UPDATE 1-US rig survivors recall gas cloud and deadly blast
* Ironically blasts came as BP officials celebrated safety
* Professor discloses transcripts of survivors' interviews
* Offshore rig leased by BP exploded and sank last month (Adds BP confirmation of executives on board rig)
HOUSTON, May 8 (Reuters) - A natural gas cloud enveloped a Gulf of Mexico offshore oil rig leased by BP Plc (BP.L) and exploded just as visiting BP officials were celebrating seven accident-free years in the rig's crew quarters, according to accounts by survivors of the accident.
Robert Bea, a University of California Berkeley engineering professor, said on Saturday he obtained transcripts of interviews from three rig workers familiar with the explosion, including two who were in the rig's crew quarters at the time.
The interviews were done as part of BP's internal investigation of the accident but had not been made public.
Swiss-based Transocean Ltd's RIGN.S (RIG.N) Deepwater Horizon rig, under contract with BP, exploded and caught fire on April 20 while it was putting the finishing touches on a well about a mile (1.6 km) beneath the ocean surface. It sank two days later.
The accident has triggered a huge oil spill posing an environmental and economic disaster to the U.S. Gulf Coast.
As rig workers and engineers on the deck of the rig were finishing a delicate operation to cement the metal well casing in place, something went horribly wrong.
According to the transcripts, a giant bubble of methane gas rushed up the drill pipe and enveloped the deck of the drilling platform in a cloud of highly flammable gas, followed by a scalding flood of crude oil that spilled onto the drill deck and ignited.
Nine workers on the drill deck were killed, along with two others working in the nearby mud storage room, Bea said.
"Gas and oil rushed up the riser. There was little wind and a gas cloud got all over the rig," according to a transcript from a rig worker recited by Bea in a telephone interview.
The drill deck itself is "intrinsically safe," meaning it is free of any sources that can cause a spark such as a spark plug in a generator, Bea said.
But the methane flood that rushed up the drill pipe was so immense it soon spread across the entire length of the massive drilling platform, Bea said.
When the gas reached a room where mud used in well-drilling was stored, sparks from engines running pumps there ignited the gas cloud, according to the transcript.
"When the main inductions of the engines got a whiff (of the gas) they ran away and exploded -- blew them right off the rig," the transcript said. "This set everything on fire."
In the living quarters next to the mud storage room, off-duty crew and visiting BP officials were celebrating seven years of accident-free operation of the rig, according to the transcript. The explosion blew the walls off the crew quarters and sent furniture and people flying.
The blast "took out all of the interior walls where everyone was hanging out having -- I am not making this up -- a party to celebrate seven years of accident-free work on this rig," the transcript said.
"The furniture and walls trapped some, broke some bones, but they all managed to get in the lifeboats with assistance from others," the transcript said.
A BP executive confirmed on Saturday that seven BP employees had been on the rig, from young engineers to a vice president for Gulf of Mexico drilling operations. All escaped safely, said Doug Suttles, BP's chief operating officer.
"One of the reasons the more senior members were there was to discuss the safety performance of this rig," Suttles told reporters at a briefing in Robert, Louisiana. "This rig had an outstanding record. I think it had gone over 2,500 days without a significant safety accident." (Editing by Will Dunham and Todd Eastham)
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