Technical and Economic Evaluation of a Gulf of Mexico PlatForm for Reuse in the North Sea

Abstract

Abstract Reuse of Gulf of Mexico platforms is common practice if the facility is also destined for the Gulf of Mexico. However, reuse of a Gulf of Mexico platform in the North Sea is uncommon. An existing Gulf of Mexico platform installed in 1990 in 303 ft water depth was evaluated for reuse in the North Sea. While the water depth was similar at both sites, regulatory and environmental conditions differed. This paper summarizes the evaluation process which covered feasibility level analyses of in-place storm, in-place fatigue, in-place damage, lifting, transport strength and transport fatigue. The evaluation also included in-depth consideration of material properties and weld procedures. A cost and schedule comparison between reuse of the Gulf of Mexico platform and fabrication of a new platform showed that the reuse option provided a lower cost and shorter schedule compared to fabrication of a new platform. While not all Gulf of Mexico platforms can be used in the North Sea, this paper points out that the newer generation Gulf of Mexico platform designs can exhibit the necessary design features and level of safety that is required for North Sea operations. Introduction Platform reuse, which is common in the Gulf of Mexico and also other areas of the world, has been a viable field development scenario for many years. For example, the first OTC conference in 1969 included a paper1 which described the successful decommissioning, removal, and re-installation of a 165 foot water depth Gulf of Mexico platform point-to-point in 1966. More recently, the larger amount of new North Sea fields considered economically marginal, coupled with declining production from existing fields, has led to platform reuse being considered as an alternative decommissioning field development scenario Amoco UK is considering the development of several fields in the UK North Sea with one scenario including subsea completions fed to a gas compression platform. Amoco has developed cost estimates for several options including a newbuild platform, a retrofit on an existing platform, and a reused platform. This paper discusses preliminary technical work performed to determine the feasibility of reusing a modem (1990's vintage) Gulf of Mexico platform to the North Sea. Consideration was given to elements involved with the development of a Safety Case, however, only preliminary discussions have been held with UK verification agents. A cost estimate was developed for reusing a specific existing platform and is compared with the costs of a new-build platform. Platform Configuration Figure I shows a comparison between the original existing platform for Gulf of Mexico (GOM) operations and the proposed upgraded platform for North Sea (NS) operations. The original GOM configuration was a four leg configuration and was originally installed in 1990. The platform follows the Amoco TAMPA2 (tender assisted minimum platform arrangement) design standard. The platform has 57 ½ inch diameter legs containing 54 inch diameter piles driven to approximately 345 feet penetration. The pile were not grouted to the legs in its original GOM location. The platform is vertically X-braced with 20 to 24 inch diameter braces. Horizontal bracing consists of 20 inch diameter X-bracing at each major elevation; however, exterior face horizontal framing exists only at the first elevation below the waterline and at the mudline (other elevations have horizontal framing for lifting purposes).