Dalia Subsea Production System

Abstract

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18541, "Dalia Subsea Production System, Presentation and Challenges," by J.L. Lafitte, M. Perrot, J. Lesgent, J. Bouville, and S. Le Pennec, Total E&P Angola; P. Dahl, Hydro; and S. Lindseth, AKS, prepared for the 2007 Offshore Technology Conference, Houston, 30 April-3 May. In 2002, the Dalia subsea production system was the largest subsea order placed with a single contractor in off-shore history. The production system had 71 wells, 71 drill-through horizontal trees, nine six-slot production manifolds, two workover systems, flowline-connection systems, and a complex control and chemical-injection system. Heat conservation and hydrate prevention were paramount concerns during the subsea-production-system detailed-design and construction phases. Subsea Production System Fig. 1 shows the general field layout with the 71 subsea wells and nine production manifolds and the flowlines, risers, and control umbilicals that cover a subsea footprint of approximately 100 km2. Production-Well-Cluster Layout. Various layouts were examined during detailed design to find the optimum solution. A typical production-well cluster comprises the manifold with three wells on one side and three wells on the other, all connected by rigid jumpers between the Christmas tree and the manifold. When the manifold is in the center of a flow loop, rigid spools connect production flowlines at both ends of the manifold. If the manifold is at the end of a flow loop, a pigging loop is mounted on the open side of the manifold, permitting roundtrip pigging. The control umbilical essentially is daisy chained from one manifold to the next. The rigid jumpers are not in contact with the seabed but float approximately 2.5 m above it because of attached buoyancy aids. Advances in directional horizontal drilling and the use of simplified completions have enabled the wells to be grouped in six-well clusters, which not only facilitates batch drilling, where necessary, but also reduces the number of subsea flowline and umbilical connections. Water and Gas Injection. Similar to Girassol, the same single-line header design is used to bring water and gas to the relevant wells. Control umbilicals are laid adjacent to the flowlines with subsea distribution units (SDUs) connecting the trees by electrical and hydraulic flying leads deployed by a remotely operated vehicle (ROV). Wellhead, Christmas Tree, and Tubing Hanger. Horizontal-Christmas-tree technology has been selected that is compatible with a 7-in. full-bore completion and permits an efficient sequence of operations, giving the possibility of batch drilling with a minimum number of rig moves. Additionally, the trees have the drill-through capability that for a light-architecture-type well, lets the tree be run before the blowout preventer (BOP). With the tree and BOP in place, the well then can to be drilled, cased, and completed.