Exxon, U.S.A., has installed a mud slide resistant drilling and production platform in 119 ft. of water in the Main Pass area, block 72. Mud slides as deep as 50 ft. are possible in this area where rapid deposition has produced a weak, unstable, under consolidated clay. The major configurationally difference between this platform and conventional Gulf of Mexico platforms is the four 154 in. diameter jacket legs required to resist the lateral forces caused by soil movement and to form a protective housing for the conductors. The foundation consists of four 144 in. diameter piles with 72 in. diameter insert piles driven in two legs and twelve 22 in. diameter conductor piles inside each of the other two legs. The design and installation of this structure are especially noteworthy due to loads imposed by the unstable bottom conditions of the site.
Exxon's Main Pass 72 platform (Figure 1) is in the state-owned portion of the block and is approximately 6 statute miles from the mouth of Pass A Loutre in water depth of 119 ft. The regional seafloor slope is to the east-northeast and averages0.4°. Under consolidation and high pore pressure have decreased the sediment strength sufficiently to make the delta front prone to mass sediment movements, which are categorized as either creep, slide, or oscillation. Creep is a gradual soil deformation with time, typically associated with a slope. A slide is a sudden movement of a mass of soil down a slope. Oscillation is a cyclic movement of the soil, including both horizontal and vertical motion, driven by wave pressures on the soil surface. Consideration of soil motion in conjunction with the wave loading leads to the design concept of encasing the well conductors within the large diameter leg members. Most other platforms (e.g Reference 1) for mud slide areas have this common design characteristic. However, the platforms are quite different, primarily due to the large array of concepts for jacket and piling installation.
The structural design of Main Pass 72 platform was initiated in 1977. The design goal was to maximize the use of proven technology, particularly that required during the offshore installation.
The Main Pass 72 platform (Figure 2) was designed to accommodate 24 wells and provide 16,900 sq. ft. of deck space on each of two levels for drilling and production. The platform was designed to be installed in 119 ft. of water and to withstand hurricane storm loads and those additional loads caused by moving soil.
The jacket has four plumb 154 in. diameter legs whose centers in plan form an 85 ft. square. The leg members penetrate 50 ft. into the mud to stabilize the jacket during the early stages of pile driving. Each leg is grouted to a 144 in. diameter pile driven to a penetration of 200 ft.
