Abstract:Increasing demands for oil and gas exploration in deep water with soft seabed conditions are resulting in the size and weight of subsea shallow foundations stretching the capabilities of installation technologies. One innovation to reduce foundation footprints involves designing foundations to move in a tolerable manner to absorb applied loads rather than being engineered to resist these loads and remain stationary. Critical design considerations are the evolution of foundation capacity and the mode of foundation displacement. The foundation should be designed to slide with acceptable settlement and rotation to prevent overstressing the joints with connected pipelines. This paper presents observations from centrifuge model tests of a mat foundation designed to slide under applied loading. The foundation is subjected to a simulated lifetime of operation, with many cycles of sliding and intervening periods of consolidation. The results provide insights to assist design, including a remarkable rise in the lateral foundation resistance over the sliding events, through repeated episodes of shearing and reconsolidation, and quantification of the accumulated settlements and rotations. The foundation is shown to translate with minimal rotation.The settlement between sliding events is more significant. This is due to the tendency for soft clay to contract on shearing, as excess pore pressures generated during sliding subsequently dissipate.The sliding-induced consolidation settlements control the tolerability of the performance of the mobile foundation.

Abstract:Tolerable mobility of subsea foundations and pipelines supporting offshore oil and gas developments has recently become an accepted design concept. It enables a smaller foundation footprint and so is a potential costsaving alternative to conventionally engineered 'fixed' seabed foundations. Dominant sources of loading on subsea infrastructure arise from connection misalignment or thermal and pressure induced expansion and these are reduced if the structure is permitted to displace while ensuring that additional loading is not induced by excessive settlements. A sound prediction of the resulting sliding response will provide a robust design basis for mobile subsea infrastructure. This paper presents a theoretical model based on critical state soil mechanics to predict the performance of a subsea installation that is founded on soft normally consolidated or lightly overconsolidated soil, and subjected to intermittent horizontal sliding movements. The framework is validated against centrifuge test results and is shown to capture the essential elements of the soil-structure interaction, which include (i) the changing soil strength from cycles of sliding and pore pressure generation, (ii) the regain in strength due to dissipation of excess pore pressure (consolidation), and (iii) the soil contraction and consequent settlement of the foundation caused by the consolidation process.

Solutions for lateral breakout and axial response of submarine pipelines are well established if the 20 undrained shear strength conditions of the soil are known and defined simply (such as uniform or increasing proportionally with depth). In reality, the geometry of the free surface and the 22 distribution of undrained shear strength around a submarine pipeline post-lay are affected by the 23 lay process. This is because of soil berms that form adjacent to the pipe, and remoulding and 24 subsequent reconsolidation of the seabed. The effect of post-lay consolidation on the subsequent 25 lateral and axial response of submarine pipelines has not been previously investigated through 26 physical model testing. 27 This paper presents results from centrifuge model tests describing lateral breakout behaviour of a 28 pipe on soft clay as a function of (i) pipe installation conditions, (ii) post-lay pipe weight and (iii) 29 consolidation prior to break out. In addition, the effect of post-lay consolidation on axial pipe 30 response is studied. The experimental results are compared with available numerical and 31 analytical predictions. 32 The results quantify the influence of the installation process, pipe weight and post-installation 33 consolidation on the lateral break out resistance and trajectory of the pipe and also the axial pipe 34 response, and show how existing prediction methods can capture these effects.

This paper considers an alternative approach for multi-planar loading and multi degree-of-freedom movement in geotechnical centrifuge model tests. The multi degree-of-freedom loading system allows for vertical load control on the vertical axis, and either displacement or load control on the two horizontal axes, whilst allowing rotation about these axes. The system is described in detail and the system performance is validated through results from a centrifuge test comparing observed results with analytical and numerical solutions. The validation of the system considers a mudmat foundation under large amplitude lateral displacement, where two displacement degrees-of-freedom and two rotational degrees-of-freedom were of interest. However, the apparatus is versatile and can be used for testing other foundation types or pipelines, with up to six degrees-offreedom. 1. INTRODUCTION Offshore structures are typically subjected to multi-directional loading and respond with displacement in multiple degrees of freedom. Foundations of fixed-base structures, oil and gas platforms or wind turbines, experience a combination of vertical load from the self-weight of the structure, horizontal loads from the action of wind, waves and currents, and moment loading from the height offset between the action of the horizontal loads and the foundation; foundations of subsea structures can experience complex multi-directional loading from multiple pipeline and spool expansion loads acting at vertical and horizontal eccentricities to the centroid of the foundation (Randolph 2012, Feng et al. 2014); offshore pipelines are subject to vertical self-weight loads, multi-directional installation loads and thermally induced axial and lateral loads during operation and respond with settlement/burial, axial walking and lateral buckling. Independent control of loading and acquisition of displacement, or vice versa, in all six degrees of freedom poses quite an experimental challenge for actuation systems. This is more achievable at 1g than in a centrifuge as the space requirements for the actuation and position measurement systems can be more easily accommodated on the laboratory floor than within the constrained space available on a centrifuge package (Byrne 2014). Centrifuge actuators typically have two or three displacement degrees of freedom (DoF) along the horizontal and vertical planes, although actuation systems that add a rotational DoF have also been developed

A sound understanding of near-surface soil strength is essential for the accurate prediction of the response of structures laid on or shallowly embedded in the seabed. However, characterisation of the uppermost region of the seabed, which is typically very soft and at a low-stress state, is extremely challenging. This paper demonstrates a novel technique for characterising the in situ undrained shear strength of near-surface soils using a newly-developed pile penetrometer. The pile penetrometer is vertically embedded into the near-surface soil and is driven laterally. A simple calculation of the resistance mobilised over the embedded depth of the pile penetrometer is presented along with its application to the continuous measurement of spatial variation in near-surface strength in virgin and disturbed regions of soil.