HP/HT Semisubmersible for 10,000 ft in Gulf of Mexico
Authors: Banon Hugh , Nygard Magne , Loken Rolf , Lu Roger , Garrett David 
Published: October 2008

The paper addresses the main results of the work executed during the Phase VII of DeepStar Project for CTR 7404: Semisubmersibles for Ultra Deepwater. The integrated system of the risers, polyester mooring and hull is discussed, and results for application of steel catenary risers (SCRs) are shown. The overall response of the risers, especially at the touchdown region, is a key focus during the design of the semi as this is largely affected by the motions of the semi. The floater is designed as a "Deep Draft Semi", essentially suppressing a large part of the motions by keeping the pontoons away from the wave loading zone. The pipe-in-pipe (PIP) is the most attractive riser system from a response perspective, but poses challenges to installation. The most significant limitation is the lack of vessel capacity to install heavy pipe-in-pipe risers at this water depth. Application of High Integrity Pressure Protection System (HIPPS) reduces the riser pressure significantly and can mitigate this problem. The conclusion of the study is that the semi with SCRs is an attractive floating concept in ultra deepwater. The attractiveness is due to a tried and tested floating system that can be designed to have acceptable motions for SCRs. In addition, the in-shore integration of a semi hull and topsides can reduce the cost and schedule of a development.

Introduction

The DeepStar program is a joint industry project launched in 1992, in which operators, service companies, regulators, cademia, and research institutes cooperate to develop the technologies that enable hydrocarbon production from deepwater. A status report of the JIP was published in the June 2005 Offshore magazine (Maksoud 2005).

One of the primary objectives of DeepStar is to identify technology gaps in deepwater and develop economical and afe solutions to close these gaps. In Phase VII of DeepStar, several studies on deepwater high pressure and high temperature (HP/HT) production facilities were funded. The basic development scenario is a medium to large oil field in 10,000 ft water depth, produced through a dry tree classic Spar, a truss Spar, or a wet-tree Semisubmersible.

This paper reports the work undertaken as part of the DeepStar CTR 7404: Gulf of Mexico (GoM) Semisubmersibles in Ultra Deepwater. The work has been executed by Aker Kvaerner's Deepwater Unit, in a collaborative effort with Stress Engineering (riser design and coupled analysis), and with Aker Marine Contractors (installation issues). The study addresses feasibility issues and identification of technology gaps with respect to designing an HPHT production semi in ultra deepwaters of the Gulf of Mexico. The main focus is performance of the integrated system comprising the hull, mooring and risers, including the challenges related to the system installation.

As the offshore industry progresses to deeper waters, the semi-submersible represents a tried and tested candidate for the floating production facility. The semi selection in these water depths is contingent on the feasibility of the riser systems (i.e., vessel motions that can enable the use of SCRs) as well as the overall cost of the facility.