Spar Deck Float-Over Feasibility Study for West Africa Environment Condition

Offshore deck installation may be performed by either derrick barges or by the float-over method. Either method involves a variety of technical challenges. The challenges are particularly severe in weather conditions such as West Africa where long period swell dominates the environment. Setting a deck on a floating platform such as a spar presents additional issues compared with operation on fixed platform. This paper provides a spar deck float-over feasibility study for a West Africa environmental condition. The design uses two existing barges in a catamaran configuration. The barges were engaged for both transportation and the float-over procedures. The weather window for spar deck installation and the importance of synchronized motion of spar, barge and deck during the deck load transfer are discussed in the paper. Four-body (two barge, spar and deck) motion simulations were applied. The study shows that the float-over and mating operation is feasible in the specified West African sea state. Rapid spar de-ballasting achieves 20% load transfer to avoid large impact during 0% to 20% load transfer. From the 20% to 85% deck load transfer using conventional ballasting the barges, deck and de-ballasting spar will have synchronized motion.