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The touchdown zones of steel catenary risers and lazy wave risers are fatigue hotspots, where the risers interact continuously with the seabed due to hydrodynamic loading exerted on the host vessel. The whole-life interactions can range from small-amplitude daily motion cycles to motions that involve large-amplitude cyclic interaction with the seabed during storm events. A key design challenge that affects the fatigue life of these risers is the accurate modelling of the evolution of the riser–soil stiffness, throughout the whole life of the riser and for different soil conditions, including overconsolidated conditions, which may occur due to the geological history, ageing or biochemical processes of the sediments. This paper describes centrifuge model pipe tests simulating whole-life riser–soil interaction in normally consolidated and uniform overconsolidated clay samples, under successive sequences of cyclic motions. Results confirm that the whole-life soil stiffness evolution depends strongly on cyclic amplitudes, with reconsolidation-induced soil stiffness recovery after heavy remoulding, and is also influenced by the soil overconsolidation ratio, with a reduced tendency for soil hardening at higher soil overconsolidation ratios. This study provides insights into the relevant cyclic soil stiffness to consider when assessing the whole-life design of risers interacting with overconsolidated seabed sediments. [ABSTRACT FROM AUTHOR]

Layered soil conditions are common in marine sediments, and deduction of strength properties for thin interbedded layers is often challenging. Full-flow penetrometers such as T-bar are increasingly being used in offshore site investigations and show high potential for differentiating the shear strength transition between layers. This paper reports the results from large deformation finite-element analysis undertaken to provide insight into the response of a T-bar penetrometer as it continuously penetrates through stiff–soft–stiff clay layers. The numerical results are validated against data from a centrifuge model test. Trapped soil is observed for any nonsmooth interface as the T-bar penetrates from a stiff to a softer soil layer, which affects the measured resistance of the soft layer. The effect of a cavity above the advancing T-bar was also investigated. The presence of a cavity can increase the sharpness of the transition in resistance from one layer to another, which thus affects estimation of the boundary of the soft layer. The evolution of trapped soil from the top layer within the soft layer, and then its erosion in the bottom layer, were explored. The research found that the measured resistance profile in the soft layer is sensitive to the strength ratio between the top layer and soft layer, but the resistance of the bottom layer is more reliable. Based on deduced measurement parameters from the resistance profile, a new interpretation framework is proposed for interpreting the layer boundaries and undrained shear strength of the interbedded soft layer. [ABSTRACT FROM AUTHOR]

An approximate solution for the effects of high strain rates, and gradual strength degradation, on the penetration resistance of penetrometers can be obtained by combining the strain-path method with the classical upper bound theorem. The stream path calculations require the integration of the material constitutive equation along the streamlines. Unless the geometry is simple so that the integration can be evaluated analytically, numerical procedures are required to backtrack streamlines. The strain at any location is calculated by finding the streamline that passes through the given point and integrating the strain rate along that streamline from its inlet boundary. Thus, the calculations can be complicated, and errors can be accumulated during the calculation procedure. This paper presents an efficient approach for evaluating cumulative strains around penetrometers without the need to backtrack individual streamlines. In this approach, the strain components are treated as field variables. The global solution is obtained using the streamline upwind Petrov-Galerkin method. The new method together with an Eulerian-based finite element formulation was used to study the cone penetration test and evaluate the effect of strain softening on the cone resistance. Copyright © 2014 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

One of the critical issues associated with plate anchor performance and design relates to the reduction of the loss of embedment during the keying process. As deep water offshore sediments typically exhibit an increasing soil strength with depth, the loss of embedment results in a reduction in anchor bearing capacity. A keying flap hinged to the main plate has been developed and adopted by industry with the aim to reduce the loss of embedment by limiting the vertical motion of the anchor. However, uncertainties remain regarding the behavior and the performance of the keying flap. This paper presents a series of numerical analyses performed to investigate the flap rotation mechanism and the condition of activation of the flap. They are compared with existing centrifuge modeling. The numerical results validate the centrifuge observations and demonstrate the nonactivation of the keying flap for typical anchor pull-out conditions. [ABSTRACT FROM AUTHOR]

A series of centrifuge model tests of the lateral response of a fixed-head single pile in soft clay is reported. Both monotonic and cyclic episodes of loading are described, with varying amplitude and with intervening periods of reconsolidation. The soil conditions are characterized by cyclic T-bar penetrometer tests. The ultimate capacity under monotonic load for virgin and for postcyclic conditions was found to be comparable with calculations based on existing design methods, including theoretical plasticity solutions and empirical methods. The lateral stiffness was observed to degrade with cycles, with the rate of degradation being greater for larger cycles. The degradation pattern has been tentatively linked to the cyclic T-bar response, by considering the 'damage' associated with the cumulative displacement and remolding, in each case. This approach provides a consistent interpretation of the tests. Although episodes of pile movement and soil remolding led to a reduction in lateral resistance, intervening periods of reconsolidation led to a similar magnitude of recovery and a reduction in the level of softening in subsequent cyclic episodes. During an initial episode of cyclic lateral movement, the stiffness degraded by a factor of 2.3, which is comparable with the strength sensitivity derived from a cyclic T-bar test. In contrast, after five episodes of reconsolidation, the stiffness had recovered back to within 25% of the stiffness observed in the first cycle of the first episode, and it showed negligible degradation during subsequent cycling. This observation implies that, over a long period of cyclic loading, the lateral stiffness of a pile may tend towards a value that is independent of cycle number, and that represents a balance between the damaging effects of remolding and pore pressure generation and the healing effects of time and reconsolidation. [ABSTRACT FROM AUTHOR]

Abstract: Geo-hazard assessment of the potential damage to a pipeline caused by a submarine landslide requires a quantitative model to evaluate the impact forces on the pipeline. In contrast with typical geotechnical problems, the strain rate within the fast moving, flow-like submarine landslide is typically far higher, which will lead to enhancement of the soil strength and therefore result in larger impact forces. Generally, there are two possible predictive frameworks for strain-rate dependence: a fluid dynamics framework and a geotechnical framework. By comparison of common rheological models adopted in these two different approaches, a unified additive power-law model, a normalised form of the Herschel–Bulkley model from fluid mechanics, is explored in this paper. This model has been used in conjunction with a large deformation finite element approach to investigate the undrained limiting loads on a cylinder moving steadily through inertia-less soft rate-dependent material, in order to quantify the strain-rate effects. The flow mechanism and the effects of the shear-thinning index and Oldroyd number on the shear zones are explored. The calculated resistance factors are compared with the drag coefficients obtained from computational fluid dynamics analysis. The average rate of strain experienced by the soil flowing past the cylinder is estimated for a given flow velocity and an expression in the form of a conventional bearing capacity equation, but with shear strength linked directly to the normalised flow velocity, is proposed to predict the magnitude of the viscous force exerted by the debris flow. [Copyright &y& Elsevier]

Three-dimensional large deformation finite-element (FE) analyses were performed to investigate plate anchor capacity during vertical pullout. The remeshing and interpolation technique with small strain approach was expanded from two-dimensional to three-dimensional conditions and coupled with the FE software, ABAQUS. A modified recovery of equilibrium in patches technique was developed to map stresses after each remeshing. Continuous pullout of rectangular plate anchors was simulated and the large deformation results for strip, circular, and rectangular anchors were compared with model test data, small strain FE results, and plastic limit solutions. Interface conditions of no breakaway (bonded) and immediate breakaway (no tension) were considered at the anchor base. The effects of anchor roughness, aspect ratio, soil properties, and soil overburden pressure were investigated. It was found that the anchor roughness had minimal effect on anchor performance. For square and circular deep anchors under immediate breakaway conditions, the maximum uplift capacity increased with soil elastic modulus, which suggests that lower bound limit analysis and small strain FE analysis may overestimate the capacity. The soil beneath the anchor base separates from the anchor at a certain embedment depth near the mudline, once tensile stresses were generated. The ratio of separation depth to anchor width was found to increase linearly with the ratio of soil undrained shear strength to the product of soil effective unit weight and anchor width and was independent of the initial anchor embedment depth. [ABSTRACT FROM AUTHOR]

Vertically installed plate anchors have been investigated in this paper by numerical analysis and centrifuge modeling. In the numerical analysis, the large deformation finite-element method (remeshing and interpolation technique with small strain) was used to simulate strip plate anchor rotation. In the centrifuge model tests, transparent soils were used to observe square anchor rotation. The loss in anchor embedment during anchor keying was assessed for anchors in uniform and normally consolidated soils with anchor pullout angle varying from 30° to 90° to the horizontal. It was found that the loss in anchor embedment during anchor keying may be expressed in terms of a nondimensional anchor geometry factor, which is a function of the eccentricity of the padeye, angle of loading, and the net moment applied to the anchor at the stage where the applied load balances the anchor weight. However, once the anchor geometry factor reaches a certain value, the loss in anchor embedment stabilizes at 0.25–0.5 times the anchor width. The loss in anchor embedment decreases linearly with decreasing pullout angle. Simple formulae and design procedures have been proposed to estimate the loss in anchor embedment during keying. [ABSTRACT FROM AUTHOR]

Spudcan foundations for offshore mobile drilling rigs are large saucer-shaped foundations that can penetrate several tens of meters into soft sediments. The penetration depth is typically predicted by considering a wished-in-place foundation at different depths and following traditional bearing capacity approaches to assess the depth at which the estimated capacity matches the applied loading. However, the geometry of the spudcan and its progressive mode of penetration lead to soil failure mechanisms that differ markedly from those relevant to onshore practice. This paper presents a new rational design approach for assessing spudcan penetration in single layer clays based on a study combining centrifuge model testing and large deformation finite-element (FE) analysis. The design approach takes account of the evolving failure mechanisms in the soil, which start with cavity formation and surface heave at shallow penetration, gradually transforming to backflow of soil over the spudcan. A detailed FE parametric study has explored the relevant range of normalized strength, strength nonhomogeneity, and spudcan base roughness, with results validated against centrifuge model test data. The penetration response curves are presented in terms of profiles of bearing capacity factors, forming nondimensional design charts along with simplified expressions for convenient use in practice. Comparisons with approaches suggested in the SNAME design code suggest an urgent need to update current practice. [ABSTRACT FROM AUTHOR]

The undrained remolded shear strength of soft clays is of importance in geosystem design, particularly for offshore structures. Common methods to estimate remolded shear strength, such as correlations with cone penetration data, direct measurement with an in situ field vane shear device, and laboratory measurements, produce varied results and can be particularly costly and time consuming. Full-flow penetrometers (T-bar and Ball) provide an alternative rapid method to estimate remolded shear strength and soil sensitivity through remolding soil by repeated cycling of the penetrometer up and down over a given depth interval. The cyclic penetration resistance degradation curve inherently contains information regarding remolded strength and sensitivity. The objective of this paper is to assess the ability of full-flow penetrometers to predict remolded strength and soil sensitivity, and to develop a suite of predictive correlations in which these properties can be estimated in the absence of complementary laboratory or in situ test data. To accomplish this, full-flow penetration profiles and cyclic tests were performed at five well characterized soft clay sites, which together represent the broad range of soils in which the penetrometers will be often used. A previously developed model for the reduction in penetration resistance with cycling is modified to predict the entire degradation curve, including the remolded penetration resistance using only measurements obtained during initial penetrometer penetration and extraction. Using field vane shear strength as the reference measurement, correlations are developed to predict soil sensitivity and remolded shear strength based solely on full-flow penetrometer data, which is particularly useful in site investigation programs where site specific data are not yet available or are sparse. Finally, the usefulness of these relationships is demonstrated by implementing them for two additional soft clay sites. [ABSTRACT FROM AUTHOR]

In practice, natural soils exhibit strain-rate dependency and also soften as they are sheared and remolded. Traditional design approaches, however, have been developed based on idealized rate-independent perfectly plastic soil. This paper presents results for deep penetration of spudcan foundations in strain-softening, rate-dependent, soft clays quantifying the effects relative to results for ideal soil. The analysis was carried out using a large deformation finite-element (LDFE) approach, modifying the simple elastic-perfectly plastic Tresca soil model to allow strain softening, and incorporate strain-rate dependency of the shear strength. Parametric analyses were undertaken varying the strain-rate parameter, the sensitivity and ductility of the soil, the normalized penetration rate, and the soil strength nonhomogeneity. Overall, penetration resistance for rate-dependent, strain-softening clays lay below that for ideal soil. Increased brittleness of the soil led to marked reduction in penetration resistance, only partly compensated by strain-rate dependency. Key results have been presented in the form of design charts, fitted by simple expressions to estimate the limiting cavity depth above the advancing spudcan and the limiting bearing capacity factor at depth. [ABSTRACT FROM AUTHOR]

This paper addresses numerical simulation of deep penetration of full-flow penetrometers in strain-softening, rate-dependent, cohesive soil, and the observed phenomenon of periodic shear bands. The analysis was conducted using a large deformation finite element approach, modifying the simple elastic–perfectly plastic Tresca soil model to allow strain-softening, with strain-rate dependency being incorporated in order to avoid spurious mesh dependency. Parametric analyses were carried out varying the strain-softening parameters (hence the relative brittleness of the soil), the rigidity index of the soil, and the strain-rate parameter. Increased brittleness of the soil led to reduction in the penetration resistance, but also to increasingly significant oscillations in the resistance–penetration responses. The oscillation was found to result from periodic shear bands evolving cyclically ahead of the advancing cylindrical and spherical penetrometers. Analyses with different values of rigidity index confirmed further that the periodic shear bands were a real material phenomenon, rather than due to errors in numerical simulation. Similar phenomena have been observed for continuous flow problems in granular materials. However, rising strain-rate dependency tended to suppress the oscillations. [ABSTRACT FROM AUTHOR]

Large deformation finite element (LDFE) analyses were performed to study the installation of caissons by suction and jacking in normally consolidated clay. The penetration of the caisson wall was modelled between depths of one and four diameters using an axisymmetric LDFE approach, which falls in the category of arbitrary Lagrangian–Eulerian (ALE) methods. The results allowed quantification of differences in the behaviour of caissons installed entirely by jacking compared with a combination of self-weight and suction as is used in the field. For jacked installation, over the penetration range of one to four diameters, the proportion of caisson wall accommodated by inward soil flow reduced from around 45% at the start to zero at about four diameters embedment; by contrast, the proportion for suction installation stayed essentially constant, oscillating around 65% through the depth of penetration. This difference was also evident in the local incremental displacements of the soil beneath the caisson tip. During continuous penetration, the induced increases in radial and mean total stresses around the caisson wall are some 10%–15% smaller for suction installation than for jacked installation, with the difference growing with increasing penetration. In addition, an obvious difference was found in the caisson tip resistance between these two installation methods. [ABSTRACT FROM AUTHOR]

This paper discusses the effects of penetration rate on the penetration resistance in soft clay for various shaped penetrometers (cone, T-bar, ball, and plate) and for T-bars with different aspect ratios. Constant rate (“normal”) and variable rate (“twitch”) penetration tests, where the penetration rate was successively halved over eight steps with the penetrometer advanced by one or two diameters in each step, were undertaken in the beam centrifuge at the University of Western Australia. The tests were conducted on samples reconstituted from clay collected from the Burswood site in Western Australia. The twitch tests showed higher penetration resistance than the corresponding normal tests after the penetration rate had been reduced by a factor of 16 due to cumulative effects of partial consolidation. The penetration rate at which the resistance started to increase due to partial consolidation was used to estimate the consolidation coefficient, cv, of the reconstituted clay. The interpreted cv values were similar to values estimated from other consolidation data, both in the centrifuge and from laboratory Rowe cell and constant rate of strain consolidation tests. In addition, results from in situ twitch tests at the Burswood site were examined to evaluate viscous effects on the penetration resistance. [ABSTRACT FROM AUTHOR]

The uniaxial vertical bearing capacity of square and rectangular footings resting on homogeneous undrained clay is investigated with finite element analyses, using both Tresca and von Mises soil models. Results are compared with predictions from conventional bearing capacity theory and available analytical and numerical solutions. By calibrating the finite element results against known exact solutions, best estimates of bearing capacity for rough-based rectangular footings are derived, with the shape factor fitted by a simple quadratic function of the footing aspect ratio. For a square footing, the bearing capacity is approximately 5% lower than that based on Skempton’s shape factor of 1.2. [ABSTRACT FROM AUTHOR]

The article presents the author's comments about a study on the evaluation of remolded shear strength and sensitivity of soft clay through the use of full-flow penetrometers. It stresses that use of single-digit resolution in coefficients simply reflects the opinion that the trend lines developed should be resolved to this level. It also emphasizes that the use of site-specific data will always improve estimation of coefficients.

New analytical models have recently been presented for evaluating the peak bearing capacity of circular footings penetrating sand-over-clay and the subsequent capacity once embedded in the underlying clay. However, in lieu of any analytical model, current methods of analysis use an inaccurate linear interpolation for the bearing capacity between the peak and the clay surface. This paper proposes an analytical model for calculating the bearing capacity of circular footings after the peak failure in sand-over-clay soils and before the footing enters the clay. Based on observations from visualisation experiments conducted in a geotechnical centrifuge, and consistent with the silo analysis approach used in predicting the peak capacity, the failure mechanism is a vertical frustum of sand progressively being forced into the underlying clay. Analysis of the capability of the new model to retrospectively simulate a database of 66 geotechnical centrifuge tests is provided. Motivation for improved prediction methods comes from the offshore mobile jack-up industry where concerns remain over the punch-through of large 20 m diameter spudcan footings in sand-over-clay. Accurate prediction of not just the peak bearing capacity but the subsequent load-displacement behaviour is required in order to assess the effectiveness of operational methods used to mitigate punch-through incidents. [ABSTRACT FROM AUTHOR]

This paper reports advanced three-dimensional large-deformation finite element analyses investigating the effect of the padeye offset on the behaviour of plate anchors. The analyses varied the normalized padeye offset from 0 to 0.5 and identified two counterbalancing effects on the anchor bearing capacity induced by the padeye offset. These are the change in anchor inclination with respect to the direction of loading (detrimental due to the reduction of bearing area) and the reduction of loss of embedment (beneficial, as the anchor mobilizes stronger soil). The results provide insights to optimize the design of plate anchors. The underlying concepts also have applications for other types of anchors. [ABSTRACT FROM AUTHOR]

Current design practice for pipe-seabed interaction in soft soils is generally based on the assumption of undrained behaviour throughout laying and subsequent operation. In reality, drainage and consolidation around a partially embedded pipe can have a marked effect on the vertical penetration and horizontal breakout resistance. In this paper, a large-deformation finite element methodology coupled with the 'modified Cam clay' plasticity soil model has been developed to study the coupled consolidation behaviour of soil around partially embedded seabed pipelines. Simulations of penetration show that after laying, subsequent consolidation leads to further embedment by an amount dependent on the level of drainage that occurred during laying. Also, if the pipe is embedded under undrained conditions, the waiting period between laying and operation allows the soil around the pipe to consolidate under the pipe self-weight. The consolidation process results in an increase in the strength of the soil. The lateral breakout resistance and the direction of pipe movement on breakout thus depend on the consolidated strength of the soil around the pipe, as well as the applied loading. The envelopes of vertical-lateral combined loading bearing capacity differ markedly from those predicted assuming undrained behaviour throughout. [ABSTRACT FROM AUTHOR]

Seabed pipelines must be designed to accommodate thermal expansion - which is commonly achieved through controlled lateral buckling - and to resist damage from submarine slides. In both cases, the pipe moves laterally by a significant distance and the overall pipeline response is strongly influenced by the lateral pipe-soil resistance. Here, the process of pipe penetration and lateral displacement is investigated using a large-deformation finite element method, with a softening rate-dependent soil model being incorporated. The calculated soil flow mechanisms, pipe resistances, and trajectories agree well with plasticity solutions and centrifuge test data. It was found that the lateral resistance is strongly influenced by soil heave during penetration and the berm formed ahead of the pipe during lateral displacement. For 'light' pipes, the pipe rises to the soil surface and the soil failure mechanism involves sliding at the base of the berm. In contrast, 'heavy' pipes dive downwards and a deep shearing zone is mobilized, expanding with continuing lateral movement. The different responses are reconciled by defining an 'effective embedment' that includes the effect of the soil berm or wall ahead of the pipe. The relationship between normalized lateral resistance and effective embedment is well fitted using a power law. [ABSTRACT FROM AUTHOR]

This paper presents results and analyses of the dynamic behavior of a small-diameter pile driven in very soft clay, with its tip embedded to a depth of 4.5 m with a clear distance of 1–1.5 m above a stiffer substratum. The research program included static and dynamic load tests performed over a period of 15 days, with blows from three different drop heights. The dynamic signals were analyzed using two different pile–soil interaction models. The performance of the dynamic models in predicting the shaft and base behavior of the model pile, as well as the change in bearing capacity and in some model parameters the dynamic with time, are addressed in particular. [ABSTRACT FROM AUTHOR]

Offshore developments for hydrocarbon resources have now progressed to water depths approaching 3000 m, with geotechnical design increasingly focused on soft sediments in the upper 30 m or so of the seabed. Due to the difficulties and high cost in recovering high-quality samples from deepwater sites, there is increasing dependence on in situ testing techniques for determining the geotechnical design parameters. This paper summarizes the findings from a joint industry project, undertaken by the Norwegian Geotechnical Institute and the Centre for Offshore Foundation Systems at The University of Western Australia, on the use of in situ testing for the characterization of deepwater soft soils. The project focused on theoretical and empirical studies for the interpretation of piezocone, T-bar, and ball penetration test data, linking the penetration resistance to shear strengths determined from in situ vane tests and laboratory tests on high-quality samples. Guidelines are summarized for interpreting in situ test data, particularly for estimating intact and remoulded undrained shear strengths from the penetration resistance measured by each type of penetrometer. Suggestions are also given for future development of in situ testing techniques to maximize the potential of in situ testing in characterization of deepwater soft soils. [ABSTRACT FROM AUTHOR]

[Display omitted] This paper presents a methodology to simulate the strain-softening-hardening response of sensitive clays when subjected to extensive remoulding. A non-local strain-softening technique is implemented into the Abaqus Finite Element Analysis (FEA) software through its user defined material subroutine UMAT, in the form of a critical state based strain-softening constitutive model. For the comparison of conventional and non-local formulations, a series of analyses have been performed investigating mesh dependency issues and the validity of a simple softening-scaling rule for practical applications. The non-local model significantly reduces solution mesh-dependency for small-strain analyses of biaxial shear and updated Lagrangian analyses of buried pipe uplift. Finally, the non-local model is applied to simulate undrained cycles of penetration and the consolidated-undrained capacity of a T-bar in kaolin clay. This analysis highlights the limitations of the use of standard element tests (e.g. triaxial tests) to calibrate the model input parameters that control the strain-softening aspect of the constitutive law, and the need for a constitutive law that captures the partial recovery of sensitivity during consolidation. [ABSTRACT FROM AUTHOR]

Seabed in regions, such as the Gulf of Guinea and North West Shelf of Australia, may exhibit a crust layer where the undrained shear strength can be an order of magnitude higher than that of the immediately underlying sediment. This can complicate design of steel catenary risers, where fatigue depends on the cyclic vertical stiffness of the pipe-soil interaction. Potential punch-through of the riser into the underlying soft soil may invalidate design assumptions based on the pipe-soil stiffness within the crust layer. The long-term evolution of pipe-soil stiffness within the crust layer, which exhibits similar properties to an over-consolidated soil, is also poorly understood. This paper describes centrifuge model tests undertaken in a clay sample with a crust layer, simulating the punch-through process of a pipe under load control and investigating the pipe-soil stiffness during long-term cyclic loading tests under displacement control. Results confirm that the potential for punching-through the crust layer depends strongly on the relative ratio of pipe diameter to crust layer thickness. The long-term evolution of pipe-soil stiffness showed a steady increase after an initial remoulding stage in contractile soils (normally consolidated and lightly over-consolidated), but a steady reduction in the heavily over-consolidated, more dilatant, crust. The magnitude of pipe-soil stiffness changes (during both remoulding and reconsolidation) is governed by the over-consolidation ratio of the soil and the amplitude of the cyclic displacements. This study provides insights on the relevant cyclic stiffness to consider when assessing SCR fatigue life in over-consolidated soils and soils exhibiting a superficial crust layer. • Results from a centrifuge test programme simulating the response of an element of an SCR at the touchdown zone are reported. • The long-term SCR-soil stiffness evolution is shown to depend on the soil overconsolidation ratio (OCR). • A critical state framework is used to understand the effects of OCR and cyclic amplitude on SCR-soil stiffness evolution. [ABSTRACT FROM AUTHOR]