The performance of a hybrid foundation for subsea systems, defined as a shallow skirted mat foundation featuring short piles at the mat corners, has been investigated through a series of centrifuge tests. The foundation, with and without corner piles, was subjected to eccentric monotonic loading along the x, y and z axes, resulting in combined loading over 6 degrees of freedom. Modes of yielding were identified and the contribution of the corner piles to the bearing, sliding, overturning and torsional capacities of the hybrid subsea foundation was quantified. Results revealed that (a) centrifuge modelling could capture the strain hardening arising from the plunging nature of the mat foundation yield; and (b) the addition of the corner piles to the shallow mat resulted in a change of yielding mode from shearing at the mat invert to overturning, with a significant increase in the foundation capacities. Consequently, corner piles appear to be an efficient option to reduce the size of the subsea system foundation required to withstand a given set of combined loading.
Subsea mats are used in deep waters as foundations in soft normally consolidated (or lightly overconsolidated) clay to support facilities such as pipeline terminals, jumpers, riser bases and manifolds. They are typically subjected to various combinations of loading in all six degrees of freedom. In some cases, a mat may not provide sufficient resistance against bearing, sliding or overturning failure. The use of pinned piles in each corner of the mat may then be considered to increase the sliding and overturning capacity of the foundation. Such a foundation is defined here as a hybrid subsea foundation. Hybrid subsea foundations have already been deployed in situ, but neither formal guidance nor experimental data existed at the time to assist in their design.
