Low-Heave Semi-Submersible Enabling Dry-Tree Field Development

Abstract

The objective of this paper is to present the new Offset-Pontoon Semi-submersible (OPS) for use as a drilling and production platform. The new semi-submersible features pontoons that are offset to the outside of the columns which lends the vessel a low heave response and enables the use of top tension risers with dry trees. Other advantages of the OPS are a shallow quayside draft, a large payload capacity and the ability to operate in ultra-deepwater.The primary design challenge for dry-tree semi-submersibles is to make their heave motions compatible with the stroke range of existing riser tensioners. Increasing the vessel draft to reduce the heave motions is constrained by installation requirements and stability limits. However, vessel heave motions can be reduced by modifying the hull shape.The OPS has a longer heave natural period and a smaller heave response than a conventional deep-draft semi-submersible. The heave performance of the OPS was compared with a conventional semisubmersible using frequency-and time-domain analyses.In this paper we also present a design study of the OPS for dry-tree field development offshore Brazil. The vessel was sized to support thirteen top-tension risers and a 30,000 ton topside. The paper describes design of the vessel and its global performance. Focus of the study was the wellbay loayout and the tensioner stroke requirements. For the selected design case we found ram-style riser tensioners with a stroke range of 30 ft sufficient. Integration of the hull and topside by float-over is also described.