Dry Tree Semisubmersible for Cost-Effective Deepwater Development

Abstract Dry tree floating platforms for deepwater offshore developments are typically Spars and Tension Leg Platforms (TLPs). All production semisubmersibles in operation support only wet tree systems because the hull motions are not sufficiently small to allow the use of riser tensioning systems which connect vertical production risers from the subsea wells directly to the topsides. The wet tree semisubmersible designs can potentially be improved for dry tree applications with reduced heave motions. The advantages of dry tree semisubmersibles for deepwater development are presented in this paper with focus on the hull global performance and the cost benefits. This paper presents a dry tree semisubmersible design with low heave motion and reduced vortex-induced-motion (VIM), hence named Heave and VIM Suppressed (HVS) semisubmersible. A case study of the HVS semisubmersible is performed for the dry tree application to support a relatively small topsides payload of 7,000 MT and five top tensioned risers (TTRs) in benign South East Asian environment. Through numerical simulations using Computational Fluid Dynamics (CFD) and conventional motion analysis based on potential flow theory, the hull global performance and maximum riser tensioner stroke range are provided. The stability analysis results for intact and damaged conditions are presented. The constructability and stability in pre-service conditions are briefly discussed to address the practical application of this hull with respect to cost effectiveness. The results demonstrate technical readiness of this hull design by showing its ability in supporting dry tree systems with field proven riser tensioners and its superior stability for drilling operations. The application of this dry tree semisubmersible is technically feasible today in mild-to-moderate environmental conditions and thus it is an attractive option for new deepwater development with potential cost benefits.