Foundation design criteria were developed for fixed base conventional jacket structures in Main Pass 299 in consideration of variations in the surface and subsurface soil movements expected to result from sulphur mine-induced subsidence. During subsidence, some structures will settle 60 to 65 ft and experience horizontal movements of more than 25 ft towards the center of subsidence. Vertical downdrag loads on piles will be caused by differential motions (between pile top and pile bottom) of up to 5 ft in some extraction stages; equal updrag will occur in other stages. The soil motion data necessary to the criteria development were obtained from influence function and finite element methods. The soil motions data for the 500-ft penetration obtained from the influence function method were used as input boundary movement data for finite element analyses of soil motion details in the foundation region from the mudline to about 350-ft penetration. This paper presents details of the concerted use of all of the available information in the development of foundation design criteria appropriate to mine-induced subsidence. Selected results are presented from the 15 different platform sites evaluated. Among the cases presented are examples of various relative importance for each type of soil motion criterion, ranging from minor considerations to dominant design consideration. INTRODUCTION Plans to undertake development of sulphur reserves of about 67 million long tons from deep below the seabed in Main Pass 299 have been initiated. A complex facility of drilling platforms, bridges, power plant, and related structures will be constructed to extract the sulphur. The structures will be fixed base, pile supported jacket platforms. The mining facility as it will be initially constructed is shown in plan on Fig. 1. Drilling platforms (Production 1 and 2) are located at the end of a mile-long bridge. The storage platform is at the other end of the bridge. Two other large structures, the Power Plant and the Quarters platforms are a part of the bridge. Also, there is a Heliport platform (Y-3) about halfway along the bridge. When the sulphur beneath production platforms is depleted, the platforms will be moved to new locations and reconnected to the bridge system. As many as 9 drilling platform locations may be required before complete extraction of the sulphur reserves, which is expected to take about 30 years. Main Pass 299 is located in the Gulf of Mexico about 20 miles east of the delta of the Mississippi River. Water depths in the block range from 202 ft to 222 ft, with a seafloor slope of about 7 ft per mile to the southeast. The sulphur is contained within salt dome cap rock buried beneath some 1400 ft of deltaic sediments and overlying anhydrite. The sulphur-bearing rock is expected to collapse under the weight of overlying strata as the sulphur is extracted. During the economic life of the facility the seafloor is expected to experience regional subsidence, encompassing over 3300 acres of seafloor and up to 65 ft at its deepest.

Previous mathematical descriptions of sampling using passive monitors have used Fick's First Law of diffusion and the assumption that the concentration of adsorbate in the vapor phase above the sorbent is zero. This paper shows that by introducing a simplified expression for the equilibrium vapor pressure, behavior more nearly resembling that observed for passive monitors is predicted. The theory can also be applied to the case of loss of sample from a diffusive monitor. Experimental evidence is also provided which demonstrates that the theory adequately describes the observed results.

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.

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