On November 5, 2004, approximately 1330 central standard time, a Bell 206B single-engine turbine powered helicopter, N496RL, registered to and operated by Rotorcraft Leasing Co. LLC., of Lafayette, Louisiana, sustained substantial damage during a forced autorotation landing into open ocean water near offshore platform South Timbelier (ST) 187, located in the Gulf of Mexico. The commercial pilot sustained serious injuries, one of his two passenger's sustained minor injuries, and one passenger was 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 187 and was destined for ST 161. The operator reported that the helicopter departed from the platform and climbed to an altitude of 500 feet above ground level (agl). As the 10,961-hour pilot switched radio frequencies to make a courtesy call to the destination platform, he heard a "loud bang," and then the engine lost power. The pilot initiated an autorotation and deployed the emergency skid-mounted float system. Approximately 50-60 feet above the rough ocean water, the pilot "started to flare and selected a wave to land on." The helicopter landed hard on the water, and remained upright for approximately 20 minutes before it rolled over inverted and partially submerged. The helicopter remained floating inverted near the surface. The pilot and two passengers evacuated the helicopter immediately after touchdown without deploying the emergency on-board life raft. Approximately 30 minutes after the accident, another helicopter arrived and dropped an emergency life raft into the water for the pilot and crew until further assistance could arrive. A brief examination of the helicopter by the pilot after evacuation revealed that the tailboom was separated from the fuselage. Recovery efforts were initiated on November 6, 2004. During the recovery process, the skids of the helicopter separated from the fuselage and the helicopter sank. Ocean depths were approximately 180 feet in the area of the accident and recovery efforts ceased. The helicopter was not recovered. The operator also reported that the helicopter underwent it's most recent Approved Airworthiness Inspection Program (AAIP) inspection on October 7, 2004, at an airframe total time of 8,615.7 hours and an engine time of 1351.8 hours since major overhaul. At the time of the accident, the helicopter had accumulated 114 hours since its last inspection. The reason for the loss of engine power was undetermined.