On May 13, 2005, about 1425 central daylight time, a single-engine Eurocopter France EC120B turbine powered helicopter, N588SC, registered to CFS Air LLC., of Danbury, Connecticut, and operated by ERA Helicopters LLC., of Lake Charles, Louisiana, remained undamaged during a forced autorotation landing into open ocean water near offshore platform South Timbelier ST 265, located in the Gulf of Mexico. The commercial pilot and passenger were not injured. Visual meteorological conditions prevailed and a company flight plan was filed for the 14 Code of Federal Regulations Part 135 on-demand air taxi flight. The flight originated from ST 265 and was destined for Port Fourchon (9LA) near Fourchon, Louisiana. According to the 2,117-hour pilot, while approximately 14 miles south of Port Fourchon and while in crise flight at 500 feet above the water, he observed the engine low oil pressure light illuminate and shortly thereafter the helicopter lost total engine power. The pilot elected to turn the aircraft into the wind, activate the emergency floats, and perform an autorotation to the water. After a successful autorotation, the pilot and passenger disembarked the helicopter without injury and were rescued. At the time of the incident the pilot reported the fuel gauges indicated approximately 24 gallons of fuel. The pilot further reported that the helicopter remained upright until the recovery, at which time the helicopter rolled inverted, but remained afloat. The helicopter was recovered, and on May 16, 2005 was examined by investigators from the Federal Aviation Administration (FAA), American Eurocopter, and Turbomeca USA. The investigators reported that the helicopter did not sustain damage during the incident but was submerged in salt water during the recovery. An examination of the engine on May 25, 2005, at the maintenance facilities of Turbomeca USA, in Grand Prairie, Texas, revealed the total loss of engine power was a result of an interruption of engine lubrication due to excessive swelling of the oil pump assembly's check valve piston o-ring, part number 79-24-05-01-240.