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Offshore pipelines are important infrastructure for transporting oil and gas. They are subjected to cycles of high temperature and pressure variation during operation and shutdown periods, and are therefore susceptible to the stability problem of axial walking due to the asymmetry between the expansion and contraction. Axial walking can cause damage to subsea pipeline systems, for instance to the connected manifolds and steel catenary risers. This paper provides a complete analytical solution to examine the process of pipeline walking based on underlying principles of mechanics, considering the pipeline–seabed sliding response as rigid plastic. The analytical solution allows calculation of the cumulative displacement, strain, axial force and soil resistance along the pipeline under arbitrary temperature or internal pressure change histories, accounting for the coupled contributions from seabed slopes and end tension forces. A dimensionless parameter η is proposed to explain the triggering mechanism for walking by comparing the activating force to the soil resistance. New criteria are established to judge whether pipeline axial walking is possible. Calculation examples are presented in the final part of this paper to demonstrate the analytical solution. This study can be used as a rigorous benchmark and a simple practical tool to evaluate pipeline walking under arbitrary loading histories of temperature and internal pressure variations. [ABSTRACT FROM AUTHOR]

Most offshore wind turbines (OWTs) are supported by large-diameter monopiles, installed by impact driving. OWTs are dynamically sensitive structures, with a narrow design range for the eigenfrequency. Fatigue and serviceability limit states can be critical in design, so the foundation stiffness plays an important role. The pile installation process results in changes to the soil state, investigated here through large-deformation numerical analyses using the coupled Eulerian–Lagrangian approach with a hypoplastic constitutive relation to model the sand. The soil state is then mapped to a small-strain model to evaluate the effect of the installation process on the lateral response. Complementary physical modelling results from monopile installation and lateral load testing in a centrifuge provide confidence in the numerical model. Analyses for two different sands highlight commonalities and differences. Comparison with results featuring wished-in-place or jacked installation illustrates the effect of changes in the soil state on the lateral response. The results characterise the changes in soil state and allow quantification of their effects as impact driving tends to produce a stiffer lateral response, with the expected variations due to pile dimensions and sand relative density. Further research will provide insights into the role of pore fluid response during impact driving and investigate vibro-driven piles. [ABSTRACT FROM AUTHOR]

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 sample 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 >7 orders of magnitude in strain rate. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Quantitative assessment of the risk of submarine landslides is an essential part of the design process for offshore oil and gas developments in deep water, beyond the continental shelf. Landslides may be triggered by a reduction in shear strength of subsea sediments over a given zone, caused for example by seismic activity. Simple criteria are then needed to identify critical conditions whereby the zone of weakness could grow catastrophically to cause a landslide. A number of such criteria have been developed over the last decade, based either on ideas drawn from fracture mechanics, or considering the equilibrium of the initial weakened zone and adjacent process zones of gradually softening material. Accounting for the history of the weak zone initiation is critical for derivation of reliable propagation criteria, in particular considering dynamic effects arising from accumulating kinetic energy of the failing material, which will allow the failure to propagate from a smaller initial zone of weakened sediments. Criteria are developed here for planar conditions, taking full account of such dynamic effects, which are shown to be capable of reducing the critical length of the softened zone by 20% or more compared with criteria based on static conditions. A numerical approach is used to solve the governing dynamic equations for the sliding material, the results from which justify assumptions that allow analytical criteria to be developed for the case where the initial softening occurs instantaneously. The effect of more gradual softening is also explored. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

This paper focuses on elastic stiffness parameters for axial, horizontal, and vertical motions of a pipeline relative to the seabed, with the aim of expressing these parameters in terms of fundamental elastic properties of the soil. Limited information exists in the literature on the axial elastic response of on-bottom pipelines, particularly for nonhomogeneous soil. Therefore, an approximate analytical approach was developed for axial stiffness, focusing on the case of shear modulus proportional to depth. The solution was then verified through numerical analysis. Further numerical analysis was carried out to obtain relationships for horizontal and vertical elastic stiffnesses of on-bottom pipelines. Finally, relationships among elastic stiffnesses were developed. Here recommendations are made for the selection of proper elastic stiffnesses in all three directions of motion. These recommendations allow consistent and rigorous modeling of elastic pipe-seabed interactions with application to the analysis of pipeline laying, buckling, walking, and on-bottom stability. [ABSTRACT FROM AUTHOR]

Partial drainage during a penetration test increases the penetration resistance, and also affects the excess pore pressures generated and the form of any subsequent dissipation test. This complicates interpretation of such tests, particularly in soils of intermediate consolidation characteristics, because it is difficult to obtain an accurate estimate of the coefficient of consolidation from which to assess the degree of consolidation during penetration. The effect of partial consolidation during penetration on subsequent dissipation tests is investigated here through a number of model piezocone and piezoball tests conducted in a single soil sample under centrifuge conditions. Backbone curves of penetration resistance and excess pore pressure are devised, and comparisons are made of the form of the dissipation response for the two types of penetrometers and for two different pore pressure monitoring positions on the piezoball. Different methods are proposed for assessing the degree of consolidation during the penetration phase, thus allowing the consolidation coefficient to be estimated with reasonable accuracy. Advantages of monitoring pore pressure at two different locations on the piezoball are argued. [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]

Abstract: Assessment of interaction forces between deep water pipelines and potential submarine slides, debris flows and turbidity currents is an important aspect of geohazard studies. Historically, interaction forces have tended to be expressed in terms of drag factors, within a traditional fluid mechanics framework, with the drag factors depending strongly on an equivalent Reynolds number for the non-Newtonian debris material. Here, we have followed a more geotechnical approach, allowing the interaction forces to be expressed in terms of a strain-rate dependent shear strength of the debris material, and with the inclusion of a drag term (with fixed drag coefficient) for high velocity, low strength, combinations. This superposition approach treats separately the interaction forces that arise from the strain-rate dependent strength and the inertia of the debris, rather than combining them into a single drag force. A failure envelope is proposed, allowing axial and normal interaction forces to be estimated for any angle of attack of the debris flow. [Copyright &y& Elsevier]

Abstract: The paper presents an overview of recent developments in geotechnical analysis and design associated with oil and gas developments in deep water. Typically the seabed in deep water comprises soft, lightly overconsolidated, fine grained sediments, which must support a variety of infrastructure placed on the seabed or anchored to it. A particular challenge is often the mobility of the infrastructure either during installation or during operation, and the consequent disturbance and healing of the seabed soil, leading to changes in seabed topography and strength. Novel aspects of geotechnical engineering for offshore facilities in these conditions are reviewed, including: new equipment and techniques to characterise the seabed; yield function approaches to evaluate the capacity of shallow skirted foundations; novel anchoring systems for moored floating facilities; pipeline and steel catenary riser interaction with the seabed; and submarine slides and their impact on infrastructure. Example results from sophisticated physical and numerical modelling are presented. [Copyright &y& Elsevier]

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]

In this paper a novel modelling procedure is proposed to estimate whole-life settlements of tolerably mobile sliding foundations. A new kinematic hardening-critical state-state parameter constitutive model, the Memory Surface Hardening model, is implemented in a one-dimensional analysis to predict accumulated vertical settlements under drained lateral cyclic loading. The Memory Surface Hardening model performance is compared with the Modified Cam Clay and Severn-Trent Sand models. The Memory Surface Hardening model is adopted to simulate available experimental data from centrifuge tests to predict the settlement of a sliding foundation at the final stable state (i.e. no further volume changes occur). [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]

Shallow skirted foundations can mobilize uplift resistance from end bearing in the short to medium term. However, uncertainty exists over the magnitude of reverse end bearing resistance compared with resistance in compression, and how this might be affected by a gap between the external face of the foundation skirt and the adjacent soil. The study presented in this paper explores this problem through centrifuge model tests, investigating the effect of skirt embedment ratio on ( i) the magnitude of reverse end bearing capacity compared with compression capacity, ( ii) the uplift displacement associated with spontaneous loss of suction during uplift, and ( iii) the existence of a vertical gap along the external skirt-soil interface. The results show that ( i) peak uplift capacity equivalent to compression capacity can be mobilized for a fully sealed foundation with an intact skirt-soil interface, ( ii) suction required for reverse end bearing can be maintained through considerable foundation displacement, even for a low skirt embedment ratio, and ( iii) the presence of a vertical gap along the external skirt-soil interface causes abrupt loss of suction beneath the top plate after minimal foundation displacement, with subsequent uplift capacity being markedly reduced. [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]

Abstract: Spudcan installation in sand overlying clay carries a risk of potential punch-through failure. This paper investigates the penetration resistance of spudcans on loose sand overlying clay soils using large deformation finite element analysis. Comparison with existing experimental results shows that the Mohr–Coulomb yield criterion with unchanging shear strength parameters simulates well the behaviour of loose sand, for which strain softening is not significant. The paper quantifies the effects on the punch-through behaviour of the spudcan of the main input parameters, including the undrained shear strength of the underlying clay, and the thickness, friction angle and stiffness of the upper sand. The current design guideline, SNAME , significantly underestimates the peak bearing capacity of the spudcan in comparison with the numerical results. [Copyright &y& Elsevier]

A laboratory-floor experimental study was conducted on the electrochemical treatment of calcareous sand for offshore foundations. A steel tube of diameter 200 mm and length 400 mm was used as a model caisson. Calcareous sand and seawater from the coastline of Western Australia were used in the study. Twelve electrodes made of perforated steel pipes, diameter 14 mm, length 450 mm and filled with soluble CaCl2 granules as a cementing agent, were installed around the caisson. The electrochemical treatment tests were carried out with two electrode layout configurations in parallel with a control test. The electric power was applied via pairs of the steel pipe electrodes in the first configuration whereas the caisson served as one electrode in the second configuration. The applied direct current (d.c.) voltage was 8 V for the first configuration and 6 V for the second. The pullout resistance of the caisson after the treatment was increased by 140% to reach a value of 304%, in comparison with the control. The cementation generated by the treatment was demonstrated by the formation of a soil plug inside the caisson, soil cemented to the surface of the electrodes and caisson, and the significant increases in pullout resistances. The cementation effects were confirmed by electron microscopy images, X-ray fluorescence analyses and X-ray diffraction analyses. [ABSTRACT FROM AUTHOR]

Subsea pipelines buried in the seabed may undergo large lateral displacement under environmental, operational, and accidental loads at different interaction rates and hence different drainage conditions. The undrained shear strength is commonly used in practice to assess the pipe-soil interaction assuming a sufficiently high displacement rate. This approach neglects consolidation effects and the rate-dependent response of the soil and may significantly underestimates the lateral resistance for a pipeline moving slowly relative to the ground. In this study, a coupled large deformation finite element (LDFE) framework is developed via a remeshing and interpolation technique with small strain (RITSS). A Modified Cam-Clay (MCC) model with efficient numerical integration is used. The proposed coupled LDFE framework is verified against selected physical model tests. Effects of the interaction rate and hence drainage condition on the p-y curve, excess pore pressure generation and dissipation, and failure mechanisms are discussed. An empirical relationship between the ultimate resistance and the normalized velocity of the pipe (denoting the drainage condition) is proposed, which may be applied for the integrity and safety analysis of buried pipes in landslide or fault-crossing regions. • Large deformation coupled numerical framework for soil-structure interactions considering advanced soil constitutive model. • p-y response of pipe – soil lateral interaction with large deformation and different drainage conditions. • Different failure mechanisms during pipe – soil lateral interaction with pore pressure generation and dissipation. • Assessment of ultimate resistance of pipe – soil lateral interaction considering interaction rate and drainage condition. [ABSTRACT FROM AUTHOR]