Shell Oil Company recently completed the design and installation of a three-part platform in 1025 feet of water, Gulf of Mexico. The jacket was fabricated in three separate pieces in Morgan City, Louisiana. The water depth dictated a unique design, and more particularly a unique installation concept. the lower, or base section, was installed in 1977 and connected to the seafloor with 24 pilings. The middle section was mated to the base section in July of 1978, and the top section placed on the mid section later that same month. Approximately 700 companies played a role in the construction of the Cognac platform. Most were domestic, but companies from Italy, Holland, France, Spain, Germany, Britain and Norway also contributed.
The area named "Cognac" consists of four tracts located 12 miles southeast of the mouth of the Mississippi River (Figure 1). In the March 1974 lease sale Shell and co-owners acquired three of the blocks for $214MM. Amoco and co-owners leased the other block for $81MM. A field unitization agreement was reached in July of 1977.
Exploratory drilling started in June 1975 and was completed in July of 1976. This nine-well program resulted in discovery of eight oil and gas bearing sands at depths between 5,000 and 11,000 feet.
Water depth over the four tracts ranges from 650 feet in the northwest corner to 1,650 feet in the southeast corner. In June of 1974 Shell signed a letter of intent for platform construction and installation with J. Ray McDermott, Inc., of New Orleans. A schedule calling for installation of the platform by late 1978 was developed.
Over 100 drainage points are required to develop the reserves on the four tracts. Consequently, the platform (Figure 2) has 62 well slots, including two curved conductor sleeves which are oriented towards the southeast. To develop the field in a timely manner the deck was designed to support two drilling rigs. The J-tube approach to pipeline and flowline riser installation was chosen and eleven J-tubes were built in. Of these, eight are sized to handle eight-inch or smaller flowlines, two are sized to handle 12-inch lines, and one is sized to handle a 16-inch line. This allows sufficient flexibility for future gathering lines or possible future underwater development. The platform is located close to the middle of the four-block lease, specifically 385.8 ft. south and 162.4 ft. west of the northeast corner of Mississippi Canyon Block 194.
A two-level deck, approximately 180 feet by 200 feet in plan dimensions, has a total of 66,500 feet of useable work area. The top level is 1099 feet above the mud-line, and 74 feet above the Mean Gulf Level (MGL). Clear span between the lower level and MGL is 56 feet.