Incident Overview
A helicopter returning to shore from an oil platform experienced an engine failure during approach. The pilot had just made a "5 mile advisory call" announcing his intention to land when the engine chip light illuminated, followed by an immediate loss of engine power. The pilot initiated an autorotation, inflated the floats, and made a mayday call. During the descent, he reported having "full [aircraft] control to include tail rotor authority."
Landing and Rollover
During the landing, the tail rotor blades entered the water, resulting in a sudden stoppage of the tail rotor. This twisted the tail rotor drive shaft apart. Following the landing, the helicopter remained upright. The pilot exited the helicopter after the main rotor blades came to a stop due to "waves and wind." After he moved away from the helicopter, it rolled over. The pilot reported that the wind was from 045 degrees at 15 knots gusting to 25 knots.
Engine Examination
An examination of the turbine engine revealed that the N1 gas producer would not turn. Disassembly of the engine revealed that the compressor coupling adapter was fractured. Metallurgical examination indicated that the compressor coupling adapter had failed due to fatigue initiating from fretting at the outer surface where it contacted the impeller. The source of the fretting could not be determined.