A major rescue operation was launched after a helicopter ferrying Centrica personnel to gas platforms off Morecambe Bay came down in the sea.
Six bodies have been recovered from the sea at the crash site and the search is continuing for a seventh person.
Some 400 people are employed in the Morecambe field operations
The Irish Sea gas fields operated by Centrica are a substantial supplier in terms of the UK's remaining reserves.
The fields, located about 25 miles (40 km) to the west of Blackpool, are this year expected to supply about 6% of Britain's gas demand.
The South Morecambe field was found in 1974 and the North Morecambe field two years later, with commercial production starting in 1985.
Some 400 people work in operations, with about 143 offshore at any one time.
Most staff are based at the central production complex at the South Morecambe field which draws gas from five remote platforms.
Drilling in the North Morecambe fields is usually operated remotely from either South Morecambe or Centrica's Barrow-in-Furness control room.
Centrica acquired the neighbouring Bains field, eight miles north-east of the South Morecambe field, in 2001.
Offshore workers are transported to the central production complex by helicopter from Blackpool airport and usually work a two week on, two week off, shift pattern.
An accommodation platform can sleep 170 in two-bed cabins.
Centrica says employees regularly undergo safety training and practice emergency sessions.
Nine lifeboats are fitted to the central platform and there is at least one standby vessel located within five nautical miles of the field.
The water around the platforms, which are plumbing reserves of an estimated 180 billion cubic metres of gas, is about 30 metres deep.
The two Morecambe fields are operated by a wholly owned subsidiary of Centrica, Hydrocarbon Resources Ltd.
The Bains field was developed by Centrica in partnership with Gaz de France and First Oil.
The neighbouring Millom and Dalton fields are owned by Burlington Resources Ltd but operated by Centrica.