Reliability-Based Design Criteria for Freeport McMoRan Main Pass Block 299 Sulphur Mine Platforms
Authors: Bea R.G. , Moore G.H. , Lee G.C. 
Published: October 2008

The application of reliability methods in development of design criteria for the Freeport McMoRan Resource Partners Main Pass Block 299 Sulfur Mine platforms is described and illustrated. Reliability engineering principles are outlined, including assessment of structural reliability and characterization of acceptable reliability. This development is cast in the context of the traditional methods that were used for the design of the structure and foundation elements that comprised these structures. Special attention is given to design criteria for the foundation piles as influenced by the substantial subsidence projected for several of the platforms.

INTRODUCTION

This paper will describe how reliability based methods were used to help define design criteria for the Freeport McMoRan Resource Partners (FMRP) Main Pass 299 Sulphur Mine Platforms. Reliability methods were used because of the unusual aspects of this facility. These aspects included:A 6 platform complex connected by 6,000 feet of bridges and an adjacent facilities platform that would be used to mine an offshore sulphur deposit underlying acreage some 30 miles off the Mississippi River Delta in 215 feet of water.The platforms supporting the mining facilities would be subjected to significant subsidence and deformations induced in the foundation piles due to the mining process; near the middle of the facility, the sea floor is expected to settle approximately 65 feet in 40 years. FMRP wanted to develop design criteria that would recognize these and other unique elements of these facilities, and take advantage of the most recent technical developments.

An important aspect of the criteria development was that the results should be capable of being applied in the context of existing conventional design methods and processes, with as little disruption to normal engineering procedures as possible. While reliability based methods would be used to develop the criteria, reliability considerations would be transparent to the design engineer. Even though a load and resistance factor design (LRFD) format would be used to express the application of the design criteria, the basic analyses performed by the engineer would remain unchanged.

RELIABILITY BASED DESIGN CRITERIA

Design criteria are intended to produce a structure that is economic, safe, and serviceable. In the ocean, this is a challenging engineering task because of the many uncertainties and vagaries of this hostile environment, and because of the innovative structures that are frequently used to work in this environment.

Design criteria should provide the engineer with a readily applied process and set of parameters that will guide him in engineering a coastal or ocean structure to have acceptable performance characteristics. The structure must have sufficient strength to satisfy its intended purposes without undue expense or risk.

A primary concern of the platform design engineer is to provide a structure that is able to satisfactorily perform its daily tasks, providing a useful service, throughout its life. This can be termed "serviceability limit state" (SLS) performance (Figure 1).

Traditional structural design is generally focused on the serviceability capacity performance characteristics.