstract: Geotechnical design considerations for offshore pipelines, foundations, and submarine slides involve assessment of the strength of fine-grained soils and the degradation of that strength with disturbance and remoulding. For offshore pipelines and slides, the relevant strength may be very low (a few kilopascals or lower), relating to near-surface soils and high levels of remoulding including the entrainment of additional water. It is commonly acknowledged that soils exhibit a loss of strength when disturbed, but it is not clear how the degradation properties vary with liquidity index. To address this uncertainty, this paper describes a series of centrifuge tests on kaolin s les consolidated from slurries with an initial voids ratio of 4.0. A total of 81 cyclic T-bar tests were conducted in s les with shear strengths ranging from 0.08 to 1.7 kPa (reflecting different stages of consolidation and in situ total stresses). Large-strain consolidation numerical analyses were used to assist the interpretation of the T-bar test results. The results demonstrate that the soil ductility (a parameter controlling the rate of strength degradation) can be linearly correlated to the liquidity index. The proposed ductility–liquidity index correlation is subsequently coupled with a previously published sensitivity–liquidity index relationship for natural clays to establish a model for the strain-softening behaviour observed in a T-bar test as a function of consolidation. In turn, because the sensitivity is a function of the liquidity index, the intact soil strength is linked to the remoulded strength obtained from laboratory (e.g., fall cone or miniature vane test) and simple index tests. These provide an improved basis to characterize softening effects for inclusion in simulations of submarine slide runout and models for soil–structure interactions that involve intense remoulding.
The Impact of Submarine Slides on Pipelines: Outcomes from the COFS-MERIWA JIP
Publisher: OTC
Date: 02-05-2016
DOI: 10.4043/27034-MS
Abstract: This paper presents key outcomes of a 3-year Joint Industry Project funded by 6 Operators on the impact of submarine slides on pipelines. This JIP developed new techniques to simulate slide runout, and assess the resulting loading and deformation of seabed pipelines. The work was distilled into guidance for practical application, which has found adoption on projects. The JIP spanned (i) characterization of soils at the solid-fluid transition, (ii) computational modelling of slide runout – via depth-averaged and continuum finite element methods, (iii) physical and numerical modelling of slide runout and pipeline impact, and (iv) analytical studies of pipeline response during slide loading. These elements combine to provide an improved practical basis for quantifying the risk associated with slide-pipeline interaction. To characterize very soft seabed soils, a new geotechnically-based framework was devised based on extensive measurements of different soils. This framework spans the solid-fluid boundary that is crossed as slides evolve into a debris flow and turbidity current. It is shown that the geotechnical link between water content and shear strength extends continuously – with no phase transformation – far into the fluid domain, allowing a single rheology to be applied throughout. Computational modelling of slide runout used a hierarchy of methods, from large deformation finite element analysis (LDFE) (with rate effects and softening at soil element level), through depth-averaged runout, to energy-based analytical solutions. In some regimes of behavior the simpler methods suffice, allowing efficient use of Monte Carlo methods to tackle uncertainty. More complex runout modes can be replicated by newly-developed LDFE techniques. From a runout analysis results, pipeline impact loads can be assessed using new solutions for the bearing capacity and drag forces on pipelines developed from numerical and physical modelling, which again unify concepts from fluid dynamics and geotechnics. Finally, simple analytical methods for assessing the structural response of a pipeline to a known slide loading are provided. These solutions allow rapid assessment of the response of a pipeline to a specified slide loading. These advances improve the methods available for quantitative assessment of slide runout and slide-pipeline interaction, allowing better determination of the resulting geohazard risk.
Upper bound plasticity analysis of a partially-embedded pipe under combined vertical and horizontal loading
Publisher: Elsevier BV
Date: 02-2008
DOI: 10.3208/SANDF.48.133
Strength of fine-grained soils at the solid-fluid transition
Publisher: Thomas Telford Ltd.
Date: 03-2012
DOI: 10.1680/GEOT.9.P.069
Abstract: Deepwater offshore oil and gas developments require an assessment to be made of the risk of infrastructure damage from submarine slides. The likelihood and magnitude of submarine slides, and the consequent impact loading on seabed infrastructure in the path of the debris from the slide, must be estimated. Export pipelines are especially vulnerable to impact from submarine slides, because of their length and the need to cross canyons and other seabed features that are potential paths for the flowing debris. Characterising the debris material represents a particular challenge, as the original soil, which is typically characterised using conventional geotechnical methods, evolves through remoulding and water entrainment into a viscous fluid. Because of this transition from soil to fluid, characterisation of the strength of flowing fine-grained sediment has been addressed separately within a soil mechanics framework and a fluid mechanics framework, resulting in two different approaches for expressing the strain-rate-dependent strength of debris flows, and the consequential impact loads on pipelines. In this paper we compare the two approaches, and show that the geotechnical characterisation of fine-grained sediments can be extended into the liquid range in a continuous fashion. This is supported by a series of undrained shear strength measurements on two different remoulded soils, from fall cone tests, vane shear (including viscometer) tests, T-bar and ball penetrometer tests. Analysis of the results shows that the variation in shear strength over the solid and liquid ranges can be described by a unique function of water content, for a given soil. Furthermore, the effects of rate of shearing are well captured by a dimensionless function of the normalised strain rate. The geotechnical approach also accounts for the observed strength reduction due to intense shearing.
Improved Image-Based Deformation Measurement in the Centrifuge Environment
Publisher: ASTM International
Date: 13-09-2013
DOI: 10.1520/GTJ20130044
Numerical modelling of seepage and tension beneath plate anchors
Publisher: Elsevier BV
Date: 04-2019
Life cycle changes in p-y stiffness for a conductor pile installed in carbonate silt
Publisher: Springer Singapore
Date: 25-09-2019
Penetrometer testing in a calcareous silt to explore changes in soil strength
Publisher: Thomas Telford Ltd.
Date: 12-2020
Abstract: This paper describes a centrifuge study using a range of penetrometer tests (T-bar, piezocone and free-fall piezocone) to explore strength changes in a reconstituted, normally consolidated, natural calcareous silt. Various penetrometer test procedures were applied to measure the penetration resistances including monotonic, cyclic and twitch-type movements as well as pauses for pore pressure dissipation. These mobilised combinations of partial or full remoulding, strain softening, consolidation and viscous rate effects. The penetrometer resistance – representing a proxy for strength – reduced by a factor of 4·1 from drained to undrained conditions (at the lowest fully undrained penetration rate). In undrained conditions, viscous enhancement of the penetration resistance raised the tip and shaft resistance in free-fall piezocone tests by ∼2·8 and ∼3·6 times, respectively. The ‘restart’ resistance immediately after the dissipation tests was ∼2·5 times higher than the resistance prior to dissipation, giving an indication of consolidation-induced strength gain. The ‘twitch’ test (using sequential steps decreasing the velocity) captured drainage and viscous rate effects, and also gave a ‘restart’ resistance that showed even greater consolidation effects than from a dissipation test. Overall, the different penetrometer test types and procedures measured resistances in the same soil s le that varied by a factor exceeding 20 from highest to lowest, resulting from different penetration rates and history, due to strain rate, strain level (or remoulding) and consolidation. An expression for the monotonic penetration resistance combining drainage and viscous rate effects was fitted to the response of all tests, spanning orders of magnitude in strain rate.
PSD measurement using the single particle optical sizing (SPOS) method
Publisher: Thomas Telford Ltd.
Date: 04-2003
Improved image-based deformation measurement for geotechnical applications
Publisher: Canadian Science Publishing
Date: 05-2016
Abstract: This paper describes and benchmarks a new implementation of image-based deformation measurement for geotechnical applications. The updated approach combines a range of advances in image analysis algorithms and techniques best suited to geotechnical applications. Performance benchmarking of the new approach has used a series of artificial images subjected to prescribed spatially varying displacement fields. An improvement by at least a factor of 10 in measurement precision is achieved relative to the most commonly used particle image velocimetry (PIV) approach for all deformation modes, including rigid-body displacements, rotations, and strains (compressive and shear). Lastly, an ex le analysis of a centrifuge model test is used to demonstrate the capabilities of the new approach. The strain field generated by penetration of a flat footing and an entrapped sand plug into an underlying clay layer is computed and compared for both the current and updated algorithms. This analysis demonstrates that the enhanced measurement precision improves the clarity of the interpretation.
Centrifuge modelling of pipe penetration due to dynamic lay effects
Publisher: ASMEDC
Date: 2008
Abstract: On-bottom pipelines are subjected to small- litude oscillations during installation due to dynamic motion of the pipe within the touchdown zone. This motion is driven by the movement of the lay vessel and hydrodynamic excitation of the suspended length of pipe. The increase in as-laid embedment caused by these dynamic lay effects has a significant influence on various aspects of the performance of the pipeline. A series of centrifuge model tests was conducted to assess the change in embedment of an unburied pipeline subjected to small- litude horizontal oscillations. Two clay s les were used — kaolin clay and a soft high plasticity clay recovered from offshore Angola. Dynamic lay effects were simulated by applying a constant vertical load to the model pipeline then imposing packets of cycles of horizontal motion, representing the pipe movement within the touchdown zone. The results demonstrate that a few (& ) small litude cycles (±0.05D) can double or triple the pipe embedment, and a number of cycles that might represent the entire lay process (∼100) could increase the pipe embedment by up to 8 times relative to the static embedment under the same vertical load. The amount of penetration is governed by the vertical load level relative to the bearing capacity, and the oscillation litude. Significant differences were observed between the two clays. The high plasticity clay was much more susceptible to softening and increased embedment under dynamic loading.
Shallow Penetrometer Penetration Resistance
Publisher: American Society of Civil Engineers (ASCE)
Date: 03-2015
Effect of a strong middle layer on spudcan penetration
Publisher: American Society of Mechanical Engineers
Date: 08-06-2014
Abstract: The rapid penetration of one or more of the foundations of a mobile jack-up rig into the seabed is an ongoing major problem in the offshore industry, with the potential to cause major damage to the structure and endangering any personnel on board. A recent ex le is the jack-up drilling rig Perro Negro 6 incident happened near the mouth of the Congo river in July 2013 with one of the rig’s crew of 103 reported missing and six others injured. This uncontrollable displacement is due to a form of failure known as punch through failure and commonly occurs on stratified seabed profiles. It has been reported that unexpected punch-through accidents have resulted in both rig damage and lost drilling time at a rate of 1 incident per annum with consequential costs estimated at between US$1 and US$10 million [1]. This paper presents the bearing capacity profiles and associated soil flow mechanisms of a common spudcan foundation penetrating into a three layer soft-stiff-soft clay soil through the use of large deformation finite element (LDFE) analysis. The Remeshing and Interpolation with Small Strain (RITSS) [2, 3] technique was implemented in the software package AFENA [4] to conduct the LDFE analysis. Both soil layer thickness and soil layer strength ratios were varied to study their effect on the spudcan penetration responses. The LDFE results of spudcan penetration into the soft-stiff-soft clay soils were calibrated by existing centrifuge test data. A parametric study was then conducted to study the bearing capacity responses and soil flow mechanisms during spudcan large penetrations by varying the soil layer strength ratio and relative layer thickness to the diameter of spudcan. It was found that there were three types of bearing responses during continuous penetration of spudcan: (a) when the top soft layer is relatively thin, the spudcan bearing response was similar to that of two layer soils with stiff over soft clays (b) when the top soil layer thickness is medium, a peak resistance is observed when spudcan penetrates into the middle stiff layer followed by reduction (c) when the soil layer is thick, the peak resistance occurs when spudcan gets into the bottom soft soil layer. The critical thickness of top soil layer is a function of soil strength ratio and middle stiff soil layer thickness. The bearing response types were also corresponding to the soil cavity formations during spudcan initial penetration.
Shallow penetrometer tests - theoretical and experimental modelling of penetration and dissipation stages
Publisher: Canadian Science Publishing
Date: 04-2020
Abstract: Shallow penetrometers are devices that penetrate into and measure the properties of surficial offshore sediments via multi-phase tests involving penetration, dissipation, and rotation stages. In fine-grained soils such as silts and clays, these testing stages yield undrained strength, consolidation, and friction properties relevant to subsea pipeline and shallow foundation design. This paper describes toroid and hemiball devices of the scale for use in box-core s les and associated interpretation methods for the penetration and dissipation stages. The aim of the paper is to provide all tools needed to design and interpret these tests. New large-deformation finite element (LDFE) dissipation solutions are presented, which can be used for back-analysis of the dissipation stage. Results of an extensive laboratory proof testing exercise in kaolin clay, for both the hemiball and toroid penetrometers, are also reported. These results highlight the potential of the two devices to quickly and economically assess strength and consolidation characteristics of fine-grained sediments in box-core s les recovered to the deck of a site investigation vessel.
Drained breakout resistance of a pipeline on a mobile seabed
Publisher: American Society of Mechanical Engineers
Date: 31-05-2015
Abstract: This paper describes a numerical study investigating the effect of sediment transport and associated changes in the local seabed profile on the drained breakout resistance of subsea pipelines. Limit analyses were conducted assessing the breakout response of a pipeline placed on a cohesionless Mohr-Coulomb material considering different seabed profiles around the pipeline. These profiles were determined from surveys of a pipeline on an erodible seabed. The parametric study shows the relative importance of various parameters describing the seabed profile geometry, including the local pipe embedment and the adjacent slope of the seabed. Significant changes in drained resistance occur due to changes in local pipeline embedment resulting from scour induced pipeline lowering and/or sedimentation. The seabed slope local to the pipeline also has a strong impact. The assumption of a flat seabed can lead to predicted seabed resistance that differs significantly from the actual value, accounting for a more natural seabed profile.
Shallow penetrometer tests: Theoretical and experimental modelling of the rotation stage
Publisher: Canadian Science Publishing
Date: 04-2020
Abstract: Shallow penetrometers are a new type of device that measures the properties of surficial offshore sediments via multi-phase tests involving penetration, dissipation, and rotation stages. In fine-grained soils such as silts and clays, these testing stages yield properties relevant to subsea pipeline and shallow foundation design namely, undrained strength, consolidation, and interface friction. This paper describes the fundamentals of the rotation stage, including models required for data interpretation, encompassing both a total and an effective stress framework. Additionally, new relationships to evaluate the pore pressure scaling factor, which is a key interpretation parameter required to convert discrete measurements of pore pressure on the penetrometers to an average pore pressure over the contact area, are developed based on large-deformation finite element simulations. Results from an experimental c aign using kaolin clay s les are presented, illustrating the potential of the devices to rapidly and repeatably measure interface friction properties of fine-grained sediments offshore. The results compare well with comparative measures obtained from shear box tests conducted at similarly low effective stress levels. Recommendations regarding future in situ applications are given at end of the paper.
Uplift resistance of buried pipelines: The contribution of seepage forces
Publisher: Elsevier BV
Date: 04-2022
Relationships between In situ and roller-integrated compaction measurements for granular soils
Publisher: American Society of Civil Engineers (ASCE)
Date: 12-2008
The value of Subsea pipelines to marine biodiversity
Publisher: OTC
Date: 20-03-2018
DOI: 10.4043/28240-MS
Abstract: As offshore oil and gas infrastructure reaches the end of its operational life, owners and regulators will question the best options available to decommission it, with decisions requiring information about the potential ecological value of these structures and the environmental impact of their removal. Using a combination of existing industrial underwater Remotely Operated Vehicle video data and marine scientific remote stereo-video surveys, we describe and compare the fish assemblage on pipelines with those on the adjacent natural seafloor of the North West Shelf of Western Australia. Using these video observations of commercially important fish species on pipelines we were able to summarise the potential monetary value of commercial fish found on pipelines to commercial fisheries. Greater numbers of fish were observed on pipelines, compared with that seen on adjacent seafloor areas, and the monetary value of the fish found on the pipelines was estimated to be ca. six times greater than that of fish in surrounding areas lacking subsea infrastructure. Pipeline spans had high fish abundance, with fish appearing to utilise these spans as refuges. These results, together with allied assessments of marine growth on pipelines, suggest over the course of their operating lives, pipelines gain ecological and fishery value, enhancing the ersity and abundance of fish (including important commercial species) which can be translated to a monetary value for commercial fisheries. With an extensive array of pipeline infrastructure spread across the North West Shelf of Western Australia, knowledge of the ecological and fisheries value of subsea infrastructure is imperative to understanding the environmental and economical consequences of removing infrastructure as part of decommissioning. Where pipelines add value to marine bio ersity and fisheries in Western Australia, engineers and ecologists can work towards quantifying this value and preserving it – and potentially also enhancing it – via novel evidence-based abandonment strategies.
Subsea cable stability on rocky seabeds - Comparison of field observations against conventional and novel design methods
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: As offshore renewable energy projects progress from concept demonstration to commercial-scale developments there is a need for improved approaches beyond conventional cable engineering design methods that have evolved from larger diameter pipelines for the oil and gas industry. New approaches are needed to capture the relevant physics for small diameter cables on rocky seabeds to reduce the costs and risks of power transmission and increase operational reliability. This paper reports on subsea cables that MeyGen installed for Phase 1a of the Pentland Firth Inner Sound tidal stream energy project. These cables are located on rocky seabeds in an area where severe metocean conditions occur. ROV field observation of these cables shows them to be stable on the seabed with little or no movement occurring over almost all of the cable routes, despite conventional engineering methods predicting significant dynamic movement. We cite recent research undertaken by the University of Western Australia (UWA) to more accurately assess the hydrodynamic forces and geotechnical interaction of cables on rocky seabeds. We quantify the conformity between the cables and the undulating rocky seabed, and the distributions of cable-seabed contact and spanning via simulations of the centimetric-scale seabed bathymetry. This analysis leads to calculated profiles of lift, drag and seabed friction along the cable, which show that all of these load and reaction components are modelled in an over-conservative way by conventional pipeline engineering techniques. Overall, our analysis highlights that current cable stability design can be unnecessarily conservative on rocky seabeds. Our work foreshadows a new design approach that offers more efficient cable design to reduce project capex and enhance through-life integrity management.
Engineering and legal considerations for decommissioning of offshore oil and gas infrastructure in Australia
Publisher: Elsevier BV
Date: 02-2017
Lateral stress changes and shaft friction for model displacement piles in sand
Publisher: Canadian Science Publishing
Date: 08-2005
DOI: 10.1139/T05-023
Abstract: The paper describes a series of tests performed in a drum centrifuge on instrumented model displacement piles in normally consolidated sand. These tests examined the influence of the pile installation method, the stress level, and the pile aspect ratio on the increase in lateral effective stress on the pile shaft during static load testing to failure. A parallel series of constant normal load and constant normal stiffness (CNS) laboratory interface shear experiments was performed to assist interpretation of the centrifuge tests. It is shown that although the cycling associated with pile installation results in a progressive reduction in the stationary horizontal effective stress acting on a pile shaft and densification of the sand in a shear band close to the pile shaft, this sand dilates strongly during subsequent shearing to failure in a static load test. The dilation (the amount of which depends on the cyclic history) is constrained by the surrounding soil and therefore leads to large increases in lateral effective stresses and hence to large increases in mobilized shaft friction. The increase in lateral stress is shown to be related to the radial stiffness of the soil mass constraining dilation of the shear band and to be consistent with measurements made in appropriate CNS interface shear tests. The paper's findings assist in the extrapolation of model-scale pile test results to full-scale conditions.Key words: sand, displacement pile, centrifuge tests, shaft friction.
Exploring the bifurcation between sedimentation versus scour onset below pipelines in unidirectional currents
Publisher: CRC Press
Date: 29-08-2016
Investigation of scour onset under seabed pipelines with geometric irregularities
Publisher: CRC Press
Date: 29-08-2016
Video observations of dynamic embedment during pipelaying in soft clay
Publisher: ASMEDC
Date: 2009
Abstract: The as-laid embedment of a pipeline is an important parameter that affects many aspects of pipeline design. Offshore pipelaying is a dynamic process that causes pipelines to embed deeper than predicted based on static penetration models. This additional embedment is due to cyclic pipeline motions at the seabed that soften and displace the soil. This study presents observations from an ROV-based video survey of dynamic pipeline motion and as-laid embedment. The pipeline had a diameter of 12 inches and was installed from a J-lay vessel in 140 m water depth onto soft clay. The observations illustrate typical trends of embedment and seabed disturbance. Different lay conditions were encountered including downtime events and laydown of the final section of pipeline using a chain. From these observations, the effect of the dynamic behavior is quantified. The results show that in this case dynamic lay effects resulted in an as-laid embedment that was an order of magnitude greater than that from the submerged pipeline self weight alone, and was up to five times greater than the static embedment accounting for the vertical stress concentration from the pipeline catenary.
Parametric Solutions for Slide Impact on Pipelines
Publisher: American Society of Civil Engineers (ASCE)
Date: 07-2010
A methodology to calibrate the PICSI cyclic p-y model using experimental results and optimisation
Publisher: Elsevier BV
Date: 11-2023
Limiting cavity depth for spudcan foundations penetrating clay
Publisher: Thomas Telford Ltd.
Date: 11-2005
DOI: 10.1680/GEOT.2005.55.9.679
Abstract: Centrifuge model tests and finite element (FE) analysis have been conducted to study the penetration of spudcan foundations in uniform clay with nominally constant strength with depth. In particular, the transition between shallow penetration, with soil heaving to the ground surface, and deep penetration, with a localised flow-round mechanism, has been investigated. This transition governs the onset of back-flow and hence the depth of soil lying on the installed spudcan, which in turn influences the bearing capacity and also the potential for suction to develop and hence the uplift capacity and moment resistance of the foundation. The maximum cavity depth above the spudcan prior to any back-flow is therefore a critical issue for spudcan assessment in clay. In the centrifuge model tests, a half-spudcan model penetrating against a transparent window has been used to visualise the soil flow mechanisms around the spudcan during penetration. The formation of a cavity above the spudcan is revealed by both centrifuge modelling and FE analysis. It is found that there are three distinct penetration mechanisms during spudcan installation: during initial penetration, an open cavity is formed with vertical walls with further penetration, soil flows partially around the spudcan into the cavity during deep penetration, the spudcan is fully embedded and the soil flow mechanism is entirely localised. Over the wide range of normalised soil strengths explored, the soil back-flow in the second stage was shown to be due to a flow failure that was triggered by the spudcan penetration and not by wall failure, that is, the collapse of the vertical sides of the soil cavity. This observation is supported by FE analysis. The cavity depth due to flow failure is much shallower than the criterion for wall failure that is incorporated in current design guidelines. Instead, a new design chart and expression is suggested with the normalised cavity depth expressed as a function of the soil shear strength, normalised by the effective unit weight of the soil and the spudcan diameter.
Comparison of failure modes below footings on carbonate and silica sands
Publisher: Thomas Telford Ltd.
Date: 03-2013
Abstract: To investigate the differences in failure mechanisms beneath vertically loaded shallow foundations on silica and carbonate sands by way of particle image velocimetry analysis, 1g and centrifuge model tests have been performed. A comparison of silica and carbonate sand behaviour at low stresses and low initial soil density was made possible by 1g scaling conditions, while centrifuge conditions focused on high stresses and high soil density. This strategy provided separate insights into the effects of the compressibility and crushability of both sands. Results indicated a distinct difference in failure mode between the two sands. The footing on silica sand exhibited a shallow failure mode with large localised strains close to the soil surface. In contrast, the footing on carbonate sand mobilised strains lower in magnitude over a greater depth. The increased stress level in the centrifuge reduced the surface heave and resulted in deeper failure mechanisms in both sands. The bearing capacity factor for the experimental results could be back-calculated with simple limit analysis solutions, provided that different friction angles were used to account for the variation of stress levels. However, such an approach masks the influence of compressibility on the failure mechanism in the loose and compressible cases by requiring a lower friction angle than is realistically possible.
Quantifying the increase in lateral capacity of monopiles in sand due to cyclic loading
Publisher: Thomas Telford Ltd.
Date: 09-2017
Abstract: The contribution of this paper is a simple method to predict the higher moment capacity of a monopile in dense silica sand after drained cyclic loading. The method accounts for the effect of cyclic load magnitude, symmetry and number of cycles, and is calibrated against a series of single gravity and centrifuge tests. The agreement between the model test data and the predictions is typically within 2%. Application of the method shows that the moment capacity of a monopile in dense sand, for the conditions tested here, is up to 36% higher after cycling. This contrasts with a 10% reduction that would be predicted using the existing industry standard p–y approach for cyclic loading in sand.
Elastoplastic consolidation beneath shallowly embedded offshore pipelines
Publisher: Thomas Telford Ltd.
Date: 26-04-2012
Abstract: The consolidation response of partially embedded seabed pipelines has been investigated in the past using small-strain finite-element (SSFE) analysis with elastic soil response. This paper presents results from large-deformation finite-element (LDFE) analyses, using the modified Cam Clay plasticity model for the soil, to address the problem more realistically, including initial plastic penetration of the pipe. The effects of initial embedment, pipe–soil interface friction and strength gradient have been explored in a systematic manner and comparisons with the elastic solutions show the influence of non-linearity. Effects of local soil heave on consolidation rate are quantified by comparing results from SSFE and LDFE analyses. The dissipation responses are fitted by simple equations to aid application in design.
A tool for ROV-based seabed friction measurement
Publisher: Elsevier BV
Date: 03-2015
An effective stress analysis for predicting the evolution of SCR–seabed stiffness accounting for consolidation
Publisher: Thomas Telford Ltd.
Date: 05-2020
Abstract: Steel catenary risers (SCRs) are an efficient solution to transfer hydrocarbons from deep-water seabeds to floating facilities. SCR design requires an assessment of the fatigue life in the touchdown zone, where the riser interacts with the seabed, which relies on reliable estimates of the SCR–seabed stiffness over the design life. Current models for SCR–seabed stiffness consider only undrained conditions, neglecting the development and dissipation of excess pore pressures that occur over the life of the SCR. This consolidation process alters the seabed strength and consequently the SCR–seabed stiffness. This paper summarises experimental data that show that long-term cyclic vertical motion of an SCR at the touchdown zone leads to a reduction in seabed strength due to remoulding and water entrainment, but that this degradation is eclipsed by the regain in soil strength during consolidation. The main focus of this paper is on prediction of the temporal changes in seabed strength and stiffness due to long-term cyclic shearing and consolidation, to support calculations of SCR–seabed interaction. The predictions are obtained using a framework that considers the change in effective stress and hence soil strength using critical state concepts, and that considers the soil domain as a one-dimensional column of elements. The merit of the model is assessed by way of simulations of SCR centrifuge model tests with over 3000 cycles of repeated undrained vertical cycles in normally consolidated kaolin clay. Comparisons of the simulated and measured profiles of SCR penetration resistance reveal that the model can capture accurately the observed changes in SCR–seabed stiffness. Ex le simulations show the merit of the model as a tool to assess the timescale in field conditions over which this order of magnitude change in seabed stiffness occurs. It is concluded that current design practice may underestimate the seabed stiffness significantly, but the new approach allows rapid checking of this for particular combinations of SCR and soil conditions.
Performance of a shallow skirted foundation for TLP mooring in carbonate silt
Publisher: CRC Press
Date: 14-05-0182
DOI: 10.1201/B18442-20
Numerical modelling of plate anchors under sustained load
Publisher: Elsevier BV
Date: 06-2023
High resolution imaging and quantitative analysis of HV cable and pipeline trenching in the marine environment
Publisher: European Association of Geoscientists & Engineers
Date: 2019
Plate anchors for mooring floating facilities - a view towards unlocking cost and risk benefits
Publisher: Society of Underwater Technology
Date: 2017
DOI: 10.3723/OSIG17.978
Reply to the discussion by McCarron on “Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design”
Publisher: Canadian Science Publishing
Date: 10-2017
The hydrodynamics of a recirculating (O-tube) flume
Publisher: CRC Press
Date: 29-08-2016
Numerical simulations of pipe-soil interaction during large lateral movements on clay
Publisher: Thomas Telford Ltd.
Date: 08-2012
Abstract: The soil resistance during large lateral movements of pipelines across the seabed is an important input to design solutions for the management of thermal and pressure-induced expansions. To investigate this behaviour, a large-deformation finite-element (LDFE) analysis method involving frequent remeshing was employed. The LDFE method allows the changing geometry of the seabed when disturbed by the pipeline to be incorporated. Also, the effects of strain rate and strain-softening on the undrained shear strength of the soil were accounted for. A back-analysis of a centrifuge modelling simulation was first performed, for validation, and then a parametric study varying the pipe weight and initial embedment was undertaken. The results show that a steady state is generally reached at large displacements, reflecting a balance between the growth of a soil berm ahead of the pipe and the softening of the disturbed soil. The initial breakout response matched well with previously established failure envelopes, and a new interpretation has been proposed to capture the large-displacement response. The ‘effective embedment' concept is used to rationalise the influence of the soil berm ahead of the pipe. This leads to simple new relationships for predicting the steady-state residual lateral resistance, which provide more accurate predictions of the LDFE response than previously established solutions. The complete load–displacement response over large movements was also shown to be well fitted by an exponential relationship, albeit for the specific case of lateral movements under constant vertical load.
Consolidation around partially embedded seabed pipelines
Publisher: Thomas Telford Ltd.
Date: 02-2011
DOI: 10.1680/GEOT.8.T.015
Abstract: When a pipeline is laid on a soft clay seabed, excess pore pressure is generated. During the subsequent dissipation process, the effective stress at the pipe–soil interface and the available axial pipe–soil resistance rise. This ‘set-up' of axial resistance is an important consideration in various aspects of pipeline design, including the mitigation of thermal and pressure-induced expansion, the stability of the pipeline on sloping ground and the assessment of pipe–soil forces during installation. A set of finite-element analyses has been conducted to assess the pore pressure dissipation and consolidation beneath partially embedded seabed pipelines, extending existing solutions for strip footings. It is shown that the curved shape of a pipeline increases the normalised rate of consolidation compared with a strip footing. Dissipation curves for various levels of embedment are presented and the calculated response is shown to compare well with data from a field test conducted on a soft clay. The dissipation curves have been used to derive the development of effective contact force between the pipe and the seabed as consolidation progresses. These results highlight the significant enhancement of this force – and therefore the available axial resistance – that arises from a ‘wedging effect' related to the curvature of the pipe–soil contact surface. This wedging effect leads to a beneficial enhancement of the axial resistance.
Large-scale modelling of soil-pipe interaction during large amplitude cyclic movements of partially embedded pipelines
Publisher: Canadian Science Publishing
Date: 08-2007
DOI: 10.1139/T07-037
Abstract: As the development of offshore hydrocarbons moves into deeper water, pipelines form an increasingly significant part of the required infrastructure. High-temperature high-pressure pipelines must be designed to accommodate thermal expansion and potential lateral buckling. A novel design approach is to control the formation of pre-engineered lateral buckles to relieve the expansion. The litude of these buckles is typically several pipe diameters. Assessment of the force–displacement interaction between the on-bottom pipeline and the seabed is crucial for design. A series of large-scale plane strain model tests has been conducted to measure the response of a pipe segment partially embedded in soft clay, during large litude cyclic movements, mimicking consecutive thermal expansion and contraction at a bend in a pipeline. Four key stages in the force–displacement response have been identified: (i) breakout, (ii) suction release, (iii) resistance against a steadily growing active berm, and (iv) additional resistance during collection of a pre-existing dormant berm. A simple upper bound solution is proposed to model the observed response. This solution captures the experimental trends including growth of the active berm and collection of dormant berms. This approach is the first attempt to quantitatively model the mechanisms underlying the response during large-displacement lateral sweeps of an on-bottom pipeline, accounting for the growth of soil berms.
Vermiculate artefacts in image analysis of granular materials
Publisher: Elsevier BV
Date: 02-2016
Seabed friction on carbonate soils
Publisher: OTC
Date: 05-05-2014
DOI: 10.4043/25398-MS
Abstract: This paper is concerned with the ‘friction’ and ‘sliding resistance’ that can be mobilised on the unusual carbonate sediments prevalent across many parts of the world, including Offshore Australia. Physical centrifuge model testing was conducted to evaluate the axial pipe-soil resistance on a soil obtained from the North West Shelf, offshore Australia. Tests were performed at a range of speeds, spanning drained/undrained conditions, interspersed with consolidation periods. The resulting axial friction factors are generally higher than is often assumed in conventional design practice for pipeline end expansion or buckling and walking analysis, which is consistent with the high friction angles and dilatancy of these angular carbonate soils. In addition, the results demonstrate the effect that soil drainage response and history of pipeline sliding movements can have on the axial-pipe friction. Overall, the results highlight the importance of making site-specific assessments of seabed sliding resistance, rather than rely on non-site-specific results (particularly from different regions) that may display a very different soil response. Lastly, recommendations are made for design assessments of the sliding resistance of carbonate soils.
Experiments using a novel penetrometer to assess changing strength of clay during remolding and reconsolidation
Publisher: American Society of Civil Engineers (ASCE)
Date: 04-2017
Effect of lateral pipe-soil interaction on controlled lateral buckling using pre-deformed pipeline
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: A novel approach to eliminate the onset of global buckling in pipelines is investigated in the paper. The method is based on pre-deforming a pipeline continuously with a specific wavelength and litude prior to installation on the seabed. The response of the pipeline to applied high temperature and pressure was studied in conjunction with variations in the lateral pipe-soil interaction (PSI) — both as uniform friction along the pipe and also with locally varying friction. Pipe and seabed parameters representing a typical wet-insulated infield flow line on soft clay are used. The pre-deformed pipeline has a higher buckle initiation temperature compared to a straight pipeline due to the reduced effective axial force build-up resulting from the low axial stiffness generated by the pre-deformed lobes along the pipeline. The results from this paper show that the strains in the pre-deformed pipeline are not significantly affected by the local variability of lateral PSI but rather by the global mean PSI. At a typical lateral soil resistance, i.e. a friction coefficient of 0.5, lateral buckling occurs at a very high temperature level that is not common in the subsea operation. At a very low friction, i.e. 0.1, lateral buckling occurs at a lower operating temperature but the strain is insignificant. The longitudinal strain of the pipeline is not highly sensitive to the lateral PSI, which is a quite different response to an initially straight pipeline. Therefore, this method could prove to be a valuable tool for the subsea industry as it enables the pipeline to be installed and operated safely at very high temperatures without the need for lateral buckling design and installation of expensive structures as buckle initiators. Even if the pre-deformed pipeline buckles at a very high temperature, during cycles of heat-up and cool-down the buckle shape ‘shakes down’ by geometric rearrangement to minimize the energy, and in doing so creates a series of ‘short pipelines’ in which the longitudinal strain is self-controlled. The system is therefore shown to be very robust in the conditions investigated and not affected by one of the biggest unknowns in seabed pipeline engineering, which is the local variability in lateral PSI.
Displacement and strain paths during plane-strain model pile installation in sand
Publisher: Thomas Telford Ltd.
Date: 08-2004
DOI: 10.1680/GEOT.2004.54.6.375
Abstract: The underlying mechanisms governing the behaviour of displacement piles in sand are not well understood, leading to unreliability in design methods. A series of plane-strain calibration chamber tests has been conducted in order to quantify the penetration mechanism around the pile tip, and the response of the interface layer adjacent to the shaft during further penetration. A series of eight tests is reported, examining the influence of soil type, initial state, pile breadth and the use of a driving shoe. A novel image-based deformation measurement technique has been used to observe the displacement and strain paths, which are found to be relatively independent of soil type. The measured strain paths are similar to predictions made by the strain path method, and contrast sharply with assumptions implicit in cavity expansion solutions. An interface zone adjacent to the pile shaft comprising fine broken soil particles was observed to contract while shearing along the pile–soil interface. This mechanism offers an explanation for the degradation of shaft friction at a given soil horizon with increased pile penetration (‘friction fatigue’), and a subsequent recovery of capacity over time (‘set-up’).
Rapid assessment of cement and fiber-stabilized soil using roller-integrated compaction monitoring
Publisher: SAGE Publications
Date: 2008
DOI: 10.3141/2059-11
Abstract: Test sections of high-early strength (Type III) portland cement and polypropylene monofilament fibers were constructed at the Bradshaw Field Training Area in the Northern Territory (NT), Australia as part of a Joint Rapid Airfield Construction (JRAC) project. Aprons, taxi-ways, and a helipad were stabilized using these materials in combination with screened native soil. The purpose of the test sections was to (a) evaluate the resulting properties for different stabilization dosage rates (b) develop construction methods, criteria (including limits), and quality control guidelines and (c) provide a hands-on training opportunity for the joint United States and Australia military construction team. Testing and monitoring consisted of roller-integrated compaction monitoring (global position systems monitoring pass coverages and compaction machine values) and in situ testing, which included dynamic cone penetration tests, Clegg impact tests, and light-weight deflectometer tests. After the test sections, construction of the helipad helped refine the construction methods and quality control testing for the selected stabilization dosage rates and machine speed. Lessons learned on the helipad were applied to the subsequent aircraft parking aprons and taxiways. Recommendations were developed for rapid stabilization construction procedures and quality control testing using Clegg impact values and light-weight deflectometer for cement-fiber stabilized soils, and the application of roller-integrated compaction technology was demonstrated to document compaction effort and uniformity.
Penetration resistance and stiffness factors for hemispherical and toroidal penetrometers in uniform clay
Publisher: American Society of Civil Engineers (ASCE)
Date: 08-2011
Press-in piling
Publisher: American Society of Civil Engineers
Date: 02-2002
DOI: 10.1061/40601(256)26
Pipe–Seabed Interaction
Publisher: Wiley
Date: 24-01-2018
DOI: 10.1002/9781118476406.EMOE536
Abstract: Subsea pipelines rest on or in the seabed, and therefore their behavior and condition depends on pipe–seabed interactions. Pipelines expand and contract during operation and can also be loaded by hydrodynamic action, turbidity flows, or debris flows. These actions are resisted by seabed reaction forces in the vertical, axial, and lateral directions. In some situations, the pipeline may be deliberately buried, for ex le, to allow overtrawling, provide thermal insulation, or prevent expansion‐induced buckling. However, in many cases, the pipeline may be laid directly onto the seabed. It may self‐bury through the action of sediment transport, remain only partially embedded into the soil, or some intermittent combination of these effects along its route. Assessments of pipe–seabed interaction forces for all the abovementioned burial conditions begin with estimation of the as‐laid (or as‐constructed) embedment. Classical methods such as limit plasticity underpin solutions for embedment and the limiting pipe–soil resistance in each direction. The critical uncertainties are usually the operative soil strength—which is affected by the seabed disturbance during the lay (or backfilling) process and throughout the operating life—and the pipeline embedment (or cover depth). Specific geotechnical testing technologies have evolved to reduce uncertainties in pipe–seabed assessments, including tools for determining the near‐seabed (and disturbed) soil strength and the pipe–soil interface strength at very low stresses. The associated analysis methods also aim to quantify the variability of the seabed reaction forces along the pipeline. The abovementioned predictions of pipe–seabed interaction behavior are performed to feed into structural analyses of the pipeline, which often use a reliability‐based framework. Pipe–soil interaction assessments require close collaboration between the pipeline and geotechnical experts due to the inherent influence of soil–structure interaction, as well as the aim of tailoring the geotechnical assessment to address the particular design criticalities, which vary depending on the seabed and pipeline conditions.
Reclaimed hydrated fly ash as a geomaterial
Publisher: American Society of Civil Engineers (ASCE)
Date: 04-2006
An effective stress framework for estimating penetration resistance accounting for changes in soil strength from maintained load, remoulding and reconsolidation
Publisher: Thomas Telford Ltd.
Date: 2019
Abstract: Some offshore foundations are subjected to intermittent episodes of remoulding and reconsolidation during installation and operational processes. The maintained and cyclic loads, and subsequent reconsolidation processes, cause changes in the geotechnical capacity, particularly in soft clays. This changing capacity affects the in-service behaviour, including changes to the safety margin, the extraction resistance, the stiffness and structural fatigue rates and also the overall system reliability. This paper provides a new analysis framework to capture these effects, based on estimation of the changing soil strength. The framework is developed using critical state concepts in the effective stress domain, and by discretising the soil domain as a one-dimensional column of soil elements. This framework is designed as the simplest basis on which to capture spatially varying changes in strength due to maintained and cyclic loads, and the associated remoulding and reconsolidation processes. The framework can be used to interpret cyclic penetrometer tests, as well as foundation behaviour. This provides a basis for the approach to be used in design, by scaling directly from penetrometer tests to foundation behaviour. Centrifuge tests are used to illustrate the performance of this approach. The penetration resistance during cyclic T-bar penetrometer tests and spudcan footing installation with periods of maintained loading and consolidation is accurately captured. The framework therefore provides a basis to predict the significant changes in penetration resistance caused by changing soil strength, and can bridge between in situ penetrometer tests and design assessments of soil–structure interaction.
Extension of Mohr-Coulomb Model into State Dependent Softening of Sand and Its Application in Large Deformation Analysis
Publisher: Springer Berlin Heidelberg
Date: 2013
Suction caisson extraction resistance in Gulf of Guinea clay
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-19
Sediment transport and trench development beneath a cylinder oscillating normal to a sandy seabed
Publisher: Elsevier BV
Date: 10-2018
Development of a prototype autonomous inspection robot for offshore riser cables
Publisher: Elsevier BV
Date: 08-2022
Modelling spatial variability in as-laid embedment for high pressure and high temperature (HPHT) pipeline design1
Publisher: Canadian Science Publishing
Date: 11-2016
Abstract: Subsea pipelines are being designed to accommodate higher temperatures and pressures. Current modelling approaches that adopt constant lateral seabed resistance along the pipeline do not capture the high spatial variability in as-laid pipeline embedment from field observations, which strongly affects the lateral resistance. Ignoring spatial variability when designing pipelines with engineered buckles leads to higher predictions of axial force along the pipeline, with reduced likelihood of buckle formation. This can result in excessive mitigation measures being adopted, such as sleepers or counteract structures, which significantly increase project costs. Spatial variability of pipeline embedment is not currently handled rationally in design because an understanding of the physical mechanisms that cause as-laid embedment and methods for accurately predicting it have only recently emerged. This paper illustrates how the influence of these physical mechanisms that drive embedment can be extracted from field survey data and then modelled synthetically in design analyses. The impact of embedment variability and the resulting variation in lateral seabed resistance on the lateral buckling response is illustrated. The framework represents an improvement in the way geotechnical uncertainty and variability is handled in pipeline–seabed interaction analyses for use in pipeline design, and has already begun to be implemented in practice.
Improvements in plate anchor capacity due to cyclic and maintained loads combined with consolidation
Publisher: Thomas Telford Ltd.
Date: 08-2020
Abstract: Plate anchor technology is an efficient solution for mooring offshore floating facilities for oil and gas or renewable energy projects. When used with a taut mooring, the anchor is typically subjected to a maintained load component and intermittent episodes of cyclic loading throughout the design life. These loads, and the associated shearing, remoulding and consolidation processes, cause changes in the anchor capacity, particularly in soft, fine-grained soils. The changing anchor capacity affects the mooring performance by changing the safety margin and also the overall system reliability. In this paper the changing anchor capacity in reconstituted, normally consolidated natural carbonate silt was assessed through a series of beam centrifuge tests on horizontally loaded circular plate anchors. The results demonstrate that full consolidation under a typical maintained load leads to a 50% gain in the anchor capacity, and subsequent cyclic loading and reconsolidation can triple this increase. An effective stress framework based on critical state concepts is employed to explain and support the experimental observations. This study shows that, when viewed from a whole-life reliability perspective, maintained and cyclic loading provide a long-term enhancement of anchor capacity in soft, fine-grained soils. This beneficial effect is currently overlooked in design practice, but can be predicted using the framework shown here, which can form the basis for a digital twin that monitors the through-life integrity of a plate anchor.
Lateral boundary effects in centrifuge foundation tests
Publisher: Thomas Telford Ltd.
Date: 09-2017
Abstract: Large deformation finite-element analyses were conducted to explore the effect of the container lateral boundary in centrifuge tests where a large offshore foundation is tested. A spudcan foundation, typically used to support jack-up drilling rigs, was penetrated into uniform clay, uniform sand and sand overlying clay stratigraphies under rough and smooth lateral boundary conditions. The effect that the proximity of the container lateral boundary has on the measured load–penetration response is quantified. The boundary impact varies with the soil profile and boundary roughness, and for sand-only or sand-over-clay conditions the required separation between the foundation and the container boundary is far greater than in clay-only conditions. In most cases, during large penetration of the spudcan foundation, the largest influence from the lateral boundary was on peak resistance, and diminished during the subsequent deeper penetration. For practical use, a centrifuge container design chart is proposed to check whether the dimensions are adequate for the foundation and model size to be used in an experiment. It is shown that the results from many earlier studies could have been potentially influenced by container boundary effects.
Modelling the embedment process during offshore pipe-laying on fine-grained soils
Publisher: Canadian Science Publishing
Date: 2013
Abstract: Subsea pipelines are becoming an increasingly significant element of offshore hydrocarbon developments as exploration moves into deep-water environments further from shore. During the lay process, pipelines are subject to small litude vertical and horizontal oscillations, driven by the sea state and lay vessel motions. Centrifuge model tests have been used to simulate these small- litude lay effects, with varying degrees of idealization relative to the real lay process. In the soft soils found in deep water, pipe embedment can exceed a diameter or more, thus significantly affecting the lateral pipe–soil interaction, axial resistance, and thermal insulation. In this paper, results from centrifuge model tests are used to calibrate a model for calculating the dynamic embedment of a subsea pipeline. The model uses elements of plasticity theory to capture the effects of combined vertical and horizontal loading, and incorporates the softening of the surrounding soil as it is remoulded due to the pipeline motions. Influences from the lay rate, lay geometry, and sea state are included in the calculation process. The model is compared with observed as-laid pipeline embedment data from field surveys at three different offshore sites. Using site-specific soil parameters obtained from in situ testing and idealized pipe loads and motions to represent the load and displacement patterns during offshore pipe-laying, respectively, the model is shown to capture well the final as-laid embedment measured in the field surveys.
Evolving Soil-Conductor Stiffness Due to Multiple-Episode Cyclic Loading
Publisher: Springer Singapore
Date: 2022
Effect of wave boundary layer on hydrodynamic forces on small diameter pipelines
Publisher: Elsevier BV
Date: 10-2016
Observed changes to the stability of a subsea pipeline caused by seabed mobility
Publisher: Elsevier BV
Date: 12-2018
A new facility for studying ocean-structure-seabed interactions: The O-tube
Publisher: Elsevier BV
Date: 12-2013
Controlling lateral buckling of subsea pipeline with sinusoidal shape pre-deformation
Publisher: Elsevier BV
Date: 03-2018
Strategies for Quantifying the Installation Reliability of Skirted Subsea Foundations
Publisher: American Society of Mechanical Engineers
Date: 31-05-2015
Abstract: An increasing number of well-related and pipeline-related subsea structures (e.g. PLETs, ILTs, buckle initiators, etc.) are being placed on the seabed. Many of these structures are founded on unskirted and skirted mudmat foundations as a cost-effective, low risk solution. Much attention has been placed in the literature on the geotechnical capacity of subsea mudmat foundations, but less attention on how these types of foundations are installed. In some soil conditions this can be a critical aspect of design: the requirement to add skirts to ensure sufficient foundation capacity comes with the penalty of increasing the necessary foundation weight to ensure that these skirts can be reliably installed. The addition of weight to a foundation increases its installation and fabrication cost and so requires careful treatment in design. As an ex le of this design optimization process, attention is given to dealing with uncertainty associated with variable soil conditions. It is shown how probabilistic geotechnical analysis can give a clearer perspective of cost and risk and how carefully targeted site investigation can also be used in this context.
Centrifuge modelling of upheaval buckling in sand
Publisher: Thomas Telford Ltd.
Date: 06-2001
DOI: 10.1680/IJPMG.2001.010202
Abstract: When operating at high temperature and internal pressure, initial imperfections in a cold laid pipeline can lead, through expansion and an increase in axial stress, to imperfection· growth and upheaval buckling. Efficient design against upheaval buckling requires accurate prediction of soil uplift resistance. However, current design methods are based on empirical uplift factors chosen from soil classification data, and do not capture the strong influence of backfill density on uplift resistance. A series of tests are reported, in which the pipe uplift is modelled in a 0.8 m diameter mini-drum centrifuge. A key advantage of the mini-drum centrifuge is that uplift tests can be carried out on small soil s les taken from boreholes along the pipeline length. Pipe uplift can be considered as a routine laboratory test. The results demonstrate the strong influence of backfill density on peak uplift resistance. Observation of the failure mechanism suggests that existing design methods based on vertical slip planes are unrepresentative of the failure. Instead, a new solution is presented, which more closely represents the observed mechanism, and provides excellent correlation with the measured data of peak uplift resistance across a range of backfill densities. Design curves are presented, which require element test results as input parameters. However, upheaval buckling is an unusual ex le in which centrifuge models, standing alone, provide the most efficient design tool.
SAFEBUCK JIP
Publisher: OTC
Date: 02-05-2011
DOI: 10.4043/21249-MS
Abstract: This paper outlines recent research into axial pipe-soil interaction from the geotechnical elements of the SAFEBUCK Joint Industry Project. The operational axial pipe-soil friction strongly influences the initiation and cyclic development of lateral buckles, and also controls the magnitude of pipeline end expansions as well as rates of axial walking. Results from model tests performed at the University of Cambridge are presented in this paper, and provide new insights into the axial pipe-soil response on fine-grained clayey soils. A simple test arrangement was used to pull an 8 m long plastic pipe axially over a bed of soft natural clay collected from a deepwater location offshore West Africa. Many axial sweeps were performed, spanning a wide range of velocities (0.001 mm/s - 5 mm/s) and a wide range of intervening pause periods (up to several days). Both of these variables had a strong influence on the axial pipe-soil resistance - or ‘friction’. The peak values of equivalent friction factor were as high as 1.5 and the residual values were generally in the range 0.2 - 0.5, but fell to below 0.1 in some cases. Higher peak values are associated with longer waiting periods between axial sweeps. The lowest residual values are associated with the fastest rates of shearing. This wide range of axial resistance was observed in a single test using the same pipe resting on the same soil, which is disconcerting from a design perspective. To identify the origin of this variability, an interpretation based on the generation and dissipation of excess pore pressure is explored. This provides a reasonable explanation for the results, but some unexpected aspects of the behavior remain. The results show the important influence of pore pressure effects, consolidation, and the level of drainage during sliding. They also highlight the complexity of axial pipe-soil interaction. For these experimental results, conventional design calculations do not provide adequate predictions of the observed behavior except for during very slow drained movements. The undrained behavior is not captured by conventional design calculations, which provides a cautionary warning to designers. In particular, in the slow-draining natural clay used in this experiment, very low equivalent axial friction factors - as low as F/W' is ~ 0.1 - can be sustained for a long period of movement. The SMARTPIPE® is a recently-developed tool for performing pipe-soil interaction tests in situ offshore, using an instrumented model pipe mounted on a seabed frame. Selected results from a SMARTPIPE® cyclic axial pipe test performed at a deep water location are also presented and discussed. The results support the proposed interpretation based on the generation and dissipation of excess pore pressure. Some differences exist between the in situ and model test data but they can be explained by the smaller magnitude of axial velocity tested, the higher coefficient of consolidation of the in-situ soil and the absence of pause periods between sweeps. Minimal data from experiments on axial pipe-soil interaction is in the public domain, so the results provided here represent a significant contribution to the available knowledge. This research is continuing within the SAFEBUCK JIP, via additional model testing using a new facility that is described in this paper. The aim is to establish new and more robust design guidance for pipe-soil interaction, to support the reliable and efficient design of seabed pipelines.
Assessment of the consolidated breakout response of partially embedded subsea pipelines
Publisher: Thomas Telford Ltd.
Date: 04-2014
Abstract: The changing soil strength due to consolidation around a subsea pipeline can alter the lateral breakout resistance. Results of elasto-plastic coupled consolidation finite-element analyses are presented that quantify the effect of consolidation on the undrained breakout resistance and trajectory of partially embedded seabed pipelines. Breakout resistance is presented in terms of failure envelopes in vertical–horizontal load space, which also allow the trajectory of the pipe during breakout to be determined by normality. Observed responses for consolidated undrained breakout are described by simple algebraic expressions as a function of initial pipe embedment, pipe self-weight (and consequent consolidation pressure) and breakout load path. Consolidation and the associated enhancement of the shear strength of the surrounding soil can have a significant effect on the breakout response. The assumption in current practice of unconsolidated undrained soil behaviour may lead to underestimation of pipeline breakout resistance and inaccurate prediction of the trajectory of the pipe during breakout.
Foundation punch-through in clay with sand: Analytical modelling
Publisher: Thomas Telford Ltd.
Date: 08-2017
Abstract: Severe punch-through of jack-up rig foundations can occur due to the presence of a stronger sand layer in a bed of relatively soft clay. Analytical estimation of the bearing capacity and leg load–penetration response on such multi-layer stratigraphies is challenging. Accurate mechanism-based models need to be established in each of the layers involved and the effects of the mechanisms in each of the layers on the response in the other layers must be captured. Based on the recently developed failure stress-dependent punch-through models for sand–clay stratigraphies, an extended model is proposed for clay–sand–clay stratigraphies. Half-spudcan particle image velocimetry centrifuge tests and full-spudcan centrifuge tests are used in developing and validating the extended model. The centrifuge test results were discussed in a companion paper and this paper focuses on the analytical developments and prediction assessment. Both spudcan peak resistance (q peak ) and spudcan punch-through depth (d punch ) can be estimated using the model. The predictions by the extended model and by the current industry guidelines are compared against the centrifuge test data. The extended model proposed in this paper outperforms the approaches suggested in the guidelines. An advantage of the proposed approach is that it can be used for either sand–clay or clay–sand–clay scenarios and exhibits excellent performance compared to the model testing dataset considered in this work for both cases. The resulting penetration resistance model is a useful design tool for routine punch-through risk assessment.
Pipe-soil interaction: Recent and future improvements in practice
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-42
Foundation punch-through in clay with interbedded sand: centrifuge modelling
Publisher: Thomas Telford Ltd.
Date: 10-2017
Abstract: This paper is concerned with the vertical penetration resistance of conical spudcan and flat footings in layered soils. Centrifuge tests are reported for a clay bed with strength increasing with depth interbedded with dense and medium dense sand. Both non-visualising (full-model) and visualising (half-model) tests were conducted with high-quality digital images captured and analysed using the particle image velocimetry technique for the latter. The load–displacement curves often show a reduction in resistance on passing through the sand layers, which creates a risk of punch-through failure for the foundations when supporting a jack-up drilling unit. For a given foundation, the peak punch-through capacity (q peak ) is dependent on the thickness of both the overlying clay and the sand layer. The failure mechanism associated with the peak resistance in the sand layer involves entrapment of a thin band of top clay above the sand layer that subsequently shears along an inclined failure surface before being pushed into the underlying clay. The top clay height when normalised by the foundation diameter affects the soil failure pattern in this layer and along with the sand layer thickness controls the severity of the punch-through failure (i.e. the additional penetration before the resistance returns to the peak value). Comparisons are made with current industry guidelines for predicting q peak and the risk of punch-through failure for sand overlying clay. These methods are shown to be conservative in their prediction of q peak but inconsistent in predicting punch-through.
An extended interpretation of the free-fall piezocone test in clay
Publisher: Thomas Telford Ltd.
Date: 20-03-2017
Analysis of the added mass term in soil bearing capacity problems
Publisher: Thomas Telford Ltd.
Date: 03-2021
An extended Prandtl solution for analytical modelling of the bearing capacity of a shallow foundation on a spatially variable undrained clay
Publisher: Thomas Telford Ltd.
Date: 09-2022
Abstract: Classical bearing capacity theory was developed mainly based on spatially uniform soil properties, which cannot account for the influence of inherent soil variability. If the soil strength is heterogeneous, then using the average strength may overestimate the bearing capacity of foundations, because the failure mechanism may preferentially mobilise the weaker soils. In this study the aim is to establish a theoretical model using upper-bound solutions applied to the bearing capacity analysis of shallow foundations on undrained clay considering spatial variability. The model is derived on the principle of least energy dissipation using a four-parameter variation on Prandtl's mechanism. The theoretical model developed is verified by the random finite-element (FE) method in spatially varying soil conditions. The results show that the model can accurately capture the effect of spatially varying strength on the shallow foundation failure mechanism. The difference of bearing capacity factor between the proposed model and the FE model is within 5%, which demonstrates that the four-parameter model has an accuracy that is comparable to FE analysis with many hundreds of degrees of freedom. Another advantage of the theoretical model is that the possible non-convergence in FE analysis can be avoided, and hence, the calculation efficiency is significantly enhanced. The model is therefore suitable for rapid quantification of bearing capacity in spatially varying soils.
Real-time natural gas explosion modeling of offshore platforms by using deep learning probability approach
Publisher: Elsevier BV
Date: 05-2023
Direct shear interface tests for pipe-soil interaction assessment
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-48
An effective stress framework for the variation in penetration resistance due to episodes of remoulding and reconsolidation
Publisher: Thomas Telford Ltd.
Date: 2013
DOI: 10.1680/GEOT.9.P.145
Abstract: Steel catenary risers (SCRs) are used to transport hydrocarbon products between offshore floating platforms and the seabed. Like many offshore structures, SCRs are subjected to gross cyclic movements during operation, which remould the seabed soil. The fatigue life of these structures is highly sensitive to the stiffness and strength of the seabed response. Accurate modelling of this behaviour requires a framework that can capture the changes in soil strength and stiffness that occur throughout the design life, accounting for remoulding during extreme events, and reconsolidation during the intervening periods. This paper describes such a framework, which is couched in effective stress terms. Soil softening during remoulding is predominantly associated with excess pore pressure generation, and the subsequent regain in strength is linked to the dissipation of excess pore pressure. The framework can describe the variation of resistance on a cylinder (i.e. a pipe) during any sequence of vertical cyclic motion, interspersed with pause periods. The framework is based on a critical state approach, with the current strength being linked to the current moisture content. The framework is shown to capture well the load–penetration response during an episodic T-bar penetrometer test. The operative soil strength is shown to vary dramatically throughout this event, with the softening effect of remoulding being almost entirely negated by a regain in strength associated with periods of partial or complete reconsolidation. The framework provides a basis for capturing these dramatic effects to aid pipeline and riser design (and other processes that involve gross remoulding and reconsolidation), without recourse to a full numerical simulation of the entire soil domain.
Solving downslope pipeline walking on non-linear soil with brittle peak strength and strain softening
Publisher: American Society of Mechanical Engineers
Date: 25-06-2017
Abstract: In 2000 the first case of pipeline walking (PW) was properly documented when this phenomenon seriously impacted a North Sea high pressure and high temperature (HP/HT) pipeline (Tornes et al. 2000). By then, the main drivers of this problem were accordingly identified for the case studied. On the other hand, to study other aspects related not only to PW, the industry joined forces in the SAFEBUCK Joint Industry Project (JIP) with academic partners. As a result, other drivers, which lead a pipeline to walk, have been identified (Bruton et al. 2010). Nowadays, during the design stage of pipelines, estimates are calculated for pipeline walking. These estimates often use a Rigid-Plastic (RP) soil idealization and the Coulomb friction principle (Carr et al. 2006). Unfortunately, this model does not reflect the real pipe-soil interaction behavior, and in practice time consuming finite element computations are often performed using an Elastic-Perfectly-Plastic (EPP) soil model. In reality, some observed axial pipe-soil responses are extremely non-linear and present a brittle peak strength before a strain softening response (White et al. 2011). This inaccuracy of the soil representation normally overestimates the Walking Rate (WR) (a rigid plastic soil model leads to greater walking). A magnified WR invariably leads to false interpretations besides being unrealistic. Finally, a distorted WR might also demand mitigating measures that could be avoided if the soil had been adequately treated. Unnecessary mitigation has a very strong and negative effect on the project as whole. It will require more financial and time investments for the entire development of the project — from design to construction activities. Therefore, having more realistic and pertinent estimates becomes valuable not only because of budgetary issues but also because of time frame limits. The present paper will show the results of a set of Finite Element Analyses (FEA) performed for a case-study pipeline. The analyses — carried out on ABAQUS software — used a specific subroutine code prepared to appropriately mimic Non-Linear Brittle Peak with Strain Softening (NLBPSS) axial pipe-soil interaction behavior. The specific subroutine code was represented in the Finite Element Models (FEMs) by a series of User Elements (UELs) attached to the pipe elements. The NLBPSS case is a late and exclusive contribution from the present work to the family of available pipeline walking solutions for different forms of axial pipe-soil interaction model. The parametric case-study results are benchmarked against theoretical calculations of pipeline walking showing that the case study results deliver a reasonable accuracy level and are reliable. The results are then distilled into a simplified method in which the WR for NLBPSS soil can be estimated by adjusting a solution derived for RP and EPP soil. The key outcome is a genuine method to correct the WR resultant from a RP soil approach to allow for peak and softening behaviour. It provides a design tool that extends beyond the previously-available solutions and allows more rapid and efficient predictions of pipeline walking to be made. This contribution clarifies, for the downslope walking case, what is the most appropriate basis to incorporate or idealize the soil characteristics within the axial Pipe-Soil Interaction (PSI) response when performing PW assessments.
Effect of drainage on upheaval buckling susceptibility of buried pipelines
Publisher: American Society of Mechanical Engineers
Date: 25-06-2017
Abstract: This paper investigates the effect of soil drainage on the uplift resistance of buried pipelines, and their susceptibility to upheaval buckling. The uplift resistance of buried pipelines is considered through analytical and numerical predictions for both drained and undrained conditions. Combinations of soil strength parameters for typical soils are estimated based on common correlations. For certain ranges of typical normally consolidated soil conditions, particularly those with high critical state friction angles, the drained uplift resistance may be lower than the undrained resistance. This observation is important because in typical practice only drained or undrained behaviour is considered depending on the general type of soil backfill used. In this case, the critical or minimum uplift resistance may be overlooked. Further, the changing undrained uplift mechanism between shallow and deep conditions is investigated. It is found that the common approach of considering the minimum of either a local (flow around) or global (vertical slip plane) failure can overestimate the uplift resistance in normally consolidated soils.
On the selection of an appropriate consolidation coefficient for offshore geotechnical design
Publisher: American Society of Mechanical Engineers
Date: 09-06-2019
Abstract: This paper addresses the selection of an appropriate consolidation coefficient for the analysis of drainage beneath foundations and pipelines in offshore geotechnical design. An emerging trend in the design of subsea infrastructure is the consideration of ‘whole life’ effects — namely the changes in soil properties and geotechnical capacity over the operating life. Seabed pipelines that undergo repeated thermal expansion and contraction cause shearing and consolidation in the underlying soil, leading to significant changes in the available seabed friction. Also, foundations that are either fixed or designed to slide on the seabed, are subjected to intermittent loads interspersed with periods of consolidation. These also cause a change in seabed strength and geotechnical capacity. To assess the time over which these effects occur, and therefore their influence on the response and the reliability of the system, it is necessary to perform consolidation calculations, using an appropriate consolidation coefficient. This paper presents observed operative consolidation coefficients drawn from recent model testing measurements and numerical analyses. It is shown that the consolidation rate can vary by more than an order of magnitude for the same soil profile under different loading conditions, due to the differences in stiffness and permeability. Meanwhile, design parameters are commonly drawn from one-dimensional oedometer compression tests. This compendium of data highlights the potential variation in consolidation coefficient for different loading types and through the ‘whole life’ of infrastructure. A key conclusion is that consolidation effects generally occur faster than is commonly assumed, meaning that changes in strength and stiffness — that are commonly beneficial in design — may be more readily relied on than is done so in current practice.
Performance Problems and Stabilization Techniques for Granular Shoulders
Publisher: American Society of Civil Engineers (ASCE)
Date: 04-2010
Research on the coupling effects between ship motions and sloshing
Publisher: American Society of Mechanical Engineers
Date: 08-06-2014
Abstract: Floating Liquefied Natural Gas (FLNG) facilities, which are a new type of floating platform, have been developed as an alternative to long pipelines for the exploitation of stranded offshore fields. FLNG vessels will be subjected to very complicated and onerous sea states in some regions and very severe motion responses must be avoided. The vessel motion can induce internal tank sloshing, which can in turn affect the global motion response of the vessel. This coupling response is of great importance for the safe design and operation of FLNG facilities in real sea states. This paper investigates the coupling effect between FLNG roll motions and sloshing. Model tests are carried out for a section of an FLNG vessel containing a tank excited in roll by band-limited white noise waves. During the model tests, the FLNG model is ballasted with fresh water and equivalent steel ballast weights respectively, to quantify the coupling effects due to the internal sloshing. Time histories of the global motions and the internal sloshing oscillations have been measured. Statistical and spectral analyses have been carried out on the measured data. The response litude operators are obtained using measured motion spectra ided by the excitation wave spectrum. The influence of the internal sloshing on the global motions has been illustrated through the comparison of the experimental results between the liquid and solid (steel) ballast weight cases. Based on the experimental results, some conclusions regarding the coupling mechanism between FLNG motions and sloshing are drawn.
Estimating compaction of cohesive soils from machine drive power
Publisher: American Society of Civil Engineers (ASCE)
Date: 12-2008
A comparison of the bearing capacity of flat and conical circular foundations on sand
Publisher: Thomas Telford Ltd.
Date: 12-2008
Abstract: This paper describes a series of centrifuge model tests investigating the vertical bearing capacity of flat and conical circular foundations on sand, which are representative of the spudcan foundations of offshore jack-up units. It is found that at shallow embedment the bearing capacity factor, N γ , mobilised by a conical footing is significantly lower—by a factor of up to 2—than that for a flat footing. Plasticity solutions indicate that this discrepancy cannot be attributed to the theoretical differences in N γ that arise from the roughness and geometry of the cone: these theoretical differences are minor ( %). Instead, it is proposed that in dense sand the pre-shearing induced by the conical shape of the footing leads to a form of progressive failure, so a lower operative friction angle is applicable to conical foundations than for flat footings. Since accurate solutions for N γ exist, the key uncertainty in the prediction of bearing capacity is the appropriate friction angle, which is affected by this progressive failure mechanism. A back-analysis that incorporates Bolton's stress–dilatancy correlations in an iterative calculation for bearing capacity is introduced, using recently published definitive values of N γ . This framework is calibrated using centrifuge model test data. The progressive failure mechanism is captured in a simple fashion by varying Bolton's m parameter, which controls the dilatant contribution to peak strength. This back-analysis is used to create simple charts that illustrate the effects of stress level, density and progressive failure on the operative friction angle and N γ . These charts are compared with existing guidance for spudcan bearing capacity.
The mechanism of steady friction between seabed pipelines and clay soils
Publisher: Thomas Telford Ltd.
Date: 12-2011
DOI: 10.1680/GEOT.8.T.036
Abstract: The large- litude lateral soil resistance between an on-bottom pipeline and the seabed is an important design parameter in assessing pipeline behaviour during lateral thermal buckling or under the impact of a submarine slide. This paper describes a series of centrifuge model tests that shed light on the underlying behaviour during large- litude lateral pipe movement. It is shown that at large displacements the lateral response is governed predominantly by the passive resistance of the growing berm of soil ahead of the pipe. Using a new analysis of this growing soil berm, based on conservation of volume, the ‘local' embedment of the pipe relative to the top of the idealised soil berm is defined. In this way, the normalised lateral pipe–soil resistance, H/s u D, from tests encompassing a range of pipe weights and initial embedments follows a single trend line. This idealisation of the response is more consistent than the usual terminology of a pipe–soil friction factor.
Temporal changes in pipeline-seabed condition, and their effect on operating behaviour
Publisher: American Society of Mechanical Engineers
Date: 31-05-2015
Abstract: It is increasingly recognized that the state of the seabed surrounding an on-bottom pipeline may change during the operating life of the pipeline. For seabed sediments that are soft and fine-grained, the strength may vary through episodes of pipeline movement due to consolidation effects. For seabed sediments that are mobile due to waves and currents, the burial state and the adjacent seabed topography may vary due to sediment transport and scour. These changes in the strength and topography of the surrounding seabed alter the exposure of the pipeline to hydrodynamic loads and ambient cooling, as well as the level of geotechnical support and insulation provided by the seabed. The design relevance of these changes in seabed condition is lified by modern design approaches in which the pipeline itself can be tolerably mobile — for ex le in a dynamic onbottom stability approach or through engineered schemes of global buckling and axial walking. This paper illustrates the interactions between the geotechnical and sediment transport processes and the resulting global pipeline behaviour. Two interactions are considered: the long-term axial walking behaviour on soft soil, and the long-term insulation and temperature profile on a mobile seabed. The ex les highlight the potential for over or underestimation of various inputs to a pipeline design when these temporal changes in pipe-seabed condition are overlooked. Emerging analysis methods for pipeline-seabed interaction that incorporate these temporal effects can lead to more reliable and cost-effective design.
Load-controlled cyclic T-bar tests
Publisher: Thomas Telford Ltd.
Date: 03-2020
Abstract: Full-flow T-bar and ball penetrometer tests are often used to measure intact and remoulded soil strengths, with the latter determined after several large- litude displacement cycles. In offshore design, the remoulded soil strength is often the governing design parameter during installation of subsea infrastructure, while a ‘cyclic strength’ applies for the less severe operational cyclic loading. This paper utilises T-bar penetrometer tests to measure both remoulded and cyclic strengths, where the latter is determined by way of a new test protocol involving cycles between load rather than displacement limits. The tests use kaolin clay and a reconstituted carbonate silt and involve three cyclic phases with intervening consolidation periods. The results demonstrate the important and beneficial role of consolidation, with the loss in strength due to remoulding sometimes surpassed by the strength recovery from consolidation. The most significant gains in strength, to 2·5 times the initial value, were measured in the load-controlled cyclic tests. These data demonstrate a novel way to characterise undrained cyclic strength, taking advantage of consolidation to reduce conservatism.
A compact high-speed image capture system for a drum centrifuge
Publisher: CRC Press
Date: 18-12-2014
DOI: 10.1201/B16200-27
Field calibration and spatial analysis of compaction-monitoring technology measurements
Publisher: SAGE Publications
Date: 2007
DOI: 10.3141/2004-08
Abstract: To implement compaction-monitoring technologies (i.e., continuous compaction control and intelligent compaction), robust and versatile specifications are needed. These specifications require field calibration of the various machine sensor outputs to in situ soil compaction measurements. The goal of this study was to provide insights into (a) the nature of compaction-monitoring measurements, (b) how the measurements are related to soil properties determined from in situ compaction control tests, and (c) how compaction-monitoring technology may be addressed in specifications for using the technology in practice. To accomplish this goal, testing was conducted on one-dimensional test strips with several nominal moisture contents for developing statistical regression models that relate machine drive power and compaction meter value data to engineering and index properties of soil. In addition, a two-dimensional test area with variable lift thickness and moisture content was constructed and tested by using both compaction-monitoring technology and in situ devices (e.g., nuclear moisture-density gauge, portable falling weight deflectometer). The spatial distribution of the data was investigated. The significance of this research is that it represents the first documented field calibration of both one-dimensional and two-dimensional tests areas on similar soils and introduces a new approach to generating pass-fail criteria based on compaction-monitoring technology.
Understanding the global scientific value of industry ROV data, to quantify marine ecology and guide offshore decommissioning strategies
Publisher: OTC
Date: 20-03-2018
DOI: 10.4043/28312-MS
Abstract: This paper describes the potential global scientific value of video and other data collected by Remotely Operated Vehicles (ROVs). ROVs are used worldwide, primarily by the offshore oil and gas industry, to monitor the integrity of subsea infrastructure and, in doing so, collect terabytes of video and in situ physical data from inaccessible regions and poorly understood marine environments. The paper begins by describing how recent ROV surveys for projects in Australia have gained a new dimension by involving marine scientists in their interpretation. A previously unrecognised influence of marine life on oil and gas pipelines was uncovered, triggering new collaborations between industry and marine science. This new collaboration prompted a team of international engineers and marine scientists to gather together with West Australian based members of the oil and gas sector and ROV operators, to examine the global scientific value of ROV-collected data. If made available for research, these data have immense value for science to quantify the marine ecology and assist good stewardship of this environment by industry. It was found that most ROV operations are conducted by industry in a way that fulfils immediate industry requirements but which can confound scientific interpretation of the data. For ex le, there is variation in video resolution, ROV speed, distance above substrate and time (e.g. both seasonal and time of day), and these variations can limit the quantitative conclusions that can be drawn about marine ecology. We examined potential cost-effective, simple enhancements to standard ROV hardware and operational procedures that will increase the value of future industrial ROV operational data, without disrupting the primary focus of these operations. The ecological value of existing ROV data represents an immense and under-utilized resource with worldwide coverage. We describe how ROVs can unravel the mysteries of our oceans, yield scientific discoveries, and provide ex les of how these data can allow quantification of the ecological value of subsea infrastructure. By using these data, we can greatly improve our knowledge of marine bio ersity on and around offshore infrastructure and their environmental impact on marine ecosystems, both of which are particularly important in the consideration and selection of decommissioning strategies. Predicting the environmental consequences of removing or retaining subsea structures after decommissioning relies on an understanding of the ecological communities that have developed in association with these structures during their operational lives. Making industrial ROV data available for scientific research, and collating it in the future using modified protocols, would provide a very positive contribution to both science and industry, allowing the environmental impacts of subsea infrastructure to be quantified. It will also allow industry to contribute to a broader scientific understanding of our oceans, given the location of ROVs in areas that can rarely be accessed by independent researchers. This would provide novel and valuable information about under-researched and little known regions of the world's oceans.
Pipeline and cable stability
Publisher: American Society of Mechanical Engineers
Date: 17-06-2018
Abstract: At OMAE in 2008 the ‘state of the art’ in pipeline on-bottom stability engineering was summarized, providing an overview of the current available knowledge for addressing pipeline stability. The aim of that work was to summarise key aspects of the pipeline stability design process and to include some historical perspective. The paper discusses the advantage and shortfalls of the different design approaches with a view to consolidate understanding, rather than to provide a ready-made solution to a complex design problem [1]. Since that time, a decade of research and further methodology refinement has extended the boundaries of the industry’s knowledge and understanding of the behaviour of subsea pipelines and cables, including geotechnics, hydrodynamics, oceanography and structural response modelling. In particular, progress has been made in: • The response of pipelines to sediment transport and scour • Understanding the behaviour of small diameter pipelines and cables within wave and current boundary layers and • The behaviour of cables on rocky seabeds in high energy marine environments. This paper summarises these innovations to enable the application of new paradigms in engineering practice and improved outcomes for initial project capital cost, reliability and operational integrity, as well as better models to predict the long-term behaviour where pipes are decommissioned in-situ. While a relatively widely studied field of engineering, there remain areas of active ongoing research to improve our understanding and ability to model and predict subsea pipeline on-bottom behavior, with a summary of the anticipated future opportunities proposed.
Drained bearing capacity of shallowly embedded pipelines
Publisher: American Society of Civil Engineers (ASCE)
Date: 11-2019
Excess pore pressure redistribution beneath pipelines: FEA investigation and effects on axial pipe-soil interaction
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-39
Analytical modelling of the steady flow of a submarine slide and consequent loading on a pipeline
Publisher: Thomas Telford Ltd.
Date: 02-2012
Abstract: This paper considers a simple one-dimensional model of a submarine slide at a steady state. From equilibrium relations, the distributions of shear stress, velocity and strain rate across the slide thickness are determined analytically for two rate-dependent soil strength models. Previous work has demonstrated that the increase in shear strength with strain rate can be adequately described using a power law or a logarithmic law model. The analytical solutions obtained with these models are compared with the ones available for a viscoplastic fluid of the Bingham or Herschel–Bulkley types. The influence of the rheological parameters, the slope angle and the slide thickness on the flow and deformation characteristics is analysed for each model. The derived analytical solutions can be viewed as representing a particular position within a slide at a given instant in time. They can be used in conjunction with numerical modelling of the entire slide to provide some insights into the flow pattern, and into the parameter sensitivity. These solutions are also applicable to determination of the loading on a pipeline that is oriented across the path of the slide. They are used to illustrate the relative contributions of the soil strength and the inertial drag, as well as the influence of the vertical position of the pipeline within the slide.
Related Organisations
University Of Southampton
Location: United Kingdom of Great Britain and Northern Ireland
University Of Cambridge
Location: United Kingdom of Great Britain and Northern Ireland
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