“Bigfoot” Direct Vertical Access Semisubmersible Model Tests and Comparison With Numerical Predictions

Abstract

The Bigfoot direct vertical access (DVA) semisubmersible is a novel floating drilling and production host that provides an attractive alternative to the spar. This concept utilizes heave plates (big feet) that improve the motion characteristics of a semisubmersible in all mild environments (Southeast Asia, West Africa, and Brazil). Bigfoot offers riser-friendly motions that enable top-tensioned risers, which is often a project requirement. This floater works in all water depths, in particular ultra-deepwater (5000 + ft) where a tension leg platform (TLP) is not an option, supports top-tensioned risers, and enables drilling and workover operations. The Bigfoot has several advantages over a spar. These include: (1) quayside topsides integration. This eliminates offshore topsides integration, a significant issue for all spar projects in terms of cost, safety, and schedule, (2) a more open deck layout compared to a spar, and (3) no fabrication location restrictions as it can be built by many yards worldwide potentially offering local content to a project. Model tests were undertaken at the Shanghai Jiao Tong University (SJTU) Offshore Basin to assess the dynamic response of the Bigfoot in waves, swell, wind, and current. Five mild non-Gulf of Mexico (GOM) environments were considered. In all the cases, the floater motions are an order of magnitude smaller than those of a conventional semisubmersible for similar deck payload, thus enabling drilling operations and top-tensioned production risers. In a parallel effort, a cosmos numerical model of the Bigfoot was developed for coupled motion analysis. The experimental results and the cosmos numerical predictions are in close agreement. In addition to measuring global motions, two heave plates were instrumented with load cells to measure forces and moments. The force measurements from the model tests are in good agreement with numerical predictions using computational fluid dynamics (CFD).