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Helical piles are being increasingly considered for offshore applications as they avoid the acoustic emissions associated with pile driving, and they provide additional capacity relative to a driven pile. In this paper, we consider the installation and tensile capacity of helical piles through a combination of centrifuge experiments and large deformation finite element analyses within a Coupled Eulerian Lagrangian framework. The experiments provide a basis for validating the numerical simulations, but also quantify the expected installation torque and undrained tensile capacity, including the variation with time after installation. The numerical simulations extend the parameter space investigated experimentally, considering the number of helices, their spacing and pitch, in addition to the ratio of pile shaft to helix diameter and the profile of undrained shear strength. Mechanisms revealed through the numerical simulations are reflected in a new analytical model for calculating undrained tensile capacity, which is seen to agree reasonably well with the numerically and experimentally determined capacities.

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.

This paper investigates the uplift capacity of horizontal plate anchors in sand through finite element analyses and centrifuge experiments. The finite element simulations adopt a sophisticated bounding surface plasticity model that accounts for stress- and density-dependent behaviour, as well as loading- and fabric-related anisotropic effects in sands. Failure mechanisms at peak anchor capacity show that failure occurs progressively, with a marked decrease in mobilised friction angle within the shear bands close to the anchor edge. Numerical simulations of a large set of centrifuge experiments on rectangular, strip, and circular plates at different relative densities and stress levels are in good agreement for dense conditions, but perform more poorly for loose conditions due mainly to the open cone yield surface in the bounding surface model. Equivalent comparisons with current limit equilibrium methods highlight the challenges in direct application of element-level strength equations. Finally, the paper proposes a modified limit equilibrium solution based on a “rigid-block” failure mechanism extending to the soil surface, but with anchor factors that encompass the results from the finite element simulations. The modified solution provides a higher level of agreement with results from a large database of plate and pipeline test data than existing limit equilibrium methods.

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.

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/pipeline/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.

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.

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.

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 amplitude. 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 amplitude cyclic loads, with each test involving at least one million cycles. In the varying amplitude 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 amplitude cyclic loads that are absent when the cyclic loads are arranged in descending order.

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.

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.

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.

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 amplitude, 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.

Pipe clamping 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 clamps onto a section of laid pipeline, with concrete ballast logs securing the clamping 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 clamping 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.

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.

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.

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.

Offshore wind turbines and their foundations experience loading from varying directions, yet understanding of the effect of changes in the direction of applied cyclic loading on foundation response remains limited. This study investigates the behaviour of a suction caisson foundation in sand over clay under various types of multidirectional cyclic loading. The findings are significant: although the caisson rotation (or tilt) is affected by changes in load direction, the caisson rotation is less than that in a unidirectional test with the same cyclic load magnitude and symmetry, meaning that a unidirectional test may serve as a conservative estimate of caisson rotation for multidirectional loading. This contrasts with previous findings for monopiles in sand, where prediction of the accumulated displacement based on unidirectional cyclic loading was substantially un-conservative for multidirectional loading. Changes in unloading stiffness due to changes in load direction are slight, varying by an amount that is no greater than the change measured over the course of a unidirectional test. Foundation stiffness and ultimate capacity following multidirectional cyclic loading are largely unaffected, unlike unidirectional cyclic loading, where consolidation of the clay layer improves both the stiffness and capacity.

Suction buckets are becoming established as a viable foundation solution for offshore wind turbines. In sand, suction-induced seepage flow reduces effective stresses at the skirt tips, whic...

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 emergence of commercial offshore floating renewable energy devices that are expected to be deployed in large integrated arrays in relatively shallow water requires anchor types that are suited to sandy seabeds. This paper considers centrifuge test data that quantify the capacity of a novel dynamically installed plate anchor in sand under drained monotonic and cyclic loading. The monotonic tests investigated the role of the eccentricity of the load attachment point from the plate, the mooring line load inclination at the seabed and the plate embedment depth, whereas the cyclic tests examined the capacity mobilisation due to both regular and irregular drained cyclic loading for a fixed embedment depth and seabed load inclination. The test data indicate that an optimal anchor design would employ a load attachment point that is eccentric from the plate by at least 0.6 times the plate height, and that the anchor provides higher capacity when the seabed load inclination is horizontal (catenary mooring) than when it is vertical (vertical taut mooring). The centrifuge data show that drained cyclic loading does not degrade anchor capacity, and may be beneficial provided the cyclic loading involves a history of lower level cyclic loading that densifies the sand.

This note describes a web based application called “Datamap” that has been developed to address the major challenge of capturing, classifying, organizing and making available geotechnical research data. Datamap is illustrated using an example project at the Australian National Field Testing Facility (NFTF). The example project illustrates the interactive and accessible nature of the web-based application for hosting in-situ and laboratory site investigation data, field testing, construction details, monitoring results, reports and bibliography of related papers.

Dynamically embedded plate anchors (DEPLAs) are a relatively new anchor type that combine the advantages of dynamically installed anchors and conventional plate anchors. Through three dimensional finite element modelling, this paper first derives the combined loading yield surface of the anchor holding capacity in clay and then uses this yield surface in plasticity analyses to study the anchor trajectory under chain loading. These analyses reveal that the anchor padeye in the current DEPLA design is not optimally located as the anchor will eventually pull out of the seabed when loaded beyond its design capacity. It also shows that there are alternative padeye locations that lead to an equilibrium condition, where the DEPLA attains an ultimate holding capacity. The plasticity analyses are supplemented by simplified closed-form equations that are used in a parametric study to calculate the ultimate DEPLA holding capacity for various padeye positions. The optimal padeye location from the analyses is at a location that has a padeye offset ratio (ratio of the normal to the parallel distance between the padeye and the centre of rotation of the plate) in the range 0.55–0.65, varying slightly with chain loading angle.

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.

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.

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-amplitude 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.

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 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.

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.

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 sample 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.

The paper investigates the effect of stress history and shallow embedment on centrifuge cone penetration tests in sand. A series of centrifuge cone penetration tests were performed in loose and dense silica sand at g-levels ranging between 20 and 100 with corresponding overconsolidation ratio (OCR) between 1 and 5. Based on the measured cone tip resistance (qc) profiles, improved empirical correlations have been proposed with depth factors (fD) to impart additional flexibility in accurately back predicting sand relative density (RD) at shallow embedment in normally consolidated (NC) sands. The qc - RD correlations are then extended to capture overconsolidation effects in cone tip resistance, which is broadly consistent with the changes in compressibility and in-situ lateral stresses taking place in sands with increasing OCR levels. The proposed expressions allow accurate quantification of depth corrected CPT profiles in soils of varying overconsolidation ratio, for application in the interpretation of model tests on shallow foundations and anchors and in shallowly buried structures such as pipelines. The expressions also have application for interpretation of field CPT profiles where the thickness of interbedded layers is of similar order of magnitude to the cone diameter.

Suction caissons have recently been considered as potential anchoring solutions for floating renewable devices (wind turbines and wave energy converters) in shallow waters, where—unlike for...

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.

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 campaign 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.

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.

This paper describes centrifuge tests in which a model free-fall sphere was allowed to free fall in water before dynamically embedding within reconstituted samples 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.

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 hamper 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.

© 2018 Taylor & Francis Group, London. Suction buckets are an increasingly considered foundation option for offshore wind turbines. Although the required suction can be predicted well using existing methods, uncertainty remains around some input parameters, because the effects of suction installation on the soil state are not understood in detail. This paper visualises the mechanisms governing both initial self-weight penetration and following suction-assisted installation in dense sand. Pioneering particle image velocimetry measurements in a centrifuge environment underpinned the investigation, with details of the experimental apparatus offered in the paper. Changes in the deformation mechanisms governing the installation process and in the soil properties are revealed. The findings have an impact on the understanding of the formation of internal plug heave – the cause of premature refusal – and the prediction of the installation response. Revealing changes in void ratio and permeability also present implications on the accumulated displacements under the metocean loading, which may conflict with serviceability requirements.

© 2018 Taylor & Francis Group, London. As part of a research project to develop a framework for quantifying cyclic effects on piles, centrifuge model tests on single piles and pile groups in dry silica sand have been carried out at the Centre for Offshore Foundation Systems (COFS) in Perth, Australia. The piles were subjected to lateral 1-way and 2-way cyclic loading, investigating the vertical and horizontal load-displacement behaviour. Strain gauges located along the pile shaft provided information about the bending behaviour. This paper presents the tests on single piles and highlights the influence of load type, cyclic load amplitude and number of cycles on the observed response. As a characteristic value for the general bearing behaviour, the cyclic performance of the modulus of subgrade reaction is calculated additionally.

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.

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.

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.