Novel Dry-Tree Semisubmersible Solutions for Deepwater Marginal Field Development

Abstract

AbstractOften the requirements for dry—tree completion, direct vertical access (DVA) to production wells, as well as 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 stroke limitations on the currently qualified tensioners’, TTRs are required to be hosted on a floating system with minimal heave response. To date, the floating systems of choice to host TTRs were Spars and TLPs, with the former being the floater of choice for water depth beyond 6,000ft.Conventional semisubmersible design has a number of significant advantages. These include provision of significantly larger and more ventilated deck area; ability to integrate the hull and topside at the quayside or via dry-mating at the fab yard; and lower risk and better economic for offshore installation operations 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 characteristics for 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 every phase. This includes realizing the savings from using DVA to wells, adopting quayside integration or dry-mating of topside to hull and avoiding the need for offshore lift operations. These measures can be achieved by developing semisubmersible designs which provide better motion characteristics suitable for TTRs and Dry-Trees.Two state-of-the-art high performance semisubmersible designs have been presented here and evaluated for the development of marginal fields in deepwater Gulf of Mexico (GoM). These designs are the Fee-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 and their 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 when developing marginal fields using the two presented novel designs.