The Holstein spar hull is the first truss spar hull to be built in two pieces and the pieces mated in water. During mating, various critical primary structural truss members, risers, and appurtenances had to be connected. This also included aligning and welding of structural leg and X-brace members that were located underwater. Various structures and tools were used during mating to achieve alignment between the two sections and create an underwater dry habitat for welding structural members. These included the mating tank with numerous compartments to support and control the floatation of the truss, open-top cofferdams to create underwater dry habitats for welding truss legs and X-braces, mating guides to pull and align the truss leg members and temporary platforms to support and align appurtenances being mated. The criticality and the thickness of the truss legs in the mating area required double sided welding, requiring access inside the legs. Pup-pieces were used to mate different members except the truss legs.
The welding root gap and alignment of the truss legs were achieved by pulling the two structures together using mating guides and ballasting the mating tank and soft tank. The alignment of members being mated and dimensional control were complicated by the fact that both the sections were floating with weight and buoyancy varying during the operation. The alignment of different structures required different ballasting configuration and also was dependent on what had already been mated. The mating of members had to be done in sequence and prevented many simultaneous operations due to: different ballasting requirements for different members, different environment criteria, land and water crane access, HSE concerns for dropped objects and diving.
This paper describes the mating operation of a truss spar, addresses the challenges and issues with mating in water and discusses how these challenges were met through analyses, proper sequencing and employment of dimensional control techniques.
The Holstein truss spar hull due to its diameter of 45.5 m (150 ft) and 227.36 m (746 ft.) length could not be carried as a single piece, from the fabrication yard in Finland to the offloading site in Gulf of Mexico, on the largest heavy lift vessel available at the time of fabrication. As a result, the spar hull was fabricated in two pieces and in two different fabrication yards. The truss and the soft tank were fabricated on land as one-piece in Technip's GMF yard in South Texas, loaded out and offloaded in sheltered waters next to the fabrication yard using a launch barge (Ref. 1, 2). The hard tank of the hull was built in Technip's TOF yard in Finland and transported to the Gulf of Mexico on a heavy lift vessel and offloaded in sheltered water near the truss fabrication yard. The two floating sections were joined (mated) and welded together over water next to the fabrication yard. The hard tank section of the hull is shown in Figure 1.