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The Main Pass Mine Production Platform No. 1 (PP1) experienced failure of a conductor guide framing level due to buoyancy-induced fatigue. The Mine is unique because several of the platforms are expected to settle up to 60 feet. An inaccurate water depth survey contributed to a horizontal level of PP1 being located exactly at the waterline, which was higher than planned. A replacement horizontal framing level was constructed and installed five feet above the damaged horizontal level and was designed with adequate fatigue life for movement through the waterline. Detailed deterministic and spectral fatigue analyses were performed along with typical wave analyses. Consideration was given to platform settlement, cyclic buoyancy, wave slamming, and wave loads on anodes and plated conductor guides. INTRODUCTION Freeport Sulphur Co.'s Main Pass Mine (18 platforms and 13 bridges) is located 20 miles east of Venice, La., in 210 ft. water depth. The Production No. 1 Platform (PP1) is one of two sulphur drilling and producing platforms. A perspective view of PP1 is shown in Fig. 1. The extraction of sulphur causes the seafloor to subside significantly. PP1 was designed for a maximum vertical subsidence of 36 ft. For the Main Pass Mine platforms that subside significantly, some horizontal framing levels must pass through the waterline. Offshore structural engineering practice is to locate horizontal platform levels away from the waterline. This is partly because the complex behavior of waves and tides in this region is not fidly understood and very difficult to quantity. In addition, wave slamming is a phenomena thatoccurs in the near waterline region which creates significant vertical wave forces on structural members. In the case of PP1, all leveIs were located such that none would pass through the waterline. The El.(-) 7'-6 horizontal framing, which contains 76 conductor slots, was such a level on PP1. Fig. 2 is aplan view of this level. Subsidence of the seafloor was predicted to occur swiftly, 5 ft. in the first year for PP1 (from the start of mining operations), whichwould have placed this level 12 ft. below the water. However, due to shallower water at the site and the lack of mudmat penetration during installation, the platform was set 7 ft. higher than the design datum. Therefore, the El. (-)7'-6 level is actually located at the waterline. Also, subsidence has not occurred since the June 1991 installation. The cause of the El. (-) 7'-6 level failure (discovered in June 1993) is believed to be fatigue damage due in great part to the constant immersion in water of this level, creating buoyancy induced stresses by even the smallest waves and tide variations. Fatigue was determined to be the cause of failure based on metallurgical analyses. Fatigue calculations showthat tubular member buoyancy was a prime factorin fatigue of joints. The initial design of PP1 included strength checks of the El. (-) 7'-6 framing for wave slamming and pressure based on maximum (waterline) vertical.

The fixed-base steel platform experienced unusual deck vibrations soon after installation during Eddy current episodes. The approximate 70 cycles/minute oscillation of the deck resulted in the temporary evacuation of personnel. The platform is a skirt-piled structure in 430 ft. of water. The paper will present analyses performed, current measurement methods employed, motion detection devices utilized and corrective measures investigated and selected.The identification of the cause of the vibrations will help in understanding and preventing future occurrences. The corrective measures investigated and utilized will assist operators encountering similar behavior on existing facilities.Eddy currents are capable of producing vibrations on fixed-based platforms when the platform geometry is prone to excitation from steady high currents due to the shedding of vortices from submerged elements. Current-created vortices shed from platform members and appurtenances created a resonant condition on the platform J-tubes. The J-tubes fundamental period of vibration matched one of the deck's vibration modes, causing the 70 cycles/minute motion. This type of platform behavior resulting from the influence of high Eddy currents was essentially new to the industry and is not found in the literature. Field data, results of computer analyses, field measurements and details of corrective measures investigated and selected are included in the paper. Dynamic, fatigue and vortex shedding analyses were performed utilizing commercial and proprietary software. Computational Fluid Dynamics simulations were also employed. Mass dampers, although not employed, were investigated and devices designed for the platform.The paper provides critical new technical knowledge to resolve an important problem: Identification of platform geometry prone to Eddy current excitation.  Analytical methods best suited to solving the problem.  Corrective methods utilized to arrest the motion.  Other corrective methods investigated but not utilized on this platform.  Current measuring and motion detection devices utilized to obtain applicable data.

The South Marsh Island 205-A platform is located in the Gulf of Mexico off the coast of Louisiana in a water depth of 437 ft. Instead of launching the jacket from the transportation barge, the jacket was lifted from the barge with two derrick barges. The jacket consists of four main legs which frame directly into four skirt sleeves. This is the first such jacket to be designed for the Gulf of Mexico. It is the largest jacket by weight and water depth to be lifted in the Gulf of Mexico and is the deepest four pile jacket installed in the Gulf of Mexico to date. The design and configuration of the jacket, the development and analysis of the skirt sleeve to leg connection and the installation of the platform are presented in this paper. INTRODUCTION South Marsh Island block 205 is operated by Mobil Exploration & Producing U. S., Inc. The contract for the design of the South Marsh Island 205-A platform was awarded to McDermott, Inc. in March, 1988. In an effort to reduce fabrication and installation costs and to enhance the financial return of the field, the feasibility of a lifted jacket with four skirt piles and no main piles was investigated. Gulf of Mexico fabrication and installation contractors were contacted for input concerning fabrication capabilities, limitations, marine equipment capacity and availability. At the same time, Mobil Research and Development Corporation (MRDC) was contacted for information on a skirt sleeve to leg connection that had been developed for the North Sea. Through preliminary analysis along with contractor and MRDC input, it was concluded that the concept was feasible and that cost savings could be realized. In another effort to reduce costs, an existing deck was salvaged and refurbished for reuse. The deck was a two level, eight leg drilling/production deck that was originally designed for simultaneous operations. For the removal of the deck, it was cut off 2 ft. below the cellar deck and placed on 2 in. circular plates to be later installed on top of the South Marsh Island 205-A jacket. Elevation views of the platform are shown in Figure 1. The eight leg deck is supported by the four corner legs of the jacket and four false legs which are braced into the corner legs below the waterline. The legs are tapered from 36 in. diameter at the top to match the deck legs to 80 in. diameter at the bottom. The spacing of the legs at the top of the jacket is 45 ft. × 125 ft. The legs are battered 1:6 in the transverse direction and 1:12 in the longitudinal direction which results in an essentially square base that is 210 ft. × 208 ft. The jacket weighs 3810 s. tons in air as weighed with electronic load cells prior to loadout. In order to lower the weight of the jacket for lifting purposes as well as on-bottom weight, approximately 45% of the jacket steel is 50 ksi material.