A Tension Leg Platform poses unique challenges with respect to its dry transportability. The engineering that is required before such a transport can be effected is given step by step. The development of a new type of cribbing block is described. The predicted extreme design environmental conditions and resulting extreme motions are compared with the actual monitored weather conditions and motions. From the correlation, conclusions are drawn.
On May 26, 1989, the first TLWP (Tension Leg Wellhead Platform) was delivered in the U.S. Gulf by a self-propelled semisubmersible heavy-lift ship. This ship departed Singapore only 41 days earlier after loading the TLWP by means of the float-on method, see figure 1. The dry transport of this platform broke new ground in various areas. The unique nature of the TLWP required various unorthodox solutions. Conventional design methods were revised and new materials were developed.
The Green Canyon block 184 TLWP was built by a shipyard in Singapore, Republic of Singapore, which contracted with a HeavyLift Transporter for the dry transport of the TLWP to Pascagoula, Mississippi, where it was to be final outfitted before installation in the U.S. Gulf.
Dry transport by a self-propelled heavylift vessel offers the highest degree of safety in combination with a minimum of environmental loads on the cargo.
A wet tow was not a realistic option given the distance of over 12,000 miles and an anticipated tow speed of 2 - 3 knots. Transportation by tug/barge was considered, but the increased safety of the self-propelled option proved to be the most attractive.
The TLWP is constructed from stiffened cylindrical shells. The truss deck is carried by four columns of 12.2 meters in diameter, 46.2 meters in height, spaced 42.67 meters apart (center to center). Pontoons, 7 meters in diameter, connect the four columns near the base.
At the moment of load-out, the TLWP's displacement was 8,400 T with its center of gravity at 31.8 meters above its base. The draft measured 8.3 meters, leaving a freeboard on the pontoons of 1.2 meters. These pontoons were outfitted with bollards to facilitate the connection of tugboats and winch wires during on- and off-loading.
Given the relatively small footprints of the columns, in combination with the fact that these would mostly overhang the carrier's 40 meter wide deck, four sponsors were required to increase the support area. Much attention was paid to the footprint loads in order to obtain the optimum support configuration.