Present Troll production is governed by 106 (of a planned 115) subsea wells, producing into 27 clusters and with more than 200 kilometers flexible flow lines at water depths of 320 to 350 meters. All the wells are horizontal and individually controlled. Of these wells, 29 are multilaterals with up to five branches. The total producing interval in the field adds up to 297 km. This paper highlights field experience with operation and maintenance of the Troll subsea production system since its start-up in 1995. Results have been good with high well availability and efficiency. Emphasis is given to intervention -maintenance philosophy - and production optimization of one of the world's largest subsea production systems. Background for the system-design and flow line layout will also be presented, describing the flexibility and redundancy of the control system. Examples of interventions performed will be given together with availability- and MTBF figures (1995 - 2004). Presently, Troll production is limited by the process gas-handling capacities and, for some clusters, by flow line capacities. This paper focuses on the challenges of optimizing production from a large number of interacting gas coning wells, exposing different degrees of rate dependent GORs. This includes a description of tools and methods for production optimization based on the marginal GOR-concept.
The Troll Oil field is located in the North Sea 80 km west of the Norwegian west coast. The Troll Oil subsea system is one of the world's largest subsea developments. Water depths on the field vary from 315 to 340 m. The development of the field was carried out in two phases (See Appendix 1 for a full map over the area).
The Troll West Oil Province (TWOP) and southern part of Troll West Gas Province (TWGP) are developed with 26 subsea satellite oil producers, out of which 20 are located on the TWOP and six in the TWGP. In addition, one vertical gas injector is drilled on the TWOP. The wells are divided into four groups in the TWOP, named the D to G clusters, and one group in TWGP, named H cluster. The main reason for choosing a cluster design with satellite wells connected to subsea manifold stations was that this design allowed for wide flexibility when the wells' spud point was decided. Figure 1. Troll B subsea layout TWOP (Available in full paper)
This subsea system has a flexibility to tie in up to 32 satellite wells. The wells in each group are connected with 200 m to 2,000 m flexible flow lines to a subsea manifold station. From each of these manifold stations, the fluid (oil/gas/water) is transported to the semi-submersible production unit, Troll B, by two parallel 9-10" flexible gathering lines and dynamic flexible risers per manifold station. The length of the gathering lines varies between 4,000 m to 10,000 m. Using two rather than one flow line provides the necessary flexibility to operate low and high performance wells and to perform testing of a well without any major reduction of total production from the well cluster.