Development Of An Alignment System For The Three-Part COGNAC Platform Jacket
E.S. Piter, Gordon H. Sterling, Bart Cox · Offshore Technology Conference · 1979
ABSTRACT This paper discusses the system incorporated into the three-part Cognac "A" Platform jacket to guide the sections into lateral alignment as they were positioned offshore. A brief description of the alignment appurtenances and data on the in-place misalignment are given. INTRODUCTION During the early phases of the Cognac design, the decision was made to fabricate the Cognac Platform in three separate sections (Reference 1 and 2), (Figures 1, 2, and 3=Available in full paper). The Base Section was launched from a barge in an upright position. It was then lowered to the seafloor and anchored by 24 large diameter skirt piling (Reference 3). This section was 178½ feet in height measured from the centerline of the bottom horizontal to the top of the upward looking mating flanges. The Mid Section was fabricated and launched in a horizontal position, ballasted to vertical, lowered and set on top of the Base Section. This brought the total height of the jacket to 498½ feet. The Top Section, which was 527½ feet in height, was fabricated and installed similar to the midsection and extended the platform so that the jacket :walkway was 14 feet above the waterline. The primary mating components were the ten jacket legs through which large diameter tubular connection pins were installed and grouted in place (Figure 4). These pins extended through all three jacket sections and terminate just above the mudline at the bottom of the Base Section. Two of the jacket legs (Rows XL and X4) terminate at approximately 400 feet below the waterline. The other eight legs extend above the waterline. At each mating interface, the jacket legs terminate with large mating flanges that contain a rubber seal for containment of the grout as it was pumped into the jacket leg/pin annulus. Other items which were fabricated as an integral part of each jacket section, and had to line-up after installation, included eleven pipeline pulltube risers, sixty straight conductor pipe guides and two curved conductor pipe sleeve guides. The proper alignment of these items was not necessary to insure the structural integrity of the platform; however, each of them could be utilized only if properly aligned. Precepts to be followed during the design and selection of the jacket alignment system were developed and alternate alignment systems were studied. Relative cost comparisons were made, taking into account the impact on the construction schedule that each system would have. The two most important functions of the alignment system were:To provide a guiding system that would allow the upper jacket section to be set down upon the landing supports of the lower section and released from the lowering lines without structural damage resulting from an underwater collision between the mating jacket sections.To provide a method of aligning the corresponding jacket legs of the mating sections to an accuracy that would allow installation of the leg connection pins.