Neptune Project: Spar Hull, Mooring and Topsides Installation

Abstract This paper describes the installation of the Neptune Spar at Viosca Knoll Block 826, approximately 90 miles south of Mobile, Alabama in the Gulf of Mexico. Water depth at the project site is approximately 1,930 feet The installation of the Spar was accomplished in two distinct phases. The first phase of the operations were completed about 3 months ahead of the arrival of the Spar hull at site. Inthis phase, the six anchor piles were driven and the six mooring lines were laid on the sea bed. In the second phase of installation operations, hull towout to the site, ballasting and upending of the Spar hull on location, the connection of the hull to the six mooring lines and setting the production facilities were accomplished. The Spar hull was towed to site horizontally and upended at site by selectively ballasting hull compartments. The mooring lines were then attached to the hull. The mooring lines are a combination of spiral strand wire and chain with a sea beddriven pile at the far end. The placement of the buoyancy cans inside the Spar moonpool area (center well bay) which support the production risers, then completed the installation effort. Introduction The Neptune Spar hun construction was started in February 1995, the Phase I of the installation work was completed inJuly 1996, and the Phase II of the installation work was completed in September 1996. The facility was turned over to the client in November 1996 and the first oil was produced in February 1997. roughly two years after the construction and installation contract signing. Reference 1 provides further details of the management of me contracting, construction and installation strategy. The hull was constructed by Aker Rauma Offshore (Reference 2) in Finland in two sections and transported to a shipyard in the Gulf of Mexico in two sections. J. Ray McDermott was responsible for fabricating the production facilities, towing thehull to site. upending it, installing the mooring lines, buoyancy cans, export risers and the production facilities. Both in Phase I and Phase II installation operations, J. Ray McDermott's dynamically positioned derrick. barge DB-50 was used extensively. Being the first of its kind, a variety of options for each phase of the installation have been considered. The result of some ofthe work completed to date will be valuable to other Spar type floating production system designs. Phase I Operations Phase I operations consisted of the placement of the anchor piles. the bottom chain and the spiral wire on the sea bed. Two flexible export risers were also installed following the mooring wire installation operation, however, the production riser installation operations are nm addressed here. Piled Anchors The initial design of the Neptune Spar mooring system was based on the principle of holding the Spar in place by using high capacity gravity anchors. A drag embedment type of anchor, most frequently found in semisubmersible drilling rigs, would be utilized.