Evaluation of the Reliability of a Conventional Platform Installed in South Pass Block 47 of the Mississippi River
Authors: Bea Robert  G. 
Published: October 2008

In August 1995, the Energy Development Corporation (EDC) installed a conventional drilling and production platform in South Pass Block 47 (SP 47). Due to its proximity to the delta of the Mississippi River, this platform will be subject to the environmental forces developed by hurricanes and movements of the sea floor. This paper summarizes results from probability based study of the reliability characteristics of a ?conventional' platform installed in SP 47. Bases for evaluation of the acceptability of the reliability of the platform are developed. This paper shows how reliability methods can be used to help improve the efficiency of offshore platforms. Application of traditional engineering approaches indicated the need for a mudslide resistant platform. Given that a mudslide resistant platform were required, then the gas reserves could not have been developed.

Introduction

SP 47 lies approximately 5 miles due south of the South Pass of the Mississippi River Delta. The platform site (Fig. 1) is located in a water depth of 224 ft. on a gently sloping soft clay sea floor.

The platform is a conventional 4-leg, steel, template-type drilling and production platform that will support six well conductors (Fig. 2). Four gas wells will be drilled and completed from this platform. The produced gas will be transported through a pipeline routed from the platform northto the vicinity of South Pass. The platform will be manned only during drilling and work over operations and intermittently during production operations. The life of the field is expected to be 4 to 6 years.

The platfom was designed by Paragon Engineering Services, Inc. (PES) and constructed by Akker Gulf Marine and Gulf Island Fabrication according to current API Working Stress Design (WSD) guidelines.1

Special provisions were made in design of the pile and conductor foundation to accommodate projected movements of the sea floor induced by hurricane waves. Temporary mudmats were designed into that platform that were removed after the installation of the platform, The supporting structure for the temporary mudmats was located at an elevation that will clear the expected thickness of potential over-run mud lobe soils. The location of the lower mudline framing and the removal of the mudmats substantially reduced the mudslide forces.

Geotechnical Conditions

An extensive study of geotechnical conditions in the vicinity of the platform location was performed by Fugro-McClelland Inc. (FMI). 2

The soils in the top 200 ft. of the soil profile are soft to firm gray clays that have been deposited by the Mississippi River on top of the Pleistocene contact that was formed at the time of the last low standing sea level. The soil boring indicates that the soils have a typical ?crust' shear strength profile that has a maximum shear strength of approximately 800 pounds per square foot (psf) at a penetration of about 75 ft. The presence of this crust profile indicates that the soils at this location have been relatively stable during recent times (last 30 to 40 years).