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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Gerwick Jr., B.C. U. of California-Berkeley Potter, R. Potter, R. Sohio Petroleum Corp. Matlock, H. ERTEC Western Inc. Mast, R. SPE ABAM Engineers Inc. Bea, R.G. SPE PMB Systems Engineering Inc. PMB Systems Engineering Inc. October 1984 Summary The critical design parameter for many concrete and steel gravity-based platforms in shallow and moderate depth waters is their resistance to sliding under high lateral forces such as those imposed by ice or earthquakes. Conventional methods of increasing sliding resistance include adding ballast, enlarging base size, and the provision of skirts. All of these have limitations because of draft, cost, or overloading of the foundation as well as problems in seating and removal. Multiple large-diameter steel spuds or dowels, individually penetrated after seating of the structure, offer a viable penetrated after seating of the structure, offer a viable alternative for the transfer of lateral shear forces into the foundation. They are especially suitable to many arctic locations where water depths are shallow and weak seafloor soils overlie more competent soils. Typical spud sizes range from 60 to 96 in. [150 to 240 cm] in diameter and may penetrate 20 to 40 ft [6 to 12 m] below the seafloor. They are installed individually by vibratory or impact hammers augmented by jetting or drilling. Assessment of ultimate shear resistance must consider several potential modes of failure. These can be balanced in each specific case to fail under overload in a desirable sequence, first in sliding and then in bending, before damage to the structure itself. The spuds are free to move vertically relative to the structure, hence the structure will maintain contact with the soil during any consolidation settlements and can rotate slightly (tilt) as necessary to develop bearing resistance to overturning moments. Under impact loads such as those from a multiyear ice floe, the compliance of the spuds in bending and the soils in strain absorbs significant energy, thus reducing the maximum loads. In seismic zones, spuds can be employed to develop high shear resistance without significant increase in the attracted inertial force. Multiple spuds appear to be a practicable solution to provision of sliding resistance to high lateral forces for provision of sliding resistance to high lateral forces for structures in shallow water, weak soils, or where relocation is anticipated as with exploratory structures. Because the number and penetration depth of the spuds can be increased or decreased penetration depth of the spuds can be increased or decreased readily, the concept is very flexible in meeting varying soil, ice, and seismic criteria. Introduction The structures for current and planned developments in the arctic are faced with the principal problem of resisting high lateral forces from sea ice. In the shallow waters of the Beaufort Sea, and presumably in the Chukchi Sea and Norton Sound as well, the critical failure mode is that of sliding. The lateral forces from the ice must be transferred through the structure and into the foundation soil. This same problem occurs in North Sea gravity structures under storm waves. There the probable failure plane is usually on or close to the interface between the base of the structure and the seafloor. Enlarged bases and steel skirts have been the adopted solution, with the skirts penetrating several meters through the weaker soils to mobilize increased shear strength below. In the water depths typical of the North Sea, there is enough net weight of structure and ballast available during installation to "dove" the skirts into the seafloor the required distance. Gravity-based caissons recently have been proposed for shallow water sites in other parts of the world where the foundation soils consist of partially consolidated sands. Some of these sites are subject to strong earthquakes. The resistance to displacement by sliding is a function of the net submerged weight of the structure and the friction factor at or just below the structure/soil interface. Although adding solid ballast to the structure can increase this resistance, it also increases the inertial forces developed-especially if more structure and more volumetric displacement are required to contain the ballast. While a true description of the seismic response of such a structure/water column/soil system is very complex and the matter of potential liquefaction of the sands must be addressed also, it appears that the addition of an enlarged base and solid ballast may not be the most effective solution because of the increased seismic forces generated. In the southern Beaufort Sea, which is the area of principal interest in this paper, there are a number of areas (including Harrison Bay) where 10 to 30 ft [3 to 9 m] of recent silts and clays overlie the relatively strong Pleistocene sands. The upper portion of these Pleistocene soils is relict permafrost, ice portion of these Pleistocene soils is relict permafrost, ice bonded to varying degrees, in temperature balance (30.2 to 28.4F [-1 to -2C]) with the recent silts above it. JPT P. 1719