Dry Tree Semisubmersibles for Gulf of Mexico

Abstract

Abstract The offshore industry has expressed a desire for combining production activities with drilling activities on semisubmersible platforms with the wellheads on the platform. This combination of activities allows for an earlier start of production while the drilling program is ongoing and/or allows well maintenance to be performed during production. These systems are typically referred to as dry tree semisubmersibles. Drilling semisubmersibles are successfully operational around the world and production semisubmersibles with subsea wells are operational in the Gulf of Mexico, the North Sea, and offshore Brazil. However, a dry tree semisubmersible solution for hurricane environments that is both technically and economically viable is in various stages of maturity. The challenge for these structures in hurricane environments is the magnitude of the stroke of the top tensioned risers (TTR) which is significant, and but is now within proven technology used on deepwater semisubmersible drilling rigs. This issue can be resolved technically by either reducing the vertical motion of the semisubmersible, by increasing the allowable tensioner stroke, or by a combination of both. An evaluation and discussion is provided on both solution paths with a focus not only on the technical challenges but also on the cost aspects of the proposed solution paths. Cost is affected by the tensioners, construction, integration, transportation, and installation issues. This paper will present one solution to the design challenge of a dry tree semisubmersible in hurricane conditions, and the resulting compromises will be discussed. The dry tree semisubmersible has promise to be more cost effective than spar or TLP solutions in many development scenarios, and will be an important component in developing ultra-deepwater reservoirs. Introduction The offshore industry has approached deep water oil and gas production with a mix of wet tree (subsea) and dry tree systems. Wet tree solutions typically have remote subsea wellheads/trees which are located away from the vessel and which are connected to the vessel by means of steel or flexible catenary riser systems, while the dry tree systems typically are positioned directly over the well locations so that the well is extended to the surface with a top-tensioned riser. The wet tree systems include all variations of floating vessels, including FPSO's, semisubmersibles, TLP's, and simple barges. The currently installed dry tree solutions are limited to TLP's and spars.