BP Probe: Blowout Triggered by Methane Gas

The deadly blowout of an oil rig in the Gulf of Mexico was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before exploding, according to interviews with rig workers conducted during BP's internal investigation.

The timeline described in the documents provides the most detailed account of what may have caused the April 20 blast that killed 11 workers and touched off the underwater gusher that has poured more than 3 million gallons of crude into the Gulf.

Portions of the interviews, two written and one taped, were read to an Associated Press reporter by Robert Bea, a University of California Berkeley engineering professor who serves on a National Academy of Engineering panel on oil pipeline safety and worked for BP PLC as a risk assessment consultant during the 1990s. He received them from industry friends seeking his expert opinion.

Complete Coverage: Disaster in the Gulf

Seven BP executives were on board the Deepwater Horizon rig celebrating the project's safety record, according to the transcripts. Meanwhile, far below, the rig was being converted from an exploration well to a production well.

As the workers removed pressure from the drilling column and introduced heat to set the cement seal around the wellhead, the chemical reaction created a gas bubble and the cement around the pipe destabilized, according to the interviews.

Deep beneath the seafloor, methane gas is in a slushy, crystalline form. But as the bubble rose up the drill column from the high-pressure environs of the deep to the less pressurized shallows, it intensified and grew, breaking through the rig's various safety barriers, the interviews said.

"A small bubble becomes a really big bubble," Bea said. "So the expanding bubble becomes like a cannon shooting the gas into your face."

Up on the rig, the first thing workers noticed was the seawater in the drill column suddenly shooting back out at them, rocketing 240 feet in the air. Then, the gas surfaced. Then the oil.

"What we had learned when I worked as a drill rig laborer was swoosh, boom, run," Bea said. "The swoosh is the gas, boom is the explosion and run is what you better be doing."

The gas flooded into an adjoining room with exposed ignition sources, he said.

"That's where the first explosion happened," said Bea, who worked for Shell Oil in the 1960s during the last big northern Gulf of Mexico oil well blowout. "The mud room was next to the quarters where the party was. Then there was a series of explosions that subsequently ignited the oil that was coming from below."

The executives were injured but survived. Nine rig crew on the rig floor and two engineers died, according to one account.

"The furniture and walls trapped some and broke some bones but they managed to get in the life boats with assistance from others," said the transcript.

The reports made the 73-year-old industry veteran cry.

"It sure as hell is painful," he said. "Tears of frustration and anger."

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