The Holstein truss spar hull, the largest ever built, was fabricated in two sections in Finland and the U.S. The Truss section with the soft tank was fabricated in the U.S. and required transferring from land to water at the fabrication yard before it could be mated with the hard tank section. The challenges due to the size, schedule and government regulations resulted in selecting a unique controlled launch submergence method to offload the truss section into the water. The offloading operation involved inclining the H-627 launch barge to unusually high angles while ballast operators were onboard the vessel, moving the truss down the barge in a controlled fashion with winches rigged to skid shoes and finally submerging the barge to create separation. The truss section was loaded on the barge from the bow and launched over the tilt beam at the stern. Discontinuous removable skid shoes were used instead of traditional continuous permanent launch cradles.
The truss section travel over the barge, and speed control during launch was achieved by employing two sets of winches, Pull Winches and Hold Back Winches, located on the Barge Deck. Structural modifications had to be made to the barge, skid beams and soft tank structure to make the operation feasible. Additional barge marine systems had to be added for the unusual ballasting and floating configuration required for the operation. An active mooring system along with the winch line tensions was used to control the barge and the truss during the operation.
The Holstein truss spar hull (Ref. 1) due to its large diameter of 45.5 m (about 150 ft.) and 227.38 m (746 ft.) length, and weight 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 (referred to only as truss structure from here on) were fabricated on land as one-piece in Technip's GMF yard in South Texas. 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 in the water next to the fabrication yard (Ref. 2).
The truss structure had to be offloaded into the water next to the fabrication yard for mating with the hard tank. The truss structure with the skid shoes weighed approximately 11,600 short tons. Various options were investigated to offload the truss and soft tank. Some of the options investigated are as follows: