Theoretical framework for predicting the response of tolerably mobile subsea installations

Cocjin, M. L.; Gourvenec, Susan; White, David; Randolph, Mark

doi:10.1680/jgeot.16.p.137

Cited by 23 publications

(27 citation statements)

References 21 publications

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“…Based on the above, a gain in s u, and therefore in the net geotechnical resistance, is achieved if full consolidation occurs after low-magnitude cyclic loading. Similar to observations from this study, previous experimental studies have shown that cyclic displacements or cyclic loading on model riser pipes on soft normally or lightly overconsolidated clay may lead to an increase of the undrained shear strength of the underlying soil, if adequate dissipation of the excess pore water pressure is allowed after the remoulding of the soil (Hodder et al 2013, Yuan et al 2015, Clukey et al 2017, Cocjin et al 2017 and that the undrained sliding of a shallow foundation followed by adequate consolidation may cause enhancement of the foundation monotonic vertical capacity and sliding resistance respectively (Cocjin et al 2014). In these examples the cyclically induced gains are greater than under equivalent monotonic loading.…”

Section: Effect Of Low-magnitude Cyclic Loading and Consolidation On supporting

confidence: 89%

Vertical cyclic loading response of shallow skirted foundation in soft normally consolidated clay

2019

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Skirted foundations are a potential foundation solution for a range of offshore structures, including hydrocarbon and renewable energy platforms and subsea structures. Offshore foundations can be subject to cyclic loading from environmental, installation, and operational events affecting the geotechnical response. A series of centrifuge tests have been performed on a shallow skirted foundation on normally consolidated kaolin clay under a range of vertical cyclic load sequences to investigate the effect of tensile or compressive average stress, the magnitude of the applied stress, and the effect of cyclic loading of low magnitude followed by consolidation on the foundation response. Results are presented as vertical foundation displacements normalized by the foundation geometry and interpreted within the traditional shear-strain contour approach. The findings indicate that the average, rather than maximum, vertical stress defines the foundation vertical displacement response and failure mode, a threshold stress exists below which a steady state is maintained even at a high number of cycles, and geotechnical resistance increases as a result of low-level cyclic loading followed by consolidation.

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Section: Effect Of Low-magnitude Cyclic Loading and Consolidation On supporting

confidence: 89%

Vertical cyclic loading response of shallow skirted foundation in soft normally consolidated clay

2019

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“…The steady state coefficient of sliding friction derived from the test observation agrees well with analytical and numerical predictions based on critical state soil mechanics (Cocjin et al 2016;Feng & Gourvenec 2015). shows that a long term sliding resistance given by tanϕ′ where ϕ′ = 23.5° is the internal angle of soil friction, is eventually achieved when the soil has undergone sufficient cycles of sliding (and hence shearing), pore pressure generation and reconsolidation to reach critical state conditions, resulting in no further contraction and excess pore pressure generation (Cocjin et al 2015, Cocjin et al 2016, Feng & Gourvenec 2016.…”

Section: Example Application Of the Multi-dof Loading Systemsupporting

confidence: 70%

A Simple Approach to Multi-Degree-of-Freedom Loading in a Geotechnical Centrifuge

O’Loughlin

Cocjin²,

Gourvenec

et al. 2018

Geotechnical Testing Journal

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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

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“…Hodder et al (2013) used a one-dimensional discretisation to analyse the remoulding and reconsolidation processes in a cyclic penetrometer test, with the device moving vertically through the layers. Corti et al (2017) and Cocjin et al (2017) analysed sliding surface foundations using the same form of discretisation, with the foundation applying varying levels of shear stress to the different layers.…”

Section: Background and Motivationmentioning

confidence: 99%

“…The 'packet' of excess pore pressure generated by each episode of undrained shearing or change in maintained load, q, is dissipated separately, with the subscript 'i' used to denote an individual packet. The dissipation response is assumed to follow a simple hyperbolic equation (Chatterjee et al, 2013;Cocjin et al, 2017), which can be expressed in rate form as…”

Section: Zhou White and O'loughlinmentioning

confidence: 99%

An effective stress framework for estimating penetration resistance accounting for changes in soil strength from maintained load, remoulding and reconsolidation

2019

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Some offshore foundations are subjected to intermittent episodes of remoulding and reconsolidation during installation and operational processes. The maintained and cyclic loads, and subsequent reconsolidation processes, cause changes in the geotechnical capacity, particularly in soft clays. This changing capacity affects the in-service behaviour, including changes to the safety margin, the extraction resistance, the stiffness and structural fatigue rates and also the overall system reliability. This paper provides a new analysis framework to capture these effects, based on estimation of the changing soil strength. The framework is developed using critical state concepts in the effective stress domain, and by discretising the soil domain as a one-dimensional column of soil elements. This framework is designed as the simplest basis on which to capture spatially varying changes in strength due to maintained and cyclic loads, and the associated remoulding and reconsolidation processes. The framework can be used to interpret cyclic penetrometer tests, as well as foundation behaviour. This provides a basis for the approach to be used in design, by scaling directly from penetrometer tests to foundation behaviour. Centrifuge tests are used to illustrate the performance of this approach. The penetration resistance during cyclic T-bar penetrometer tests and spudcan footing installation with periods of maintained loading and consolidation is accurately captured. The framework therefore provides a basis to predict the significant changes in penetration resistance caused by changing soil strength, and can bridge between in situ penetrometer tests and design assessments of soil-structure interaction.

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Theoretical framework for predicting the response of tolerably mobile subsea installations

Cited by 23 publications

References 21 publications

Vertical cyclic loading response of shallow skirted foundation in soft normally consolidated clay

Vertical cyclic loading response of shallow skirted foundation in soft normally consolidated clay

A Simple Approach to Multi-Degree-of-Freedom Loading in a Geotechnical Centrifuge

An effective stress framework for estimating penetration resistance accounting for changes in soil strength from maintained load, remoulding and reconsolidation

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