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Consolidation beneath circular skirted foundations
No full textThe effect of foundation embedment on consolidation has not been considered previously in a systematic manner, although this is of particular interest for offshore foundations, where embedment is provided by skirts that enclose a compressible soil plug. For skirted foundations, critical uncertainties include what to assume in terms of the degree of drainage at skirt tip level, and the relative time scales of consolidation within the soil plug and beneath the foundation. In this paper, results from small strain finite-element analyses are used to quantify the immediate and time-dependent response of circular skirted foundations to uniaxial vertical loading. Foundations with frictionless and fully rough skirt-soil interfaces with varying ratio of embedment depth to foundation diameter are considered and the responses compared with those for surface foundations. The findings show that both skirt-soil interface roughness and embedment ratio have a significant effect on the consolidation response.</p
Coefficient of consolidation for soil - that elusive quantity
Full text linkAlthough it is accepted that the coefficient of consolidation for soil is not a true material property, but reflects the net effect of permeability and compressibility, it is a very useful parameter in day to day design. Design calculations make extensive use of elastic solutions for consolidation, such as beneath a shallow foundation or around a driven pile, but an important consideration is how to measure or estimate an appropriate coefficient of consolidation to use in those solutions. Typically the quantity is determined either from laboratory oedometer tests (generally then referred to as cv) or from field dissipation tests using a piezocone or piezoball penetrometer (generally then referred to as ch). Since the latter form of test includes a mix of stress paths, for some of which the soil has a stiffness associated with unloading and others of which involve plastic compression, the magnitude of ch for a given soil is typically 3 to 10 times the value of cv from virgin compression in laboratory oedometer tests. The paper explores the relationship between cv and ch for different boundary value problems, within the confines of soil modelled as Modified Cam Clay, for both isotropic and anisotropic permeability. Problems range among: simulated oedometer testing, field dissipation testing and pore pressure response beneath a shallow foundation. Results of finite element analysis of this range of problems are used to develop guidelines for different classes of problem, comparing the relevant coefficient of consolidation against a benchmark cv value associated with virgin compression in an oedometer. The normalised values of consolidation coefficient are expressed as functions of fundamental soil parameters used within Modified Cam Clay.</p
Interaction forces between pipelines and submarine slides - A geotechnical viewpoint
No full textAssessment 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.</p
Seabed characterisation and models for pipeline-soil interaction
No full textPipelines and flowlines represent major cost items in the development of deepwater fields. Accurate modelling of the axial and lateral pipe-soil resistance can lead to significant cost reductions by optimising design. Critical design issues include axial motion, or walking, of pipelines due to cyclic thermal changes, lateral buckling due to thermal expansion, and fatigue damage to risers in the touchdown region. Traditionally, interaction between a pipeline and the seabed has been simplified into frictional models for axial and lateral resistance during walking or buckling. Improving these models is a priority, but is hampered by difficulties in characterising the behaviour of very low strength, near-surface, seabed soils, and by a lack of detailed understanding of the soil mechanics of pipe-soil interaction. The cylindrical geometry of a pipeline invites comparison with the behaviour of tubular piles. Recent advances in pile design methods generated by considering the underlying soil mechanics indicate that the same potential exists for improving the understanding of pipeline behaviour. This paper describes recent advances in measuring the low shear strengths associated with near-surface seabed soils, using both in situ methods in the form of cylindrical (T-bar) and spherical penetrometers, and laboratory shear tests at very low effective stresses. The relationship between penetration resistance and the vertical and lateral resistance of pipelines is explored, taking account of the depth of burial and the cycles of movement. New approaches for assessing the axial and lateral resistance of on-bottom pipelines are described. Future trends and recent developments are summarised.</p
Subsea pipeline walking with velocity dependent seabed friction
No full textWith the increase in demand and supply gap in the oil and gas industry, new developments of oil and gasinfrastructure are moving into deeper water. This requires design and construction of long high temperature and high pressure pipelines from deep sea to shore. These pipelines are subjected to cyclic expansion during operating cycles. Accumulated axial movement due to repeated thermal cycles may lead to global displacement referred to as ‘walking’. Walking rates depend on the restraint associated with seabed friction. In conventional analyses, seabed friction is independent of the rate of thermal loading and expansion but it has been recognised that the sliding resistance between a pipe and the seabed varies with velocity, partly due to drainage effects. In this paper a numerical model is used to explore the effect of velocity-dependent seabed friction. A velocity-dependent friction model is implemented in commercial software ABAQUS and validated via single element and simple (flat seabed) pipeline cases. This model features upper and lower friction limits, with a transition that occurs as an exponential function of velocity. A parametric study is performed using differing rates of heating and cool-down in walking situations driven by seabed slope, SCR end tension and the difference between heat up and cool down rates. The walking behaviour is compared to cases with constant friction and solutions are proposed to express the velocity-dependent response in terms of an equivalent constant friction. These equivalent friction values can then be applied in existing simple solutions or more complex numerical analyses, as a short cut method to account for velocity-dependent friction.</p
Parametric solutions for slide impact on pipelines
No full textPipelines are frequently subjected to active loading from slide events both on land and in the offshore environment. Whether the pipeline is initially buried or lying close to the surface, and whether it crosses the unstable region or lies in the path of debris originating from further away, the main principles are unchanged. The pipeline will be subjected to active loading over some defined length, related to the width of the slide, and as it deforms will be restrained by transverse and longitudinal resistance in adjacent passive zones. Ultimately the pipeline may come to a stable deformed shape where the continued active loading from the slide is equilibrated by the membrane tension in the pipeline in addition to the passive resistance. This problem has been explored by various writers and these principles are well established. However, to date no attempt has been made to develop a standard set of parametric solutions, which is the purpose of the current paper. Both analytical and numerical solutions of the problem have been developed, initially for slides acting normal to the pipeline but later extended to general conditions with the slide impacting the pipeline at some angle. It is shown that analytical solutions based on certain idealizations maintain their accuracy over a wide parameter range, and the net effect of the slide in terms of stresses induced in the pipe wall and maximum displacement of the pipeline may be captured in appropriate dimensionless groups. Design charts are presented for slide widths of up to 10,000 times the pipeline diameter for a practical range of other parameters such as the ratios of passive normal and frictional resistance to the active loading. Although the solutions are limited by some of the idealizations, they should provide a useful starting point in design, providing a framework for a more detailed numerical analysis for the particular governing conditions.</p
Stress changes due to shape effects in the construction process of pile walls
No full textThe shaft friction of displacement piles in sand depends on the change in the horizontal stresses on the pile shaft during installation. Recent research has revealed that the sectional shape of a pile has a strong influence on the horizontal stresses, leading to great difference of the pile shaft friction. Based on these findings, an optimization idea has been proposed, referred to as the “switched-on mechanism”. The main emphasis in this paper is given to understanding the changes in horizontal stresses measured during “switching-on” from open-section (installation individually) to closed(load testing for whole pile walls) in cross section. In order to model the construction and load test process for pile walls, a dual installation system was installed into the beam centrifuge. This paper discusses the stress changes through the whole construction process and the efficiency of the switched-on mechanism, and highlights how physical modelling has allowed this highly complex problem to be quantified
Large-deformation numerical modeling of short-term compression and uplift capacity of offshore shallow foundations
Full text linkLarge-deformation finite-element analysis has been used to model the undrained response of skirted shallow foundations in uplift and compression. Large-deformation effects involve changes in embedment ratio and operative local soil shear strength with increasing foundation displacement-either in tension or compression. Centrifuge model testing has shown that these changes in geometry affect the mobilized bearing capacity and the kinematic mechanisms governing failure in undrained uplift and compression. Small-strain finite-element analysis cannot by definition capture the effects of changing foundation embedment ratio and variation in local soil strength with foundation displacement. In this paper, load-displacement relationships, ultimate capacities, and kinematic mechanisms governing failure from largedeformation finite-element analyses are compared with centrifuge model test results for circular skirted foundations with a range of embedment between 10 and 50% of the foundation diameter. The results show that the large-deformation finite-element method can replicate the loaddisplacement response of the foundations over large displacements, pre-and postyield, and also capture differences in the soil deformation patterns in uplift and compression. The findings from this study increase confidence in using advanced numerical methods for determining shallow skirted foundation behavior, particularly for load paths involving uplift.</p
Evaluation of elastic stiffness parameters for pipeline-soil interaction
No full textThis 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.</p
An experimental investigation of a shallow skirted foundation under compression and tension
No full textShallowly embedded skirted foundations are an attractive alternative to piles for jacket structures and buoyant facilities as they can resist uplift but are cheaper to install than deep foundations. Bearing capacity of shallow skirted foundations in compression is moderately well understood while there is still considerable uncertainty over uplift capacity, particularly for loading sustained over a period of time. This paper reports results from beam centrifuge tests on a shallow skirted foundation in clay, subjected to uplift and compression. Rapid and sustained loading is considered and the effects of consolidation stress level and stress history on undrained capacity and sustained load response are reported.</p
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