ABSTRACT

One of the major risks associated with jack-up rigs installed on layered seabed is the punch-through failure of the footings, especially when a strong soil layer overlays a soft soil layer. One way to mitigate the punch through hazard is to replace the widely used spudcans by skirted footings. In this study, the effect of skirt length on the bearing capacity of skirted footings on strong over weak clay layers was investigated by finite element method. Both small strain and large deformation FE analyses were carried out. The large deformation FE analysis simulated the footing continuous penetration process; while the small strain FE analysis provided the bearing capacity of the surface skirted footing with certain skirt length. The FE results showed that, relative to the bearing capacity of spudcans, a skirted footing can significantly reduce the potential of punch-through failure by extending the skirt length to the layer interface. However, the ultimate capacity of the footing was compromised at the same time.

INTRODUCTION

When a strong clay layer overlays a weak clay layer, the soil is proved hazardous to the spudcan footing of a jack-up platform during its installation. The hazard is punch-through failure, where the footing can have a sudden penetration after the peak capacity is reached (Hunt & Marsh, 2004). When a punch-through failure occurs, the footing normally punches a block of strong soil into the underlying soft soil in an uncontrolled manner. Recently, skirted footings have attracted increasing attention in offshore engineering due to their better performance under combined loadings by comparing with flat footings and spudcans (Bransby and Randolph, 1998; Hu et al., 1999; Cassidy et al., 2004; Tran and Randolph, 2008; Gourvenec et al., 2009). However, the study of skirted footings on layered soils, where the potential of punch-through failure exists, is relatively rare.

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