Skirted, or bucket, foundations are widely used offshore to support or anchor structures for the oil and gas industry and provide an attractive foundation solution for future developments. Current industry design guidelines give conservative collapse loads for offshore shallow foundation systems and loading conditions, particularly due to overlooking the beneficial effect of negative excess pore pressure developed within the soil plug during rapid uplift or overturning.
This paper outlines research being carried out at the Centre for Offsore Foundation Systems (COFS) at the University of Western Australia to investigate the response of skirted shallow foundations, to develop reliable enhanced predictions of limit states under transient loading, and to quantify the timescale over which sustained tensile loading can be withstood. The project involves using centrifuge modeling to investigate the effect on capacity of skirt dept; consolidation stress level; time after installation when uplift is applied; eccentric and lateral loading in conjuction with uplift; conditions in which gapping along the foundation skirt occur and their effect on uplift capacity; and methods of suppressing crack formation.
Skirted foundations have application for a variety of offshore structures, as shown in Figure 1. They can be embedded to depth of up two foundation diameters, although shallower embedment ratios are often associated with larger foundation diameter. Circumferential skirts confine a soil plug, allowing uplift resistance to be mobilized by suctions (i.e. negative excess pore pressures) developed within the soil plug. This is particularly relevant offshore, where environmental forces lead to large horizontal and moment components of foundation loading.
In general there is a lack of accurate solutions for general loading of skirted foundation, and there is uncertainty over, and no formal guidance regarding, the timescale for which tensile stresses can be sustained beneath a skirted foundation. Routine offshore design guidance for shallow foundation 1,2,3 is based on classical bearing capacity theory 4,5, which has several shortcomings in respect to offshore foundation systems and loading conditions. Formal design guidance does not represent the simultaneous action of horizontal load and moment 6,7,8,9, uses conservative, empirical depth factors 10,11,12 and neglect tensile capacity.
The size of offshore shallow foundation (up to 15 000m2 for a gravity-based structure) precludes direct assessment of capacity by mean of loading tests, and reliance must be placed instead on results from numerical analyses and physical model tests. The latter has been carried out under laboratory conditions but without the correct scaling of soil strength and self-weight stresses 13,14,15. This is a significant limitation for problems involving uplift, or where potential cracks or gaps may develop between structure and soil. Model tests conducted in a geotechnical centrifuge overcome this problem, but existing centrifuge data are limited in terms of soil conditions, foundation geometry and applied load combinations 16, 17, 18.
Greater progress has been made in numerical modeling 8,9,19,20. However, the main body of work is restricted to undrained soil response, plane strain geometry and surface footings with full bonding at the soil interface to represent tension capacity.