The installation of mobile jack-up platforms remains hazardous because of the potential for uncontrolled punch-through failure. Seabeds of sand overlying clay, or stiff clay over soft clay, are problematic because the embedding spudcan pushes the strong layer of soil into the underlying softer layer. Both experimental and numerical approaches have been adopted in previous studies to assess punch-through potential and to understand the factors affecting the failure mechanism. In this paper, the entire penetrating process of a spudcan is simulated using a large-deformation finite element approach - the Coupled Eulerian-Lagrangian method in the commercial package Abaqus. The numerical approach is first compared with centrifuge model test data for uniform clay, uniform dense sand and loose sand over uniform clay. Results from a series of parametric studies of spudcan penetration into sand over uniform clay sites are then described. The dependence of the penetration behaviour on the friction angle and dilation behaviour of sand, the undrained shear strength of the underlying clay and the normalised sand layer thickness are also discussed.

1. Introduction

A jack-up rig is typically used at more than one site during its service life and may encounter different soil types and environmental load conditions. Thus, the suitability of the unit for different sites, especially those consisting of sand overlying clay or stiff clay overlying soft clay, must be assessed. This is because in these conditions the spudcan can push the stronger overlying layer into the softer underlying soil. Known as a punch-through, the subsequent reduction of vertical load can cause the jack-up leg to uncontrollably penetrate. Several platform failures have been reported to be due to spudcan punchthroughs (e.g. the discussion in Hossain, 2008). This is because the soil around the spudcan undergoes significant translations and rotations during the penetration process.

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