OPTI-EX Deck Design: A Novel Approach to Deck Mating

This paper describes deck design and mating operation of the semisubmersible Floating Production Unit (FPU) OPTI-EX. It also addresses engineering challenges, and how these were met by a novel design and proper analysis. OPTI-EX is the first production semi-submersible mated by a combination of guide plates and shoe bracket at 8 contact points. The challenges of mating include dimensional control of the mating, hull deflections control by guide plates, dimensional tolerance criteria, and welding connections. Because of the flexibility of OPTI-EX pontoon in the water, the horizontal deflection of four columns is significant, which makes the adjustment of column position important. The use of half “Y” shape guide plates installed on column top helps the deck bring the column back into desired location under deck self weight. The deck self weight is shed at 8 contact points. The dimensional tolerance criteria are critical, since fatigue is sensitive in the deck column connection area. By selecting the placement of guide plates, the tolerance can be achieved. The mating operation went very well in terms of time and final tolerance. Welding work after the landing of truss deck on column is easy to access and of simple design. All the work is done in open space rather than confined in a small space. A sensitivity study was done to check the contact forces during the mating process. The results show that there will be significant changes in contact forces assuming different initial gaps present between deck and column top guides during the mating. However, all these forces can be accommodated by a proper local reinforcement design. While mating the deck and column, a built-in stress due to the loads in mating is normally involved. The support conditions change between mating stage and in-place condition. This means modeling the in-place condition only will lead to an unrealistic result. In this paper, it is shown that a prestress approach has to be utilized to predict the final in-place strength condition with the consideration of built-in stress in deck at mating stage.© 2010 ASME