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.
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.