Numerical & experimental investigation of offshore shallow skirted foundations

Divya Salliyil Kodakkattu Mana

    Research output: ThesisDoctoral Thesis

    160 Downloads (Pure)

    Abstract

    [Truncated abstract] Skirted foundations are a type of offshore shallow foundations, which consist of a plate resting on the seabed and a peripheral skirt (often supplemented by internal skirts) penetrating into the seabed and confining a soil plug. Skirted foundations are widely used for shallow and deep water oil and gas developments, and are often required to resist uplift loads or significant overturning moments. When skirted foundations are subjected to uplift, suction is developed between the underside of the foundation top plate and the soil plug confined by the skirts, which enables mobilisation of resistance from end bearing. Uncertainty exists over several aspects of this reverse end bearing potential such as its magnitude in comparison with compression capacity, the duration over which suction can be maintained and the effect of a gap at the skirt-soil interface. The main focus of the research presented in this thesis is to explore the behaviour of skirted foundations subjected to vertical uplift loading. The study was conducted through a series of centrifuge model tests and numerical analysis. Firstly, the bearing capacity factors in compression were determined for a range of foundation geometries, interface roughness and soil conditions using small strain finite element (SSFE) analysis. Further, centrifuge model tests were conducted to determine the bearing response of skirted foundations having skirt embedment to diameter ratios 0.1 to 0.5, subjected to short-term compression and uplift loading in lightly over consolidated clay. Large deformation finite element (LDFE) analysis was also used to model the undrained response. The centrifuge test results showed that peak uplift capacity equivalent to compression capacity could be mobilised for a fully sealed foundation and suction required for reverse end bearing could be maintained through considerable foundation displacement, even for low skirt embedment ratio. LDFE analysis method could replicate the load-displacement response of the foundations over large displacements, pre- and post-yield. Kinematic soil failure mechanisms around skirted foundations subjected to undrained compression and tension were investigated through digital image analysis of drum centrifuge tests and compared with predictions from finite element analyses. Analysis of images captured in the centrifuge tests showed that rather different kinematic mechanisms govern failure in uplift and compression, and a reverse end bearing mechanism was mobilised even for the shallowest embedment ratio studied. LDFE analysis could capture differences in the soil deformation patterns in uplift and compression whereas SSFE analysis failed to capture the differences...
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2013

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