ORCID Profile
0000-0002-5823-6265
Current Organisation
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 Geotechnical Engineering | Civil Engineering | Ocean Engineering | Civil engineering | Maritime Engineering | Civil geotechnical engineering | Stochastic Analysis and Modelling | Physical Oceanography |
ANZSRC Socio-Economic Objective (SEO)
Oil and Gas Extraction | Wind Energy | Expanding Knowledge in Engineering | Wave Energy | Oil and Gas Exploration | Industrial Energy Conservation and Efficiency | Geothermal Exploration | Application Tools and System Utilities | Civil Construction Design | Road Infrastructure and Networks | Rail Infrastructure and Networks
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Publications
Soil response in the wake of dynamically installed projectiles
Publisher: Thomas Telford Ltd.
Date: 09-2015
Abstract: The soil response in the wake of dynamically installed (free-fall) projectiles is poorly understood, notably with respect to the potential for the hole the projectile creates during dynamic penetration to remain open. The work reported in this paper considered this problem through centrifuge tests in which the impact of free-falling projectiles on the surface of kaolin clay was captured using a high-speed video camera. The video observations show that hole closure may occur at the same rate as the projectile penetrates, or may remain open, either fully or partially. The paper shows that hole closure is controlled by a dimensionless strength ratio, expressed in terms of the undrained shear strength at the rear of the embedded projectile, the projectile diameter and the effective unit weight of the soil. The centrifuge data agree well with an expression derived from tests on undrained, constant rate of penetration spherical penetrometer tests, demonstrating that hole closure is controlled by soil backflow at the rear of the projectile, regardless of the geometrical aspect ratio (length/diameter) of the projectile. This expression can then be used to assess hole closure assumptions made in dynamic penetration analyses, by comparing the final embedment depth with the calculated transitional depth for soil backflow.
Centrifuge modelling of seepage through tailings embankments
Publisher: Thomas Telford Ltd.
Date: 03-2016
Abstract: Tailings storage facilities (TSFs) are manmade geotechnical structures usually comprising a perimeter embankment, fill material (the tailings) and a water-level control system. The key issues often raised in TSF operation are uncertainties surrounding likely seepage to the environment and accurate prediction of seepage surfaces for input into stability assessment. Critically, TSFs are much more complex than the current numerical models conventionally assumed. This paper presents techniques for investigating steady-state and drawdown seepage behaviour of TSF embankments using a fixed-beam geotechnical centrifuge. The development of experimental equipment for centrifuge testing is described and novel methods to characterise model materials preliminarily, using a ‘desktop’ centrifuge, is presented. Good agreement is found between experimental results from the fixed-beam centrifuge and those predicted by the GeoStudio SEEP/W software package for steady-state and drawdown conditions at all tested hydraulic gradients.
Experience with a dual pore pressure element piezoball
Publisher: Thomas Telford Ltd.
Date: 09-2016
Abstract: Piezoballs, which are full-flow ball penetrometers incorporating pore pressure measurements, are an attractive soft soil characterisation tool as they allow measurement of the intact and remoulded strength and the consolidation coefficient in a single test. The merit of full-flow penetrometers as a reliable tool that is superior to the cone in quantifying the strength of soft clay is gaining acceptance. Much of the recent focus on piezoballs has been on the pore pressure measurement location. Prompted by recent studies that highlight the merit of measuring pore pressure concurrently at more than one measurement location, this paper considers a new centrifuge-scale piezoball, with simultaneous pore pressure measurement at the equator and mid-face positions. Results from centrifuge tests in normally consolidated kaolin clay form the basis for the examination and yield coefficients of consolidation that are consistent with values derived using a number of different methods. Although the mid-face position appears to be the more sensible pore pressure measurement position for dissipation tests, consideration of the values measured at both positions provide strong indications of the drainage response during penetration that is shown in the paper to be important for deriving coefficients of consolidation from subsequent dissipation phases.
Mind the gap – an experimental study on the need for grouting suction buckets in sand under vertical cyclic loading
Publisher: Thomas Telford Ltd.
Date: 09-05-2023
Abstract: Suction bucket jackets are increasingly being used to support bottom-fixed offshore wind turbines. It is generally assumed that contact between the underside of the bucket lid and the seabed is mandatory to safeguard against additional settlement. Lid contact is usually ensured by underwater grout injection, albeit this has a significant impact on the duration and costs of installation. Although grouting is routine, there are limited available data regarding the benefits of ensuring contact between the lid and the soil. This paper presents data from centrifuge tests of completely and partially installed buckets in dense sand subjected to vertical cyclic loading to investigate the role of lid contact. Lid contact was shown to have a discernible effect on displacement accumulation when buckets were loaded beyond the drained skirt capacity in compression. Under these conditions, settlement of a completely installed bucket was limited by lid contact with the soil plug, while partially installed buckets experienced continuous settlement, as expected. The physical evidence indicates complex load transfer mechanisms that are influenced significantly by the drainage conditions. Hence, an understanding of the soil drainage characteristics at a particular site would provide value when evaluating the requirement for under-lid grouting.
Assessing the penetration resistance acting on a dynamically installed anchor in normally consolidated and overconsolidated clay
Publisher: Canadian Science Publishing
Date: 2017
Abstract: Predicting the final embedment depth of a dynamically installed anchor is a key prerequisite for reliable calculation of anchor capacity. This paper investigates the embedment characteristics of dynamically installed anchors in normally consolidated and overconsolidated clay through a series of centrifuge tests involving a model anchor instrumented with a microelectric mechanical system (MEMS) accelerometer, enabling the full motion response of the anchor to be established. The data are used to assess the performance of an anchor embedment model based on strain-rate-dependent shearing resistance and fluid mechanics drag resistance. Predictions of a database of over 100 anchor installations — formed from this study and the literature — result in calculated anchor embedment depths that are within ±15% of the measurements. An interesting aspect, consistent across the entire database, relates to the strain rate dependence on frictional resistance relative to bearing resistance. The predictions reveal that strain rate dependency may indeed be higher for frictional resistance, although only if a soil strength lower than the fully remoulded strength is considered as the reference strength, which suggests that water may be entrained along a boundary layer at the anchor–soil interface during installation.
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.
Dynamic installation and monotonic pullout of a torpedo anchor in calcareous silt
Publisher: Thomas Telford Ltd.
Date: 02-2015
Abstract: Challenges associated with dynamically installed anchors include prediction of the anchor embedment depth, which dictates the anchor's holding capacity. This is particularly true for calcareous sediments, as very little performance data exist for this anchor type in these soils. This paper reports results from a series of model tests undertaken to provide insight into the behaviour of a torpedo anchor during dynamic installation and monotonic pullout in lightly overconsolidated calcareous silt. The tests were carried out in a beam centrifuge, varying the drop height and consequently the impact velocity, and the consolidation period prior to anchor pullout. The mudline load inclination was also varied to encompass various mooring configurations. The centrifuge model test data were used to calibrate: (a) an analytical dynamic embedment model, based on conventional bearing and frictional resistance factors but with strain-rate-dependent undrained shear strength for the soil and (b) an analytical quasi-static vertical pullout capacity model, accounting for reverse end bearing and frictional resistance. A total energy based expression, appropriate for calcareous silts, was proposed for predicting anchor embedment depth for a given anchor geometry, mass and impact velocity. For assessing anchor vertical holding capacity, a piezocone based direct design approach was also proposed, deriving anchor end bearing and frictional resistance from cone tip resistance and sleeve friction, respectively. Anchor capacity under inclined loading was presented as failure envelopes expressed in terms of dimensionless vertical and horizontal components of anchor net resistance, which agreed well with a finite-element based envelope developed for embedded foundations. The regain of anchor capacity was found to be in good agreement with predictions based on the cavity expansion framework.
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.
Suction caissons in dense sand, part II: vertical cyclic loading into tension
Publisher: Thomas Telford Ltd.
Date: 11-2018
Abstract: Suction caissons have been used for numerous oil and gas installations and are increasingly considered as a foundation solution for offshore wind turbines (OWTs). There can be significant differences between the two offshore energy applications in the load paths and magnitudes, soil type and caisson aspect ratio (skirt length to diameter). This paper investigates the response of suction caissons in dense sand to a range of cyclic vertical loading histories relevant to a jacket-supported OWT, with an emphasis on cyclic tensile loading. The findings are based on a series of experiments performed in a centrifuge, such that soil stresses reflect those in the prototype. The caisson was installed using suction at enhanced gravity, followed by cyclic loading and then caisson extraction. The installation and extraction results are discussed in a companion paper. This paper focuses on the caisson load–displacement response under vertical cyclic loading. The centrifuge experimental results reinforce findings from previous work, add insights into the load transfer mechanisms and provide confidence in their applicability to the prototype, both qualitatively and quantitatively. The results highlight the complexity of the caisson response, particularly under tensile loading, with the influences of average load, cyclic load litude and drainage discussed in detail.
Suction caissons in dense sand, part I: installation, limiting capacity and drainage
Publisher: Thomas Telford Ltd.
Date: 11-2018
Abstract: Suction caissons are a promising foundation concept for supporting offshore wind turbines. Compared to applications in the oil and gas industry, where most practical experience exists, significant differences arise in terms of load paths and magnitudes, soil type and caisson aspect ratio (skirt length to diameter). In a set of two companion papers, this contribution investigates the response of suction caissons in dense sand through a series of centrifuge experiments. The caisson was installed using suction, followed by sequences of cyclic loading and then extraction, all steps completed continuously in-flight. This first paper discusses installation, limiting capacities and drainage, whereas the second paper focuses on vertical cyclic loading into tension. The work demonstrates that suction caisson installation behaviour is well described by existing calculation methods. Tests performed at different installation rates demonstrate that careful assessment of the pumping rate is needed to ensure successful installation, with low pumping rates resulting in premature refusal. In the centrifuge tests, full skirt penetration was achieved without apparent loosening of the soil plug. The limiting capacity in tension, measured during the testing at both fast and slow uplift rates, was also well described by existing calculation methods.
Behaviour of vertically loaded plate anchors under sustained uplift
Publisher: Thomas Telford Ltd.
Date: 08-2016
Abstract: The use of plate anchors in permanent moorings for deep-water floating structures requires a thorough understanding of their performance under sustained and cyclic loading. This paper addresses the former aspect through a series of centrifuge tests on vertically loaded plate anchors in normally consolidated kaolin clay, supported by large-deformation finite-element analyses. The centrifuge experiments, which included particle image velocimetry measurements to identify the conditions leading to anchor failure, demonstrate that sustained loading can be maintained indefinitely if the anchor displacement at application of the sustained loading does not exceed 40% of the displacement reached at monotonic failure. This corresponds to a sustained load that is about 88% of the monotonic capacity, as identified from the numerical analyses. An important observation from both the centrifuge tests and the numerical analyses was a gap at the base of the anchor that developed under moderate load levels when the vertical effective stress at the base of the anchor reached zero. Although this so-called breakaway condition is generally associated with a lower capacity factor and a heightened potential for anchor failure, the conditions leading to breakaway also allow for consolidation-induced strength increase at the top face of the anchor and the mobilisation of partially drained or drained soil strength, both of which maintain anchor stability.
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
A Simple Approach to Multi-Degree-of-Freedom Loading in a Geotechnical Centrifuge
Publisher: ASTM International
Date: 26-10-2018
DOI: 10.1520/GTJ20180037
A new 240 g-tonne geotechnical centrifuge at the University of Western Australia
Publisher: CRC Press
Date: 11-07-2018
Sustainability in an Era of Increasing Energy Demand: Challenges for Offshore Geotechnics
Publisher: American Society of Civil Engineers
Date: 24-02-2014
Consolidation effects on monotonic and cyclic capacity of plate anchors in sand
Publisher: Thomas Telford Ltd.
Date: 08-2020
Abstract: This study investigated the change in monotonic and cyclic capacity of a plate anchor across different degrees of consolidation in dense sand. To quantify the effect of consolidation on anchor capacity, a framework is introduced and validated using centrifuge model anchor test data. The centrifuge tests considered a rectangular plate loaded at varying rates in dense sand, under both monotonic and irregular cyclic conditions, at a fixed embedment depth and with a horizontal load inclination (at the seabed). In order to vary from drained to undrained conditions, the sand was saturated using both water and a viscous pore fluid with viscosity approximately 700 times higher than water. The anchor's ultimate monotonic capacity in dense sand increased by up to 173% as the consolidation response evolved from drained to undrained with generation of dilation-induced suction. This increase in capacity across the consolidation regime can be adequately quantified using the proposed framework however, uncertainty arises in achieving the theoretical undrained capacity. Both drained and undrained irregular cyclic loading resulted in anchor capacity increases of up to 33%, attributed to soil volume changes associated with cyclic densification under drained cyclic loading and excess pore pressure dissipation under undrained cyclic loading.
Behavior of Geosynthetic-Reinforced Piled Embankments with Defective Piles
Publisher: American Society of Civil Engineers (ASCE)
Date: 11-2019
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.
Penetration of dynamically installed anchors in clay
Publisher: Thomas Telford Ltd.
Date: 09-2013
Abstract: This paper utilises centrifuge data to explore the penetration response of dynamically installed anchors in normally consolidated clay. The data indicate that for anchors with no flukes, expected anchor tip embedment depths are two to three times the anchor length for impact velocities approaching 30 m/s, with a strong dependence on the net density of the anchor and smaller dependence on the impact velocity. Total energy, defined as the sum of the kinetic energy of the anchor at the mudline and the potential energy released as it penetrates the seabed, is shown to be a useful quantity for comparing the embedment depth of anchors with markedly different geometries and mass, impacting the soil at different velocities. The centrifuge data were used to calibrate an analytical embedment model, based on strain-rate-dependent shearing resistance and fluid mechanics drag resistance. The merit of the anchor embedment model has been demonstrated by predicting the final embedment depths for a series of offshore field trials to within 4% of the measurements.
A release-to-rest model for dynamically installed anchors
Publisher: American Society of Civil Engineers (ASCE)
Date: 09-2017
Nonlinear Cone Penetration Test-Based Method for Predicting Footing Settlements on Sand
Publisher: American Society of Civil Engineers (ASCE)
Date: 03-2010
Physical Modelling of Suction Bucket Installation and Response Under Long-Term Cyclic Loading
Publisher: Society of Underwater Technology
Date: 2017
DOI: 10.3723/OSIG17.524
Consolidation-induced improvements in plate anchor capacity
Publisher: Canadian Science Publishing
Date: 08-2023
Abstract: Plate anchors are an attractive technology for mooring floating facilities as relative to piles, suction caissons, and drag anchors, they provide a much higher capacity relative to their mass. Plate anchors may experience an extreme loading event that will cause geotechnical failure, although they will still retain a residual capacity. The displacement associated with bringing the anchor to failure will induce excess pore pressures that initially reduce soil strength but will dissipate over time, leading to regains in soil strength and hence anchor capacity. This paper considers the time scales and magnitude of this anchor capacity regain through a series of model scale experiments conducted in a geotechnical centrifuge. The experiments involved vertical loading of pre-embedded horizontally orientated circular anchors in normally consolidated kaolin clay. The results show that anchor capacity regain is a function of consolidation time and the level of resistance maintained on the anchor, with the longest consolidation time and highest maintained resistance leading to a capacity regain of approximately 60%. These capacity increases are described here using a simple hyperbolic function, which provides a basis for estimating the time needed for the residual anchor capacity to regain sufficient capacity following a movement event.
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.
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.
Effect of soil biology and pore water chemistry on a lakebed sediment
Publisher: Thomas Telford Ltd.
Date: 11-2019
Abstract: This paper attempts to explain how a normally consolidated lakebed sediment can have an undrained strength ratio of approximately unity, when the lithic component of the sediment is less than 3% by volume. The structured clay framework, coupled with sensitivity measurements of the sediment, show that the structure retains a very high strength in a remoulded state. Close examination of the sediment indicates a significant microfossil presence. When considered together with the depositional environment and local water chemistry, this suggests that the soil strength arises from a structural matrix of biofilm adhering to the clay and microfossil particles, which is enhanced by cross-linking due to the presence of multi-valent cations in the local water. Laboratory-scale piezocone, piezoball and T-bar penetrometer tests show that the strength and consolidation characteristics can be significantly altered by changes in the pore water chemistry, an observation that should be considered in geotechnical design for such soils.
Anchor and Chain Reaction During Inclined Pullout in Clay
Publisher: ASMEDC
Date: 2006
Abstract: Plate anchors have attracted much attention in offshore deep water development. This paper studies anchor rotation and chain reaction during plate anchor inclined pullout when it is installed vertically in clay. Both numerical simulation of strip plate anchor and centrifuge model tests on square anchor are conducted in uniform and normally consolidated (NC) clay. In the numerical analysis, Remeshing and Interpolation Technique with Small Strain model (RITSS) is used to simulate large movements of the anchor. In the centrifuge model tests, a transparent “soil” is used to observe anchor rotation and chain reaction during anchor pullout. It is found that plate anchors reach ultimate capacity (Nc ≈ 11.7) when they are fully rotated to a position perpendicular to the pullout direction. During anchor pullout at 60° to the horizontal, the loss of embedment during keying-in ranges from 0.38 B to 0.58 B for square and strip anchors in uniform and NC soils. The loss of anchor embedment in NC clay is about 4% ∼ 23% higher than that in uniform clay depending on the soil strength profile in the NC soil.
Applicability of the strain accumulation procedure for the geotechnical foundation design of zero-radius bend triggers
Publisher: Elsevier BV
Date: 08-2019
Capacity of dynamically embedded plate anchors as assessed through field tests
Publisher: Canadian Science Publishing
Date: 2015
Abstract: A dynamically embedded plate anchor (DEPLA) is a rocket-shaped anchor that penetrates to a target depth in the seabed by the kinetic energy obtained through free-fall and by the anchor’s self-weight. After embedment, the central shaft is retrieved leaving the anchor flukes vertically embedded in the seabed. The flukes constitute the load bearing element as a plate anchor. This paper presents and considers field data on the embedment depth loss due to the plate anchor keying process and the subsequent bearing capacity factor of the plate anchor element. The loss in plate anchor embedment was significantly higher than that reported from corresponding centrifuge tests and is reflected in the larger padeye displacements required to mobilize peak capacity in the field tests. Measured plate capacities and plate rotations during keying indicate that the end of keying coincides with the peak anchor capacity. Experimental bearing capacity factors are in the range N c = 14.3–14.6, which is appreciably higher than existing solutions for vanishingly thin circular plates. The higher N c for the DEPLA is considered to be due to a combination of the cruciform fluke arrangement and the fluke (or plate) thickness.
Behaviour of OMNI-Max anchors under chain loading
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-133
Strain regularisation using a non-local method in Coupled Eulerian-Lagrangian analyses
Publisher: Elsevier BV
Date: 12-2023
Response of pile groups in sand due to lateral cyclic loading
Publisher: Thomas Telford Ltd.
Date: 11-2019
Abstract: This paper considers data from centrifuge model tests on pile groups in silica sand involving lateral one-way cyclic loading. The influence of cyclic load litude, pile spacing within the pile group and number of cycles on the observed response are highlighted. Investigating the contribution of each pile on the lateral capacity shows leading row piles exhibit highest bending moments and attract larger loads due to ‘shadowing’ effects. Cyclic loading results in a subtle change of the load distribution within the pile group, with the load gradually shifting from the leading to the trailing piles as the number of cycles increases. The effect of cyclic loading on the initial stiffness of the p–y curves is more significant initial stiffness reduces with increasing number of loading cycles at a rate that is higher in the leading row than in the trailing row. The changes in load distribution are quantified using simple analytical equations that may be used in design.
Rock Berms as Axial Restraint for Flowlines on Soft Soil: Improving Design Through Centrifuge Modelling
Publisher: OTC
Date: 25-04-2022
DOI: 10.4043/31976-MS
Abstract: Rock berms are used to restrain flowlines from moving axially by adding vertical load to enhance resistance. Should the contact pressure be reduced at the flowline-berm interface, such as in response to ‘arching’ of the rock berm due to pipe settlement, this resistance may be substantially reduced. This study shows how the development of axial restraint forces is complex, with shear (‘friction’) at the pipe-berm and pipe-soil interfaces influenced by system settlement. We examine the restraint provided by rock berms through three phases of centrifuge model testing. Phase 1 isolates the pipe-berm interaction through ‘trapdoor’ tests on a false (rigid) seabed using actual berm profiles and scaled rock. Phase 2 models a slightly overconsolidated clay seabed with load from simulated berms held constant, thereby isolating the effects of pipe settlement and cyclic hardening at the pipe-soil interface. Finally, Phase 3 - which most closely represents the actual field behaviour - models the berm ipeline/seabed system to investigate the combined effect of pipe settlement, arching and frictional response. The trapdoor tests identify that berm arching can occur in situations where simulated settlement of the flowline leads to redistribution of the rock berm weight, such that the berm weight acting on the flowline falls towards zero. Axial cycling is shown to help recover the berm's effectiveness, but is dependent on the amount of settlement - with greater settlements generating less axial restraint recovery. The constant vertical load tests confirm that axial resistance developed along the pipe-soil interface is strongly influenced by the vertical load delivered by the rock berm. Testing showed that axial resistances increase with consolidation and hardening, and also that settlements were modest and observed to be a function of vertical load. Axial resistances developed in the final phase of testing were the highest of all, as the tests include all restraining actions. These tests suggest that arching does occur under ‘realistic’ conditions - but that the effect is modest and largely eliminated by ongoing cycling. The findings reduce current design uncertainties and have already been incorporated in an offshore project where rock berms are being used to mitigate axial movements of a flowline. In particular, this novel centrifuge modelling confirms the potential for berm arching and loss of restraint, but also shows that arch collapse leads to recovery of flowline restraint - enabling the potential to reduce rock berm volume compared to the case of assuming arching leads to permanent loss of resistance.
The Dynamically Embedded Plate Anchor: Results From an Experimental and Numerical Study
Publisher: American Society of Mechanical Engineers
Date: 09-06-2013
Abstract: Dynamically embedded plate anchors are rocket shaped anchors that penetrate to a target depth in the seabed by the kinetic energy obtained through free-fall. After embedment the central shaft is retrieved leaving the anchor flukes vertically embedded in the seabed. The flukes constitute the load bearing element as a plate anchor. This paper provides an overview of an experimental and numerical study undertaken to provide the first performance data for this anchor concept. The experimental work includes geotechnical centrifuge modelling and field tests using three different reduced anchor scales, whereas the numerical work focused on investigating anchor capacity for a rage of geometries, embedment depths and seabed conditions. The experimental work indicates that expected tip embedments are in the range 2 to 3.3 times the anchor length and depend on the impact velocity, anchor mass and shear strength of the soil. As with other plate anchors, the anchor needs to key before maximum capacity can be mobilised. Both the centrifuge and field experiments show that this keying and pullout behaviour is typical of other vertically installed plate anchors, where the main issue is the loss in embedment during keying. Both the experimental and numerical studies showed that the capacity of the DEPLA is much higher than that of other dynamically installed anchors with capacities up to 40 times the dry weight of the plate and plate bearing capacity factors of about 15.
The response of suction caissons to long-term lateral cyclic loading in single-layer and layered seabeds
Publisher: Thomas Telford Ltd.
Date: 08-2018
Abstract: Suction caissons are being increasingly considered as an alternative foundation type to monopiles for offshore wind turbines. Single caisson foundations (or monopods) for offshore wind turbines are subjected to lateral cyclic loading from wind and waves acting on the structure. Recent studies have considered the response of suction caissons to such loading in sand, but have generally been limited to a few thousand cycles, whereas offshore wind turbines will generally experience millions of loading cycles over their lifetime. This paper presents the results from a programme of caisson tests in sand, clay and sand over clay seabed profiles, where each test involved about one million cycles of lateral load. The capacity and rotation response is shown to approach that measured in the sand seabed when the sand–clay interface is located at or beneath the caisson skirt tip. In contrast to previously published studies in sand, one-way cyclic loading is the most onerous loading symmetry for a layered seabed with a sand thickness equal to half the skirt length. However, the rotation for this seabed profile is essentially identical if the load is sustained or cyclic, provided that the cyclic loading remains one way. Lateral cyclic loading was seen to increase caisson capacity by up to 30% – with a bias towards clay-dominated seabed profiles – and stiffness by up to 50%. Such stiffness increases need to be considered when assessing the system dynamics for the offshore wind turbine, as demonstrated in the paper.
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.
Visualisation of mechanisms governing suction bucket installation in dense sand
Publisher: CRC Press
Date: 11-07-2018
Soil strength estimation and pore pressure dissipation for free-fall piezocone in soft clay
Publisher: Thomas Telford Ltd.
Date: 10-2014
Abstract: This paper considers centrifuge modelling of free-fall piezocones in soft clay, and interprets the test data to estimate the undrained shear strength and coefficient of consolidation. Data from the free-fall piezocones, which involved dynamic embedment in a normally consolidated kaolin clay s le from various drop heights, were compared with equivalent data from piezocone tests. The undrained shear strength, as interpreted from the acceleration trace of the free-fall piezocone, was found to be in good agreement with that derived from the piezocone penetration tests. The free-fall piezocone is also found to produce identical pore pressure dissipation behaviour as for a statically penetrated piezocone, despite a significant rise in the pore pressure at the start of the dissipation phase.
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.
Centrifuge modelling of a novel skirted spudcan for penetration in layered soil
Publisher: American Society of Mechanical Engineers
Date: 09-06-2019
Abstract: This paper reports on a series of centrifuge tests to investigate the feasibility and performance of a novel spudcan foundation design featuring a peripheral skirt with side openings. The ‘skirted spudcan’, designed and patented by Keppel, is particularly beneficial in layered seabeds with a clay layer overlaying sand. The purpose of the side openings is to enable the clay trapped within the spudcan skirt to flow outside the skirt, so the skirt can penetrate into the sand and develop the required fixities against combined vertical, horizontal and moment loading. The centrifuge tests considered different skirted spudcan designs, each 10 m in diameter, but with a skirt length of either 1.38 m or 2.76 m, two spudcan invert shapes and various configurations of side and top openings. The model skirted spudcans were penetrated into a soil s le featuring a clay layer 6 m thick (in prototype scale) with an average undrained shear strength of about 15 kPa, overlaying a silica sand layer with a relative density of about 40%. The skirted spudcan embedded the sand layer by more than the skirt height at the maximum penetration resistance of about 4 MPa. However, examination of the model after retrieval demonstrated that a clay layer remained inside the skirt, despite the openings on the side of the skirt. The comparison between the eight tests demonstrated that the thickness of the clay plug reduces with the strength of the soil and with the size of the openings. Large horizontal openings located at the top of the skirt were shown to generate the lowest plug thickness and the largest skirt embedment into the sand. Somehow surprisingly, the addition of top openings (in addition of side openings) does not have beneficial effects. In general, the results demonstrated the efficiency of the skirted spudcan with side openings concept. Although the clay plug inside the skirt compartment was only partially expelled, significant skirt embedment into the sand layer (and hence high fixities) could be achieved. The high compressive stresses within the residual clay plug should limit concerns about potential rocking of the spudcan when clay remains trapped within the skirt compartment.
Capacity of dynamically installed anchors as assessed through field testing and three-dimensional large-deformation finite element analyses
Publisher: Canadian Science Publishing
Date: 05-2015
Abstract: The capacity of dynamically installed anchors in soft normally consolidated clay was examined experimentally through a series of field tests on a 1:20 reduced-scale anchor. The anchors were installed through free fall in water, achieving tip embedment of 1.5–2.6 times the anchor length, before being loaded under undrained conditions at various load inclinations. Vertical anchor capacities were between 2.4 and 4.1 times the anchor dry weight and were satisfactorily predicted using the American Petroleum Institute approach for driven piles. Anchor capacity under inclined loading increased as the load inclination approached horizontal the field data indicated this increase to be up to 30% for the minimum achievable inclination of about 20° to the horizontal. Corresponding large-deformation finite element analyses showed a similar response, with the maximum capacity occurring at a load inclination between 30° and 45° to the horizontal. The finite element results demonstrate that, for the anchor geometry considered, an inclined load at the anchor padeye could be decomposed into ultimate vertical and moment loading at the anchor centroid. The establishment of a vertical and moment loading yield envelope for the geometry investigated forms the basis of a simple design procedure presented in the paper.
Physical modelling of active suction for offshore renewables
Publisher: CRC Press
Date: 11-07-2018
Installation of suction caissons in Gulf of Guinea clay
Publisher: CRC Press
Date: 18-12-2014
DOI: 10.1201/B16200-67
Installation of dynamically embedded plate anchors as assessed through field tests
Publisher: Canadian Science Publishing
Date: 09-2015
Abstract: A dynamically embedded plate anchor (DEPLA) is a rocket-shaped anchor that comprises a removable central shaft and a set of four flukes. The DEPLA penetrates to a target depth in the seabed by the kinetic energy obtained through free-fall in water. After embedment the central shaft is retrieved leaving the anchor flukes vertically embedded in the seabed. The flukes constitute the load-bearing element as a plate anchor. This paper focuses on the dynamic installation of the DEPLA. Net resistance and velocity profiles are derived from acceleration data measured by an inertial measurement unit during DEPLA field tests, which are compared with corresponding theoretical profiles based on strain rate–enhanced shear resistance and fluid mechanics drag resistance. Comparison of the measured net resistance force profiles with the model predictions shows fair agreement at 1:12 scale and good agreement at 1:7.2 and 1:4.5 scales. For all scales the embedment model predicts the final anchor embedment depth to a high degree of accuracy.
Rate effects on penetrometer resistance in kaolin
Publisher: Thomas Telford Ltd.
Date: 02-2009
Abstract: This paper describes the results from a centrifuge investigation into the rate dependence of T-bar and ball penetrometer resistance in kaolin. Four T-bar diameters and two ball diameters were installed in kaolin with overconsolidation ratios of 1, 2 and 5 at penetration velocities varying over five orders of magnitude. The penetration resistances are compared with the properties of kaolin as measured in element tests to assist in the development of a framework that describes this resistance over the full velocity range. Consistent trends emerge when the soil viscosity is assumed to affect the penetration resistance in both the partially drained and undrained conditions, enabling the dependence of the T-bar and ball resistances on diameter, velocity and soil characteristics to be quantified.
Influence of the installation process on the performance of suction embedded plate anchors
Publisher: Thomas Telford Ltd.
Date: 08-2006
DOI: 10.1680/GEOT.2006.56.6.381
Abstract: This paper describes a series of centrifuge tests performed in order to investigate the influence of the installation process on the capacity of a suction embedded plate anchor. A 1/145th reduced scale model suction caisson and plate anchor were used for this purpose, embedded in a normally consolidated clay s le. The full installation and retrieval process was simulated as well as loading of the anchor to failure. Results are compared with similar pullout tests where the anchor was pre-embedded manually in the s le. The keying process and the anchor capacity were also investigated. Results show a loss of performance of the suction embedded anchor immediately following the retrieval of the caisson due to weakening of the clay in the vicinity of the anchor. As the clay regains strength with time, the anchor capacity increases to match those of the jacked anchors. The normalised anchor capacity following consolidation agreed well with theoretical solutions. The loss of anchor embedment during the keying process was observed to be lower for the suction embedded anchors than for the jacked anchors, but both sets of data correlated closely with the load inclination at the anchor padeye.
A review of anchor technology for floating renewable energy devices and key design considerations
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-127
Centrifuge Tests on Dynamically Installed Anchors
Publisher: ASMEDC
Date: 2009
Abstract: Dynamically installed anchors are torpedo shaped and are designed so that after release from a designated height above the seafloor will penetrate to a target depth in the seabed by the kinetic energy obtained through free-fall and through the self-weight of the anchor. This paper presents results from an extensive series of centrifuge tests undertaken to both inform expected anchor penetrations in normally consolidated clay and form the basis for calibrating an analytical anchor embedment model. The database indicates that for anchors with no flukes, expected anchor tip embedment depths are 2 to 3 times the anchor length for impact velocities approaching 30 m/s, with a dependence on both impact velocity and to a greater extent anchor mass. The centrifuge data were used to calibrate an analytical embedment model, based on strain rate dependent shearing resistance and fluid mechanics drag resistance. Back-figured strain rate parameters increase with increasing impact velocity and are in the range 0.2–0.5 (logarithmic function) and 0.06–0.12 (power function). As the strain rates in the centrifuge tests are approximately 200 times equivalent strain rates in the field, the lower bound strain rate parameters are considered more appropriate for field conditions.
Drained cyclic capacity of plate anchors in dense sand: Experimental and theoretical observations
Publisher: Thomas Telford Ltd.
Date: 02-2015
Abstract: This paper provides experimental evidence that shows that the drained cyclic capacity of a plate anchor in dry dense sand may be higher than the equivalent monotonic capacity. The experimental data show that when cyclic loading is low relative to the monotonic capacity, increases in the eventual capacity are observed when the magnitudes of the cyclic loads are closer to the monotonic capacity, no increases in capacity are observed. These responses are explained in the paper using an elasto-plastic macro-element model extended with expandable bounding and memory surfaces that address the increase in strength or stiffness caused by changes in soil density and fabric when the anchor is subject to cyclic loading in dense sand.
The performance of dynamically embedded anchors in calcareous silt
Publisher: American Society of Mechanical Engineers
Date: 09-06-2013
Abstract: The Omni-Max anchor is a new type of dynamically installed anchor featuring a mooring arm located close to the anchor tip that is free to rotate about the anchor length. An experimental programme has been undertaken on a geotechnical centrifuge to assess the anchor performance in calcareous silt. The testing programme includes (i) anchor drops and measurement of the penetration depth, and (ii) anchor pull and assessment of the anchor trajectory. An anchor embedment model based on strain rate enhanced shearing resistance is capable of satisfactorily predicting anchor embedment using model parameters that are similar to those used for clay. The ing behaviour of the anchor, post keying, is demonstrated, provided that the initial embedment after impact is deep enough to prevent a shallow mechanism to develop during anchor keying.
Soil flow mechanism around deeply embedded plate anchors during monotonic and sustained uplifts
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-124
Design and performance of suction embedded plate anchors
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-123
Observations of the Effects of a Clay Layer on Suction Bucket Installation in Sand
Publisher: American Society of Civil Engineers (ASCE)
Date: 05-2020
Tensile monotonic capacity of helical anchors in sand
Publisher: Canadian Science Publishing
Date: 10-2019
Abstract: This note examines the interaction between the helices of a multi-helix anchor in terms of the mobilized drained capacity response in tension. Assessments are made on the basis of centrifuge tests in dense silica sand, supplemented with data from existing studies. The centrifuge tests were designed to isolate potential anchor installation effects from those due to the interactions between helices. The data show that additional helices will only contribute to anchor capacity if they are located outside the region of soil mobilized in the failure mechanism of the lower helices. In the dense sand considered in these centrifuge tests, this required helices to be separated by greater than nine diameters, and hence for the lowermost helix to be located at a depth greater than nine diameters. This separation distance is much higher than suggested in previous studies, which tended to attribute the low or nil contribution of additional helices to the soil disturbance generated during anchor installation.
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.
Torsional installation and vertical tensile capacity of helical piles in clay
Publisher: Thomas Telford Ltd.
Date: 20-06-2023
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.
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.
Installation and capacity of dynamically embedded plate anchors as assessed through centrifuge tests
Publisher: Elsevier BV
Date: 09-2014
Performance of a shallow skirted foundation for TLP mooring in carbonate silt
Publisher: CRC Press
Date: 14-05-0182
DOI: 10.1201/B18442-20
Use of a bounding surface model in predicting element tests and capacity in boundary value problems
Publisher: Canadian Science Publishing
Date: 06-2020
Abstract: The paper examines the merit of a bounding surface plasticity model at both element and system level. The governing equations are based essentially on the parent bounding surface plasticity model reported by Dafalias and Manzari in 2004 with some simple yet practical changes to enable realistic predictions for monotonic loading along different load paths. This is achieved by scaling the influence of the state parameter based on a normalised measure of anisotropy, thus leading to suitable change in dilatancy and plastic modulus for different loading directions. The paper presents a simple optimisation technique for calibrating the model parameters, providing an objective approach to reduce the uncertainties in parameter determination that leads to good agreement with responses measured in drained and undrained triaxial tests. The model has also been implemented for the boundary value problem of a buried circular plate anchor and a surface circular footing. Comparisons of the simulated responses with those measured in centrifuge tests demonstrate the potential of the model, whilst also pointing to the challenges in capturing the global response at all strain levels, even for rather simple boundary value problems.
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
Drained Capacity of a Suction Caisson in Sand under Inclined Loading
Publisher: American Society of Civil Engineers (ASCE)
Date: 02-2019
A new technique to reconstitute crust layers for model testing
Publisher: CRC Press
Date: 18-12-2014
DOI: 10.1201/B16200-40
Towards a simple design procedure for dynamically embedded plate anchors
Publisher: Thomas Telford Ltd.
Date: 09-2016
Abstract: Dynamically embedded plate anchors (DEPLAs) are a promising option for anchoring floating facilities in deep water, as relative to current technology such as suction caissons, the anchor will be much smaller and less expensive to install. This paper considers data from the final stage of an experimental c aign that has been undertaken as a preliminary step towards qualification of the DEPLA at full scale. The experiments involved field testing of a 1:4·5 reduced-scale DEPLA in approximately 50 m water depth at a site off the west coast of Scotland. Measurements in the tests included accelerations during free fall in water and embedment in soil, and pullout resistance as the DEPLA was loaded to failure and subsequently retrieved to the deck of the installation vessel. The acceleration data are used in the paper to explore the suitability of strain rate formulations for scaling the undrained shear strength to values appropriate for dynamic penetration, and to demonstrate the potential for a dynamic penetration model to predict the final anchor embedment depth. The paper finishes by summarising output from the embedment model in a simple design chart that can be used to scale a DEPLA for a given mooring line load.
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.
Centrifuge study on the cyclic performance of caissons in sand
Publisher: Thomas Telford Ltd.
Date: 12-2014
Abstract: Suction caissons are currently considered as an alternative to monopile foundations for met masts and offshore wind turbines. This paper presents the results of a series of centrifuge tests conducted on cyclically loaded suction caissons in very dense dry sand. Two representative caisson foundations were modelled at a 1∶200 scale in a geotechnical centrifuge and were subjected to a number of different cyclic loading regimes, for up to 12 000 cycles, both of which add to previous data sets available in the literature. During each test, changes in stiffness, the accumulation of rotation and settlement of the system were measured. It was found that the rotational caisson stiffness increased logarithmically with the number of loading cycles, but to a much lower extent than previously reported for monopiles. Similarly the accumulation of rotation was also observed to increase with number of cycles and was well described using a power relationship. An aggregation of rotation was also observed during two-way tests and is believed to be caused by the initial loading cycles that create a differential stiffness within the local soil. Predictions were then made as to the behaviour of a prototype structure based upon the observed test results and established influence parameters.
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.
Time-dependent processes influencing offshore foundations in clay: an experimental study on plate anchors
Publisher: Thomas Telford Ltd.
Date: 24-10-2023
Numerical study of pull-out capacities of dynamically embedded plate anchors
Publisher: Canadian Science Publishing
Date: 11-2014
Abstract: Dynamically embedded plate anchors (DEPLAs) are a type of offshore anchor that combine the capacity advantages of vertically loaded plate anchors with the installation benefits of dynamically installed anchors. DEPLA capacity under monotonic loading conditions in clay has been investigated through centrifuge and field tests. In this paper, the monotonic capacity of DEPLAs in normally consolidated clay was studied using a three-dimensional large deformation finite element approach based on frequent mesh regeneration. Results from the numerical simulations were validated by comparison with centrifuge test data and existing numerical and analytical solutions for circular and rectangular plates. The effect of anchor embedment depth, anchor roughness, fluke (or plate) thickness, plate inclination, and DEPLA geometry were investigated in a parametric study where soil was prescribed to remain attached to the DEPLA base. The findings indicate that for a horizontal anchor subjected to vertical loading, most DEPLA geometries exhibit deep behaviour at an embedment ratio of 2.5, but that this embedment ratio is dependent upon the plate inclination, with vertical plates requiring the highest embedment depth for a deep localized failure mechanism. At a shallow embedment depth equal to one plate diameter, the reduction in capacity factor as the plate inclination changes from horizontal to vertical is 23.4%, compared with 1.3% at an embedment depth equal to four plate diameters. Plate roughness and fluke thickness are shown to have a minimal effect on the anchor capacity factor for vertical loading. Analyses that considered the breakaway (no tension) at the DEPLA base demonstrated that the anchor capacity factor approaches the no breakaway value as the embedment depth increases and as the soil strength (relative to the effective unit weight of the soil) decreases. The paper proposes a simple means of approximating the anchor capacity factor for breakaway conditions, by summing the capacity factor in weightless soil (which is unique for a given DEPLA geometry) and the normalized overburden pressure.
Offshore Wind Turbine Mono-Bucket Foundations in Sand
Publisher: American Society of Mechanical Engineers
Date: 21-06-2021
Abstract: Single (or mono) suction buckets have been put forward by others as possible offshore wind turbine (OWT) foundations. This paper presents a series of centrifuge model tests conducted in dense sand to investigate their monotonic response for a range of drainage conditions. The results from the centrifuge tests suggest that the mono-bucket rotational response at large rotation in dense sand is dependent on drainage conditions but does not seem to be affected by the contact condition between the bucket invert and the seabed. A final comparison between results from an equivalent set of uplift tests suggests, however, that multi-bucket foundation systems are likely to be more efficient foundation solutions, although suggestions are made which might improve mono-bucket foundation response.
Physical Modeling of Foundation Systems
Publisher: Wiley
Date: 20-04-2017
DOI: 10.1002/9781118476406.EMOE537
Abstract: This article presents the fundamental principles associated with the physical modeling of geotechnical problems, with an emphasis on the performance and design of offshore foundations systems. The similitude principles necessary for reduced scale modeling of an offshore foundation to exhibit an identical behavior to the full‐scale structure it represents are discussed. The necessity of achieving an identical stress state between the model and the prototype is demonstrated, leading to the development of centrifuge modeling. Centrifuge technology, including model preparation, instrumentation, data acquisition, and motion control, is presented, and an ex le application related to pipeline soil interaction during laying is discussed.
Anchoring systems
Publisher: Wiley
Date: 25-01-2018
DOI: 10.1002/9781118476406.EMOE534
Abstract: Station keeping of floating facilities requires mooring lines that terminate at anchors located on or within the seabed. This article provides an overview of currently available anchoring technology, including guidance on the types of seabed and mooring configurations that each are suited to, and the considerations that need to be made in the design process. Design approaches for the various anchor types are outlined, with ex les showing how these approaches capture observed performance both during installation and in service. The information and guidance provided in this article serves as a stepping‐stone toward a complete design, which should be based on expert geotechnical advice and appropriate use of design codes, design recommendations, and the literature.
The response of suction caissons to multidirectional lateral cyclic loading in sand over clay
Publisher: Elsevier BV
Date: 12-2018
Capacity of plate anchors in clay under sustained uplift
Publisher: Elsevier BV
Date: 04-2021
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.
Setup following installation of dynamic anchors in normally consolidated clay
Publisher: American Society of Civil Engineers (ASCE)
Date: 04-2009
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.
Suction caisson extraction resistance in Gulf of Guinea clay
Publisher: CRC Press
Date: 14-05-2015
DOI: 10.1201/B18442-19
Centrifuge tests on the response of piles under cyclic lateral 1-way and 2-way loading
Publisher: CRC Press
Date: 11-07-2018
Recent Advances in Anchor Design for Floating Structures
Publisher: International Society of Offshore and Polar Engineers
Date: 03-2017
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.
Loss in anchor embedment during plate anchor keying in clay
Publisher: American Society of Civil Engineers (ASCE)
Date: 10-2009
Observations during suction bucket installation in sand
Publisher: Thomas Telford Ltd.
Date: 05-2020
Abstract: Suction buckets represent a viable solution as foundations for offshore wind turbines. Installation in sand is relatively straightforward, albeit with limited understanding of the resulting changes in soil state. This paper describes an experimental methodology that allows for visualisation and quantification of changes in soil state during suction bucket installation, validated in sand. Insights obtained from particle image velocimetry analyses, performed on images of a half-bucket installing against a Perspex window taken in a geotechnical centrifuge are discussed. Compared with the initial self-weight penetration, the deformation mechanism governing the suction-assisted phase shows a preference for the soil below the skirt tips to move inwards and upwards inside the bucket. The installation process is responsible for changes in relative density and permeability within the bucket. In these experiments, the majority of the soil plug heave can be attributed to the soil displaced inwards by the advancing skirts, with a minor contribution caused by dilation. The confidence in the experimental methodology provided through the results of suction bucket installation in sand discussed herein now enables suction bucket installation in more complex seabeds to be investigated.
Optimisation of the padeye location for dynamically embedded plate anchors
Publisher: Elsevier BV
Date: 10-2021
An extended interpretation of the free-fall piezocone test in clay
Publisher: Thomas Telford Ltd.
Date: 20-03-2017
Response of normally consolidated kaolin clay under irregular cyclic loading and comparison with predictions from the accumulation procedure
Publisher: Thomas Telford Ltd.
Date: 02-2019
Abstract: The accumulation procedure on contour diagrams of shear strain is used in geotechnical design to account for the effect of cyclic loading on offshore foundations. In this paper, contour diagrams of maximum shear strain are presented based on a series of stress-controlled, symmetrical and non-symmetrical, cyclic direct simple shear tests on normally consolidated kaolin clay that can be used to re-examine the extensive database of model testing in kaolin clay. It is shown that failure under non-symmetrical cyclic loading can be defined at a higher maximum shear strain than under symmetrical loading, offering potential opportunities for reducing foundation sizes for structures that are less sensitive to displacement. Staged tests with parcels of uniform litudes have also been performed to compare the measured shear strains with the values predicted by the accumulation procedure. It is shown that the accumulation procedure predicts the strain measured in the symmetrical staged tests well, but over-predicts shear strains measured in the non-symmetrical staged tests.
Inclined loading of horizontal plate anchors in sand
Publisher: Thomas Telford Ltd.
Date: 12-2022
Abstract: The performance of plate anchors in sand, relative to clay, is not well understood, particularly for the more realistic case of an inclined load. This paper investigates the effect of load inclination on horizontal plate anchors in sand through centrifuge tests and numerical finite-element simulations. The centrifuge tests were performed on rectangular plate anchors in loose and dense sand, at shallow embedment depths with four different load inclinations. The experiments showed that the anchor capacity of horizontal plates increased progressively as the load inclination became progressively more horizontal, with anchor capacity under pure horizontal loading being approximately 1·8 times higher than that under pure vertical loading. These experimental observations were also replicated in finite-element simulations using a bounding surface plasticity model. Investigation of the underlying failure mechanisms and stress paths showed that the slip planes become longer and the mobilised lateral stresses increase as the load inclination becomes increasingly horizontal, which leads to higher anchor capacities. Finally, the anchor resistance factors from the numerical analyses were decoupled into vertical and horizontal components and represented as interaction diagrams, providing a basis for performing hand calculations of anchor capacity for a given embedment depth, load inclination and relative density.
Consolidation effects on uplift capacity of shallow horizontal plate anchors in dilating sand
Publisher: Thomas Telford Ltd.
Date: 11-2022
Abstract: This paper examines the effect of consolidation on a shallowly embedded horizontal plate anchor in medium dense and dense sand using centrifuge tests, where the loading rate was varied over four orders of magnitude. The experimental results show a 4·8 to 5·5 times increase in anchor capacity as the consolidation condition changes from drained to undrained, driven by a steadily increasing negative excess pore pressure with increasing loading rate. At the highest loading rate, under undrained conditions, the measured maximum negative excess pore pressures reach a steady limit, suggesting the occurrence of cavitation. This increase in anchor capacity with dimensionless velocity is captured using a ‘backbone curve’ interpretation framework that describes the change in capacity between the limiting values of drained and undrained anchor capacity. Calculation of drained anchor capacity is straightforward relative to the more challenging problem of calculating undrained capacity, particularly during cavitation. This was addressed separately through a numerical parametric study (pure undrained analysis) using a bounding surface soil model involving different water depths (cavitation potential), densities and embedment ratios. The numerical results are then synthesised into a simple extended analytical solution to allow estimation of undrained anchor capacity under different densities and water depths.
Session report: Offshore geotechnics at ICPMG 2014
Publisher: Thomas Telford Ltd.
Date: 06-2015
Abstract: This paper provides an overview of the contributions to offshore geotechnics at the 8th International Conference on Physical Modelling in Geotechnics 2014. The problems addressed in the conference contributions are a reflection on the focus of the offshore industry, which is expanding into new geographical regions and marketplaces. This expansion brings a fresh set of geotechnical challenges, including seabed stratification that presents risks for the installation of mobile jack-up platforms, strict serviceability criteria for the foundations of offshore wind turbines, complex loading on shallow foundations used to support seabed infrastructure and protecting offshore pipelines from ice gouging. These problems, as addressed in the conference proceedings and elsewhere in the literature, are discussed with notable contributions highlighted.
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.
Vertical cyclic loading response of shallow skirted foundation in soft normally consolidated clay
Publisher: Canadian Science Publishing
Date: 04-2019
Abstract: Skirted foundations are a potential foundation solution for a range of offshore structures, including hydrocarbon and renewable energy platforms and subsea structures. Offshore foundations can be subject to cyclic loading from environmental, installation, and operational events affecting the geotechnical response. A series of centrifuge tests have been performed on a shallow skirted foundation on normally consolidated kaolin clay under a range of vertical cyclic load sequences to investigate the effect of tensile or compressive average stress, the magnitude of the applied stress, and the effect of cyclic loading of low magnitude followed by consolidation on the foundation response. Results are presented as vertical foundation displacements normalized by the foundation geometry and interpreted within the traditional shear-strain contour approach. The findings indicate that the average, rather than maximum, vertical stress defines the foundation vertical displacement response and failure mode, a threshold stress exists below which a steady state is maintained even at a high number of cycles, and geotechnical resistance increases as a result of low-level cyclic loading followed by consolidation.
Estimating consolidation parameters from field piezoball tests
Publisher: Thomas Telford Ltd.
Date: 04-2016
Abstract: The piezoball, a ball penetrometer featuring pore pressure measurements, is a relatively new device that is potentially superior to the more commonly used piezocone for profiling fine-grained soils. This is due to lower uncertainty in how to derive soil strength from the net penetration resistance and the option of measuring consolidation characteristics during pauses in the penetration, potentially more quickly than in a piezocone test. This paper presents results from a series of piezoball tests undertaken at a soft clay test site using a piezoball that measures pore pressure concurrently at the ball equator, tip and half-way between the tip and equator, the so-called mid-face position. Analysis of the test data provides strong arguments for measuring pore pressure at both the equator and mid-face positions. The coefficient of consolidation derived from piezoball dissipation data using recently developed numerical solutions is shown to be highly comparable to that deduced from a piezocone dissipation test. This paper shows that the penetration resistance varies significantly with the rate of penetration due to either viscous rate effects or increasing degrees of partial consolidation during penetration, both of which influence the estimation of undrained shear strength and h er interpretation of dissipation data. Guidance on assessing the drainage response during a piezoball penetration test is provided. Finally, dissipation test data presented in the paper are added to a database formed from centrifuge and field tests that is used to form a new empirical method for estimating the coefficient of consolidation.
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.
Effect of the ordering of cyclic loading on the response of suction caissons in sand
Publisher: Thomas Telford Ltd.
Date: 06-2020
Abstract: Wind and wave action on offshore wind turbines causes irregular cyclic loading on the foundation that can lead to rotation accumulation of the structure. Such loading is generally considered in design using ‘counting methods’ that decompose the time history of irregular cyclic loads into a series of cyclic load parcels of uniform litude. These parcels are ordered in magnitude, with the rotation due to previous cyclic load parcels accounted for by including an equivalent number of cycles of the current cyclic load parcel. Hence, this superposition approach adopts Miner's rule, as the accumulated rotation is considered to be independent of the ordering of the cyclic load parcels. This paper examines the validity of this assumption for moment cyclic loading of suction caissons in sand, through a series of model tests involving both constant and varying litude cyclic loads, with each test involving at least one million cycles. In the varying litude cyclic tests, the accumulated rotation approximately doubled when the load ordering changed from ascending to descending. This is considered to be due to changes in grain contacts and beneficial densification effects from lower litude cyclic loads that are absent when the cyclic loads are arranged in descending order.
Suction caisson foundations for offshore wind energy: cyclic response in sand and sand over clay
Publisher: Thomas Telford Ltd.
Date: 10-2019
Abstract: This technical note considers experimental data on the long-term response of a suction caisson in sand and sand over clay to lateral cyclic loading. Installation of the caisson under suction in a geotechnical centrifuge provides insight into the contribution of this installation process, as well as the effects that soil drainage and consolidation in the clay layer have on the accumulated caisson rotation and change in stiffness. The tests focused on sand over clay, and considered variations in the cyclic load magnitude and symmetry. One-way cyclic loading in sand over clay is seen to result in higher rotation than two-way loading, which contrasts with findings from previous studies in sand. Excess pore pressure dissipation in the clay layer leads to strength increases that stabilise caisson rotation and increase stiffness. The rate of accumulation in caisson rotation is observed to be the same from centrifuge and single gravity tests, while the initial rotation differs with stress level, drainage regime, loading magnitude, soil profile and installation method. The centrifuge tests are considered collectively with equivalent single gravity tests to form a basis for predicting the long-term response of a monopod suction caisson.
Using near-surface CPT data to predict foundation skirt embedment in partially drained carbonate sands
Publisher: CRC Press
Date: 23-06-2022
Drained response of rigid piles in sand under an inclined tensile load
Publisher: Thomas Telford Ltd.
Date: 03-2020
Abstract: The response of rigid piles in sand to an inclined tensile load is poorly documented and understood, particularly when the load is cyclic. This is becoming relevant for floating marine renewable energy devices such as wave energy converters and floating wind turbines. This paper considers centrifuge data for rigid piles in dense sand subjected to drained monotonic and cyclic loading at various inclinations. The data strongly advocate (in dense sand) the avoidance of load inclinations higher than about 60° (to the horizontal) as cyclic loading significantly deteriorates pile capacity, whereas at lower (flatter) load inclinations, there are potential benefits from cyclic densification that improve pile capacity.
Drained monotonic and cyclic capacity of a dynamically installed plate anchor in sand
Publisher: Elsevier BV
Date: 2018
A comparative field study of free-fall cone and sphere penetrometers in soft sediment
Publisher: Elsevier BV
Date: 07-2023
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.
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