Numerical analysis of reinstallation of spudcans near footprints formed during prior installations

Abstract

This paper reports the effect of the remoulded soil strength of a footprint on the reinstallation of jack-up rigs near the footprint. The footprint creation is from an earlier spudcan penetration and extraction process in which the soil is remoulded. Dissipation of pore pressures between the footprint creation and jack-up reinstallation causes further consolidation of the soil around the footprint. Large-deformation finite-element (LDFE) analyses were performed using the Coupled Eulerian-Lagrangian (CEL) approach. A recently developed novel spudcan shape with a flat base and four holes is studied to mitigate spudcan-footprint interactions during spudcan reinstallation. The installation of a whole three-legged jack-up rig is modelled considering a simplified global jack-up unit. The LDFE results were validated against existing centrifuge test data prior to undertaking a detailed parametric study, assessing the effects of consolidation occurred after footprint creation, the offset distance of spudcan installation from the footprint centre, and spudcan shape. Compared to a footprint of the same shape, but virgin intact soil strength, the footprint with remoulded soil strength enhances the maximum horizontal force, maximum moment, maximum lateral displacement, and rotation, by 12–20%. The critical offset distance is found to be one spudcan diameter with the spudcan centre being at the crest of the footprint. The novel spudcan shape is shown to be effective at easing spudcan-footprint interactions, reducing horizontal force, moment, lateral displacement and rotation by about 50%; and total stress on the jack-up leg by 41%.