Using a Temporary Semi-Submersible MODU, the Ocean Voyager, on a Pre-laid Floating Production Facility Mooring

A large, independent Operator drilled three discovery wells and installed a 24 well template at Garden Banks 388. Two 12 inch pipelines were laid to the location. The Operator also designed a Floating Production Facility (FPF) which consisted of an Ocean Victory class semi-submersible moored to a permanent, 12 leg, catenary mooring system. The FPF conversion, due for completion in January, 1995, fell behind schedule. The Operator desired to stay as close to schedule as possible by temporarily substituting a semi-submersible drilling unit to complete the discovery wells and tieing them back to the 24 well template. Each pre-laid mooring leg consisted of a 25 metric ton anchor, a spiral-strand ground wire, a dip-zone chain, a spiral-strand catenary wire, and a submersible buoy assembly. First, laying additional mooring legs between the twelve pre-laid legs was considered. This was ruled out because of the danger of tangling mooring legs and damaging the relatively fragile spiral-strand construction wire ropes in the pre-laid mooring legs. Second, mooring a semi-submersible MODU to the existing pre-laid mooring legs was evaluated and selected. The MODU Ocean Voyager, similar to the FPF prior to conversion, was used. One advantage of using the pre-laid moorings and a similar unit was that the hook-up and disconnect procedures could be tested prior to the arrival of the FPF. These procedures were refined considerably. The initial hook-up was completed in December, 1994. Two similar 6140 BHP anchor-handling, towing, supply (AHTS) vessels were rigged with an additional two drum winch and a shallow-depth, diving spread to connect the legs. Four of the pre-laid mooring legs were left idle. Despite some difficulties, the hook-up was completed in nine and one-half days which included four days of weather delay. Disconnecting the moorings was completed in May, 1995. A single AHTS vessel was rigged with an additional two drum winch and a large A-frame over the stern. Because of the revised procedures, considerably less diving was required. Using only one AHTS vessel further reduced costs and required only seven days, including two days of weather delay, to disconnect the moorings. Using the temporary semi-submersible MODU allowed the Operator to complete and test two wells and connect the wells to the template with jumpers. Thus, although the FPF arrived five months late, getting production on stream should be only two months behind schedule.