Designing and laying the FLAGS offshore line

Brent field is the largest oil and gas reservoir in the British sector of the North Sea. Above the oil there is a significant gas cap with associate natural-gas liquids. The development of the field is a joint venture of Shell U.K. Ltd. and Esso Petroleum Co. Ltd., with Shell Expro acting as operator. The Brent gas and associated gas liquids are transported to the mainland at St. Fergus, Scotland, through a 450-km-long, 36-in.-diameter subsea pipeline, which is the offshore part of the FLAGS (Far North Liquid and Associated Gas System) Project. At St. Fergus the FLAGS gas-processing plant extracts the natural-gas liquids and delivers dry methane into the British Gas Corp. crosscountry transport pipeline. The liquids will be transported overland via a 222-km, 20-in.-diameter pipeline, to a NGL fractionation plant now under construction at Mossmoran. The liquids will be separated into ethane, propane, butane, and natural gasoline. Transport of these products will be through a tanker-loading facility near Mossmoran at Braefoot Bay on the Firth of Forth. The system is designed for an initial capacity to handle about 1,000 MMscfd of methane gas, plus about 100,000 b/d of associated liquids. This two-part article reviews the construction and commissioning ofmore » the offshore gas pipeline project, with emphasis on the planning and special engineering efforts which resulted in cost savings of about pounds138 million.« less