Improving spudcan extraction from deep embedment in soft soils

Omid Kohan

    Research output: ThesisDoctoral Thesis

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    Abstract

    Self-elevating mobile jack-up units are employed for offshore exploration and development in shallow waters up to approximately 150 m deep. Jack-ups are designed to relocate to a new site after the completion of operations, which requires the extraction of the jack-up legs and their spudcan footings from the soil. Modern spudcans are generally circular or polygonal in shape and have diameters of approximately 20 m. The inability to extract spudcans from deep embedments in soft clay is having a significant financial impact on offshore operations; delays of up to ten weeks have been recorded. To mitigate this problem, this thesis focuses on 1) understanding and describing the breakout failure mechanism of deeply embedded spudcans, 2) developing an analytical method to estimate the spudcan extraction resistance, 3) understanding and describing the reduction in the maximum extraction resistance through the use of a water jetting system, and 4) understanding and describing the extraction of spudcans under cyclic loading.
    The centrifuge modelling technique was used to experimentally investigate spudcan extraction in normally consolidated kaolin clay. For the first time, embedments up to three spudcan diameters deep were modelled. Furthermore, the experiments systematically varied the operating period with and without the application of top and bottom water jetting and also when investigating the effect of extraction under regular and irregular cyclic loading.
    The results demonstrate that the extraction mechanism, which is primarily a function of the operational period, is the same in shallow and deep embedments. An analytical method for estimating the maximum extraction resistance is developed. Though based on an existing method, a new set of recommendations relate the factors that characterise the change the soil shear strength and the effects of the operational load and the strength ratio on the extraction of deeply embedded spudcans.
    It is shown that the application of water jetting at the spudcan invert reduces the peak extraction resistance, whereas the top water jetting relieves the post-breakout resistance. The validity of a current conceptual framework for evaluating the optimal bottom jetting flow rate for spudcan embedments of up to three diameters is also verified.
    The maximum extraction resistance decreases if the combination of the mean uplift cyclic load and the amplitude of the cycles on the spudcan, is sufficiently high to decrease the effective stress and shear strength by remoulding of the soil. The mean uplift and cyclic loadings are provided by pushing and pulling the leg by floating the hull in the water and allowing the impact of small amplitude waves on the hull. Two preliminary graphs are proposed to estimate the number of cycles that is required for failure as a function of the buoyancy load and the regular cyclic loads with respect to the maximum extraction resistance.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2015

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