Effect of Gapping On the Uplift Resistance of a Shallow Skirted Foundation

Mana, D.S.K. (Centre for Offshore Foundation Systems, University of Western Australia) | Gourvenec, S. (Centre for Offshore Foundation Systems, University of Western Australia) | Randolph, M.F. (Centre for Offshore Foundation Systems, University of Western Australia) | Hossain, M.S. (Centre for Offshore Foundation Systems, University of Western Australia)

OnePetro 

Abstract:

Offshore shallow foundations may be subjected to uplift due to overturning or buoyancy loading. Peripheral (and often internal) skirts can enable transient tension loads to be resisted because of negative excess pore pressures developed between the underside of the foundation top cap and the soil plug confined by the skirts. Uncertainty exists with regard to the duration over which these negative excess pore pressures can be maintained and the effect of a gap forming along the skirt-soil interface on the transient and sustained holding capacity. This paper presents results from drum centrifuge tests carried out on a shallow skirted foundation subject to transient and sustained uplift in a lightly overconsolidated clay. Results from baseline tests with an intact skirt-soil interface are compared with tests in which a gap was created along the skirt-soil interface prior to transient and sustained uplift. The results are promising, showing for example that uplift loads of 40% of the peak undrained capacity were maintained for up to two years without significant foundation displacement when an intact foundation-soil interface was maintained. However, they also reveal that the presence of a gap may halve the time to reach similar displacements.



Introduction

Shallow skirted foundations are widely used offshore to support small platforms, seabed protection structures, storage tanks, and subsea frames for oil wells and pipelines, as well as for larger fixedbottom and floating structures (e.g. Støve et al., 1992; Tjelta, 1994; Bye et al., 1995; Watson and Humpheson, 2007). Skirted foundations are also an attractive option for mooring or supporting current meters and wind turbines offshore. Skirted foundations may comprise a top plate a peripheral skirt, and sometimes internal skirts or a cluster of individual skirted units connected together. Regardless of configuration, the foundation penetrates the seabed, confining a soil plug inside the structural members.