Performance of skirted foundations for offshore structures

    Research output: ThesisDoctoral Thesis

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    Abstract

    Skirted foundations are being used increasingly as foundation systems for offshore structures. Uses include skirted gravity base structures, suction anchors (as alternatives to drag anchors) for catenary moorings, and caisson foundations for either taut leg moorings (such as tension leg platforms) or jacket structures. The behaviour of caisson foundations in calcareous sediments, subjected to combined monotonic and cyclic loading conditions, is the primary focus of this thesis. The main thrust of the research is experimental, with a wide range of model tests conducted in a geotechnical centrifuge.

    In order to allow interpretation of foundation results, the research has been extended to include a review of soil characterisation procedures, including an examination of existing strength measurement techniques, and the study of new test devices. Results from tests conducted in a range of material types and stress histories are examined, and conclusions drawn regarding the derivation of in situ soil strength.

    From tests conducted in either kaolin clay, calcareous silt or calcareous sand, the response of skirted foundations to applied monotonic vertical and horizontal loads has been examined. In addition, general numerical solutions for installation resistance, bearing capacity, tension capacity and lateral capacity are compared to the experimental results obtained. Included in the discussion is the potential for significant increases in bearing and lateral capacity arising from the application of preload. Finally, combined loading tests (in V-H load space) have been used to investigate empirically the form of a yield envelope for caisson foundations, and the potential use of an associated flow rule to define plastic deformation at yield.

    Cyclic caisson response has also been investigated in the centrifuge, with a detailed programme of model caisson testing undertaken. This has involved 2-way and 1-way lateral cyclic loading, and either compressive or tensile vertical loads. General foundation behaviour is summarised in the form of a series of contour diagrams describing foundation deformation (vertical settlement and lateral displacement) with respect to the amplitude of cyclic loading and the number of cycles performed. These diagrams are then used in the derivation of empirical prediction methods to estimate foundation response to complex loading situations, such as arising during storm loading.
    Finally, two case examples exploring the use of skirted foundations for offshore structures are examined. In each example, both the monotonic and cyclic response is investigated, and conclusions are drawn with respect to the observed foundation behaviour.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Awarding Institution
    • The University of Western Australia
    Award date10 Sep 2001
    DOIs
    Publication statusUnpublished - 1999

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