Often the requirements for dry–tree completion, direct vertical access (DVA) to production wells, as wellas the ability to perform drilling and work-over from the production platform, due to operational,economic, or reservoir requirements, necessitate the use of Top Tensioned Risers (TTRs). Due to strokelimitations on the currently qualified tensioners’, TTRs are required to be hosted on a floating system withminimal heave response. To date, the floating systems of choice to host TTRs were Spars and TLPs, withthe former being the floater of choice for water depth beyond 6,000 ft.Conventional semisubmersible design has a number of significant advantages. These include provisionof significantly larger and more ventilated deck area; ability to integrate the hull and topside at thequayside or via dry-mating at the fab yard; and lower risk and better economic for offshore installationoperations compared to the Spars where they require upending and offshore deck lift operations.Additionally, the semisubmersible platforms are more flexible with respect to maximum payload.However, conventional semisubmersibles heave response is not suitable for TTR applications.This paper presents invovative semisubmersible designs, which provides suitable motion characteristicsfor hosting TTRs, while maintaining the advantages of semisubmersibles floaters.For development of marginal fields, it is essential to minimize the total cost of the project in everyphase. This includes realizing the savings from using DVA to wells, adopting quayside integration ordry-mating of topside to hull and avoiding the need for offshore lift operations. These measures can beachieved by developing semisubmersible designs which provide better motion characteristics suitable forTTRs and Dry-Trees.Two state-of-the-art high performance semisubmersible designs have been presented here and evaluatedfor the development of marginal fields in deepwater Gulf of Mexico (GoM). These designs are theFee-Hanging Solid Ballast Semi (FHSB Semi) and the Damper Chamber Column Semi (DCC Semi).Detailed description of these novel designs and their performance in the deepwater GoM Central region,as well as the performance of the associated TTR systems are presented. Furthermore, project risks andtheir proposed mitigation measures for these two novel designs are also investigated and reported.The paper concludes with discussions on the project execution plan, cost and schedule benefits whendeveloping marginal fields using the two presented novel designs.
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