NTSB Cites Methane Gas as Culprit in Oil Rig Accident
Pilot was unaware rig was venting before takeoff.

The National Transportation Safety Board (NTSB) believes methane gas expelled from an offshore oil rig in the Gulf of Mexico caused the partial power failure on a Bell 206-L3 helicopter departing the MP61A platform on March 24, 2011. The pilot experienced the power failure at about 4:55 p.m., shortly after liftoff. The NTSB attributed the power loss to an engine compressor stall after it ingested methane gas during takeoff.