Recent advances in offshore geotechnics for deep water oil and gas developments

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 co...

Coupled consolidation analysis of pipe-soil interactions

The evolution of penetration resistance as a function of penetration depth of a pipe into a cohesive seabed is of practical importance, particularly in the areas of pipeline on-bottom stability ass...

Deep cavity flow mechanism of pipe penetration in clay

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 la...

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Tolerably mobile subsea foundations – observations of performance

This paper presents a study on the effects of shearing velocity, surface roughness and overconsolidation ratio on the strength and deformation behaviour of fine-grained soil–structure interfaces. T...

Rate effects on the interface shear behaviour of normally and overconsolidated clay

Axial pipe–soil resistance is an important aspect of deep-water pipeline design, since it influences the longitudinal and lateral buckling responses under thermally induced expansion and contraction of the pipeline. Experimental evidence has shown that the axial resistance, expressed as a proportion of the submerged pipeline weight, can vary by an order of magnitude, depending on the rate of axial movement and cumulative time. This paper provides a theoretical framework for assessing the magnitude of axial friction. The framework is developed within a critical-state context using effective stresses, applicable to any degree of drainage in the soil, quantifying the magnitude and duration of excess pore pressures generated near the pipe/soil interface. Two other aspects of behaviour are added to match the observed velocity dependence of axial resistance: (a) a damage term, leading to contractive volumetric strain at the interface; and (b) strain-rate dependence of the mobilised soil strength. Analytical exp...

Modelling the axial soil resistance on deep-water pipelines

The consolidation response of partially embedded seabed pipelines has been investigated in the past using small-strain finite-element (SSFE) analysis with elastic soil response. This paper presents results from large-deformation finite-element (LDFE) analyses, using the modified Cam Clay plasticity model for the soil, to address the problem more realistically, including initial plastic penetration of the pipe. The effects of initial embedment, pipe–soil interface friction and strength gradient have been explored in a systematic manner and comparisons with the elastic solutions show the influence of non-linearity. Effects of local soil heave on consolidation rate are quantified by comparing results from SSFE and LDFE analyses. The dissipation responses are fitted by simple equations to aid application in design.

Elastoplastic consolidation beneath shallowly embedded offshore pipelines

Finite-element analyses of the axial sliding resistance of a seabed pipeline embedded in soft normally consolidated modified Cam clay are presented. The study demonstrates the rise in axial resistance associated with episodes of movement with intervening periods of consolidation. It is shown that the excess pore pressures generated during undrained axial movements lead to significant consolidation and strength gain in the surrounding soil. For modified Cam clay, using properties representative of kaolin, the resistance rises exponentially with cycles and reaches the drained limit within approximately 10–20 episodes of movement and consolidation. The rate of gain in resistance is shown to be controlled by the volumetric stiffness ratio, κ/λ, for soil that is initially normally consolidated. Simple relationships are proposed that will assist in utilising this beneficial phenomenon in design practice. The increase in axial sliding resistance due to consolidation may mean that mitigation requirements against ...

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Cyclic consolidation and axial friction for seabed pipelines

This paper reports numerical analyses of shallowly embedded hemispherical and toroidal penetrometers under torsional and vertical load. These novel penetrometers are a new design suited to the assessment of near-surface soil strength and are aimed at the analysis of pipeline embedment and axial pipe-soil interaction. The geometry of the penetrometers avoids the complication of end effects that arise if a short pipe segment is used, as is the current practice. The operation of these devices involves vertical penetration typically of up to half a diameter, followed by rotation about the vertical axis, whereas the corresponding loads are recorded. The FE analyses explore the undrained bearing capacity and stiffness factors required for the measured loads to be converted to soil strength and stiffness for application in design. On the basis of these analyses, the geometry of the toroidal penetrometer has been optimized to minimize the size of the instrument and limiting interference across the toroid, which would hamper comparisons between the penetrometer response and a pipeline. It is shown that a relatively compact toroid can be used.

Penetration Resistance and Stiffness Factors for Hemispherical and Toroidal Penetrometers in Uniform Clay

This paper describes the initial evolution of two novel site investigation tools - shallow penetrometers - that are designed to measure the properties of surficial seabed soils, and in particular the axial resistance between pipelines and fine-grained soils. The shallow penetrometers are toroidal and hemispherical in shape, and are pushed vertically and rotated about the vertical axis. This design allows soil-interface shearing to be performed without the inconvenience of end effects. The paper describes numerical and centrifuge modelling results which show the resistance to purely vertical or torsional loading on undrained clay. The results of a cyclic torsional test using the toroidal penetrometer are shown, including detailed measurements of pore pressure build-up and dissipation. The new tools show promise and the results highlight the need to develop effective stress-based interpretation methods, to enhance the utility of these tools.

Y. Yan

Papers

Modelling the axial soil resistance on deep-water pipelines

Elastoplastic consolidation beneath shallowly embedded offshore pipelines

Cyclic consolidation and axial friction for seabed pipelines

Penetration Resistance and Stiffness Factors for Hemispherical and Toroidal Penetrometers in Uniform Clay

Investigations into novel shallow penetrometers for fine-grained soils