This paper describes the main features of the Lille-Frigg subsea development, with particular emphasis on the subsea control system Operated by Elf Petroleum Norge A/S, Lille-Frigg is a subsea satellite extension of the man Frigg Field The reservoir contains gas and condensate at a high temperature and pressure, which requires the use of special high pressure downhole equipment and remote control components
The three high pressure wellheads are located in 115 m water depth and feature a wellhead shut-in pressure of 530 bar and a wellhead flowing temperature of 110°C The wells are tied back to the Frigg TCP2 platform about 20 krn away
These demanding operating conditions resulted in the development of a dedicated subsea control system, implementing some advanced technologies such as very high pressure hydraulic components (760 barg) and specific ROV tooling
The subsea system is designed for initial installation using a combination of diver and diverless techniques Diverless intervention is feasible in connection with change out of sub-systems such as Xmas trees, choke inserts and well control pods
Extensive qualification programmes were initiated prior to the development phase in order to verify and also improve the available technology to meet with the stringent requirements of the Lille-Fngg needs.
Fig. 1 LILLE FRIGG SUBSEA (Available in full paper)
The Lille-Frigg field was discovered by the exploration well 25/2–4 drilled in 1975 in the northern part and confirmed by the appraisal well 25/2–12 & 12A drilled in 1988 in the central part The field IS located on the Norwegian shelf about 20 km north-east of the Frigg Central complex
The reservoir pressure and temperature are 670 bar and 125°C at the gas/water contact level (3678m/MSL)
Recoverable reserves are estimated to be in the order of 8.5 x 109 std m3 of wet gas and 3.5 x 106 m3 of condensate
The concept selection studies for Lille-Frigg included three basic principles for development
a wellhead platform concept
a hybrid concept (similar to North-East Frigg)
a subsea concept
The selected development scheme (see Fig 1) is based on 3 vertical subsea wells producing through one 10″ flowline to Frigg treatment and export facilities located on the TCP2 platform Dry gas is exported through the Frigg to St Fergus pipeline, while stabilized condensate is exported through the new Frostpipe line (Frigg to Oseberg pipeline) Monitoring and control of the wells is performed from the Frigg Field through an electro-hydraulic remote control system
A central subsea main fold is located in the same structure as one of the wellheads "A" The two other wellheads "B" & "C" are located in satellite protective structures and tied in to the central manifold by flexible production lines, service/inhibitor injection and remote control lines
Provisions have been made in the central manifold and also in the electrical and hydraulic distribution networks for extension of the subsea system \nth one or two additional wells if later required by production constraints
