On June 1, 1999, at 1448 central daylight time, a Bell 206B helicopter, N7131U, was substantially damaged during a forced landing, following a loss of engine power near Venice, Louisiana. The helicopter was owned and operated by Chevron USA, Inc., of New Orleans, Louisiana. The commercial pilot and the two passengers were not injured. Visual meteorological conditions prevailed, and a company VFR flight plan was filed for the 14 Code of Federal Regulations Part 91 aerial observation flight. The local flight originated from the West Delta 30 platform located in the Gulf of Mexico, at 1426. The pilot reported that he flew the helicopter at an altitude of about 150 feet agl and an airspeed of 60 knots, while inspecting a pipeline, which was reported to have a possible oil leak. After completing the inspection, as the helicopter was accelerating (to return to West Delta 30), he heard a loud "bang," and the low rotor rpm and engine out audio warnings activated. The pilot further reported that he "entered autorotation, flared the aircraft, and placed it in six to eight inches of water. It was a grassy area, so I didn't pop the floats." The passengers reported hearing a loud "bang," and the aircraft began to "shudder/shake." The passenger in the rear seat reported that after the bang, he heard a "metallic grinding screeching sound." The helicopter "lurched," yawed to the left, and "pitched forward." The pilot maneuvered the helicopter to the "edge of open water and onto a grassy mat in the marsh." The pilot "pulled pitch right before the landing and we landed with one medium bump and then followed very quickly with another medium bump with a loud sound with it." The rear seat passenger further reported that the landing attitude was "relatively normal;" however, "it was possibly a little more pitched up than normal." The rear seat passenger also reported that the landing was "reasonably soft." Examination of the helicopter by the FAA inspector revealed that the tailboom was partially separated aft of the horizontal stabilizer, and the tail rotor assembly was damaged. Both main rotor blades were damaged. A piece of blade, aft of the main spar and outboard of the trim tab, was separated from one main rotor blade. The outboard section of this blade was bent upward. A teardown examination of the Allison 250-C20J engine, serial number CAE270491, was conducted at the operator's maintenance facility in New Orleans, Louisiana, under the supervision of the FAA. A continuity check of the compressor through the accessory gearbox revealed a disconnect. Removal of the compressor section revealed that the Spur Adapter Gearshaft (SAG) was fractured at the forward splines. The accessory gearbox was torn down revealing "major" damage to the #2-1/2 roller bearing. The lower magnetic plug had a "large" amount of flaking type debris adhering to it, and the upper gearbox magnetic plug had a "small" amount of sliver type debris adhering to it. According to the engine manufacturer's metallurgist, "severe spline wear led to [the] spiral fatigue failure of the Spur Adapter Gearshaft and Compressor Adapter Coupling." The damage to the #2-1/2 bearing "appeared to be secondary" to the failure of the SAG. The metallurgist reported that "no metallurgical cause was determined for the spline wear." The compressor had accumulated a total time of 11,898.2 hours, with 2,694.9 hours since overhaul. See the excerpts of the enclosed manufacturer report for details of the examination.