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The interaction between a spudcan and an existing footprint is one of the major concerns during the reinstallation of jack-up rigs. This paper reports the results from a series of centrifuge model tests undertaken to assess the efficiency of a recently developed novel spudcan shape with a flat base and four holes through the base to ease spudcan–footprint interactions. The experimental program was carried out at 200 g in a beam centrifuge. The footprints were created either manually using a cutting tool or through spudcan penetration and extraction allowing a typical operation period as well as an elapsed time prior to reinstallation. While the former allowed a consistent evaluation of the benefits of spudcan shape, removing the additional variability due to soil strength heterogeneity, the latter allowed for examining the effect of real field footprints. For manually created shallow footprints in soft clay, comparing the responses with those on a generic spudcan, it was possible to show that the novel spudcan reduced the maximum horizontal force and moment by up to 47%–62% and 28%–36%, respectively. The critical offset distance was found at 0.75D. For manually created deep footprints in stiff clay, the reductions were 20%–59% and 9%–45%, respectively. The critical offset distance was found at 0.5D. For footprints created by penetrating and extracting a spudcan, the critical offset distance was identified at 0.75D. The efficiency of the novel spudcan was calculated with the reduction of maximum horizontal force and moment by 42%–59% and 21%–62%, respectively. [ABSTRACT FROM AUTHOR]

Assessing the potential for a punch-through failure during spudcan installation in sand-over-clay is crucial for reducing risk in the operations of mobile jack-up platforms. Typically, in the offshore industry, the peak penetration resistance and the depth at which it occurs are determined deterministically without rigorously considering the uncertainties in the soil. This paper proposes a probabilistic approach to estimate the peak resistance and the corresponding depth, as well as a Bayesian method of incorporating installation data to update the predictions. Instead of a single value in the deterministic analysis, a range of the potential peak resistances and depths can be estimated by accounting for the uncertainties in the soil, the spudcan's geometry and in the calculation method itself, with a database of 66 geotechnical centrifuge tests characterising the model. This prior probability is then updated using the monitored data, allowing a real-time update of the probabilities associated with candidate values of peak resistance and depth during the installation. The advantage of such a probabilistic updating model is shown in a retrospective simulation of a mobile jack-up platform in sand-over-clay conditions in the Gulf of Mexico. The results show that the prior estimation can be effectively refined by incorporating the monitored data. The proposed method provides a powerful tool for assisting decision-making during the installation of jack-ups offshore. [ABSTRACT FROM AUTHOR]

The use of plate anchors in permanent moorings for deep-water floating structures requires a thorough understanding of their performance under sustained and cyclic loading. This paper addresses the former aspect through a series of centrifuge tests on vertically loaded plate anchors in normally consolidated kaolin clay, supported by large-deformation finite-element analyses. The centrifuge experiments, which included particle image velocimetry measurements to identify the conditions leading to anchor failure, demonstrate that sustained loading can be maintained indefinitely if the anchor displacement at application of the sustained loading does not exceed 40% of the displacement reached at monotonic failure. This corresponds to a sustained load that is about 88% of the monotonic capacity, as identified from the numerical analyses. An important observation from both the centrifuge tests and the numerical analyses was a gap at the base of the anchor that developed under moderate load levels when the vertical effective stress at the base of the anchor reached zero. Although this so-called breakaway condition is generally associated with a lower capacity factor and a heightened potential for anchor failure, the conditions leading to breakaway also allow for consolidation-induced strength increase at the top face of the anchor and the mobilisation of partially drained or drained soil strength, both of which maintain anchor stability. [ABSTRACT FROM AUTHOR]

Centrifuge tests investigating the effect of compressive preloading and consolidation on the subsequent uplift capacity of shallow skirted foundations that rest on lightly over-consolidated kaolin clay are reported. The uplift resistance, total and pore pressures at the foundation invert were monitored during the installation, consolidation and uplift. The results show that uplift performed immediately after the preloading generates a lower capacity due to the remoulding of the clay and reduction in soil strengths that result from the application of preloading. The soil strength can be recovered and eventually enhanced by the subsequent consolidation process. A theoretical framework, where the variation of the soil operative strength with preloading and consolidation is interpreted using the critical state concept, is proposed to explain and justify the experimental observations. The framework is validated through retrospective prediction of the uplift resistance of the centrifuge model tests with good agreement for a wide range of levels of preloading and degrees of consolidation demonstrated. [ABSTRACT FROM AUTHOR]

A complete analytical method to describe the full load-penetration resistance profile of a mobile jack-up spudcan footing penetrating a sand over clay stratigraphy is described. It is based on both large deformation finite-element analyses and geotechnical centrifuge experiments. The coupled Eulerian-Lagrangian (CEL) approach is used to accommodate the large deformations of a spudcan footing penetrating sand overlying clay. Modified Mohr-Coulomb and Tresca models describe the sand and clay behaviour, with modifications accounting for the effects of strain softening on the response of the soil. The CEL results are shown to match centrifuge tests well, allowing the numerical study to be extended parametrically, and with confidence, to cover the range of layer geometries, sand relative densities and footing shapes that are of practical interest to offshore jack-ups. The results are used to ( a) assess the performance of an existing model to predict the peak resistance in the sand layer (extending its range of application to medium dense to dense sands and to conical footings of angle 0° to 21°), and ( b) develop an expression for the bearing capacity factor when the footing penetrates into the underlying clay. Using the analytical formulas proposed, retrospective simulations of centrifuge tests show that the method provides a reasonable estimate of the peak punch-through load, the behaviour in the underlying clay, as well as the punch-through distance; the latter being a basic reflection of the severity of a potential punch-through failure. [ABSTRACT FROM AUTHOR]

Accurately predicting peak penetration resistance qpeak during spudcan installation into sand overlying clay is crucial to an offshore mobile jack-up industry still suffering regular punch-through failures. This paper describes a series of spudcan penetration tests performed on medium-loose sand overlying clay and compares the response to existing centrifuge data from tests performed on dense sand overlying clay. Together these data demonstrate that punch-through is a potential problem for both dense and loose sand overlying clay soil stratigraphies. Using this experimental database, a failure-stress-dependent model has been modified to account for the embedment depth, and the depth of occurrence of qpeak is shown to be a function of the sand thickness Hs. The model then was recalibrated, taking these findings into account, for a larger range of material properties and ratios of sand thickness to spudcan diameter (Hs=D). Finally, the performance of the modified and recalibrated model is verified by comparing its predictions with those calculated using current guidelines. The comparisons show that the modified model yields more accurate predictions of qpeak over the range of Hs=D ratios of practical interest, which when used in practice will potentially mitigate the risk of unexpected punch-through on sand overlying clay stratigraphies. [ABSTRACT FROM AUTHOR]

Experimental data from loading tests of model circular footings on soft normally consolidated clay are presented. The experiments were carried out on a drum centrifuge at a radial acceleration level equivalent to 100 times Earth's gravity, ensuring conditions of stress similitude between the model and the prototype scale. The aim of the experiments was to compare the undrained response of different offshore foundations to the same loading conditions. Two different types of foundation were targeted for investigation: spudcans and suction caissons. The spudcan, typically an inverted shallow cone, is the traditional footing for mobile drilling units (also known as jack-up rigs). An alternative foundation concept that is being increasingly considered is that of foundations skirted about the perimeter and installed by suction. A loading arm that incorporated an internal hinge was used so that combinations of load appropriate to the foundations of jack-up units could be applied to the models. Although it was anticipated that, by using skirted foundations there would be increased moment capacity, this was not found to be the case (for the caisson skirt length and soil strength profile investigated). However, there was a stiffer response and additional horizontal capacity at the foundation level. Significantly the use of skirted foundations allowed for a greater combined loading capacity under tensile vertical loads. The results have been interpreted within the framework of strain-hardening plasticity theory, and comparisons with existing yield surfaces are detailed. [ABSTRACT FROM AUTHOR]