Hybrid Subsea Foundations for Subsea Equipment

A hybrid subsea foundation (HSF) is defined here as the combination of shallow and deep foundations, designed such that both shallow and deep foundation elements contribute to the total foundation capacity. In this project the type of HSF investigated is a shallow mat foundation connected to short piles, with pinned connections made close to the mat corners. The motivation for this research is the potential economic benefit to using HSFs in deepwater subsea developments, where mat sizes to resist typical subsea foundation design loads (e.g., from pipeline end terminations, manifolds, and riser bases) require ever larger installation vessels with higher associated cost. This paper assesses the design feasibility of a HSF for subsea facilities in soft clay and provides recommendations for design. The focus here is on analytical and numerical modeling, although corroborating results from physical modeling are provided. Numerical and physical modeling has shown that the proposed analytically based design approach is suitably conservative, with deformations well within typical serviceability limits at the calculated capacities. Most significantly, the paper demonstrates that significant reduction in the foundation footprint can be realized by supplementing a mat with short piles at each corner.