Incident Overview
A helicopter experienced a loud noise from the engine or transmission area during takeoff from an oil rig helideck. The pilot observed a change in engine noise and maneuvered the aircraft away from the oil rig. The pilot inflated the pop-out floats and performed an autorotation to the water.
Impact and Recovery
Upon water impact, the aircraft rolled over. While being towed to the oil rig for recovery, the helicopter sank. The wreckage was not recovered.
Investigation
Because the wreckage was not recovered, investigators were unable to determine the nature of the engine malfunction or failure. The specific cause of the noise and subsequent engine behavior remains unknown.