Design of a Conventional Platform for the South Pass Block 47 of the Mississippi River Delta
Authors: Danner Lee D. , Kreinsen Michael C. 
Published: October 2008

Energy Development Corporation (EDC) installed a ‘Conventional’ drilling and production platform in South Pass Block 47 (SP 47) in August, 1995. The platform site chosen is subject to tropical hurricanes as well as foundation mudslides and overflows, which typically necessitate a ‘non-conventional’ structure, As a result of a coordinated effort among risk, oceanographic, geotechnical, and structural consultants and the use of detailed site specific data, a conventional platform became a reality. This paper presents the structural engineering role in the project.

Introduction

The SP 47 prospect is located in 224 feet of water in the Mississippi River Delta approximately 15 miles Southeast of Bumvood, Louisiana. The site of the facility was chosen for directional drilling access to two targeted gas fields in SP 34 and SP 47 with a potential total of six wells. Even though this water depth is well within the limits of many jack-up drill rigs, the potentially unstable soils of the region prevent their use and necessitate drilling from a full platform rig.

These requirements of resisting loads from both unstable soils and tropical storm waves coupled with the necessity of supporting sizable drilling loads have traditionally resulted in large ‘unconventional’ platforms. Such structures have typically protected their wells from soil loads by drilling through the legs of the platform or through a large central caisson. Both of these options result in a sizable amount of steel being added to the structure. If wells are drilled through the jacket legs, the legs need to be vertical and larger in diameter than normal and jacket framing becomes more difficult and costly. If a large central caisson is used, the weight of the caisson itself plus the framing necessary to support and guide it must be considered. Due to the weight constraints of available lift barges, approximately 1200 to 1600 tons, both of these types of jackets would have required a launch installation. This in turn, adds additional steel weight for the launch trussing. Thus, mudslide resistant structures have tended to be heavy and costly.

The economics of the SP 47 project demanded capital investment be minimized for a successfied return from the possible six wells. Since a platform's cost is directly linked to its weight, the challenge was to minimize the steel required in the jacket. After a review of other mudslide resistant platforms, it became evident that the key to minimization was the use of independent conductors coupled with a stretch 4-leg platform

A project team of independent consultants was assembled to show that such a concept was viable. This required precise definition of the environmental and geotechnical characteristics particular to the site, their utilization in the analysis and design of the platform, and the determination of the concept's risk heliability. This process, once completed, would be presented to the Minerals Management Service (MMS) for their approval to proceed with the detail design and fabrication.