Million gallons of oil a day gush into Gulf of Mexico

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Interviews with surviving Deepwater Horizon rig workers show how explosions led to what may be the world's worst oil spill

An extraordinary account of how the Deepwater Horizon disaster occurred emerged yesterday in leaked interviews with surviving workers from the rig. They said that a methane gas bubble had formed, rocketed to the surface and caused a series of fires and explosions which destroyed the rig and began the gushing of millions of gallons of oil into the Gulf of Mexico, threatening wildlife and coastal livelihoods. Oil-covered birds caught by the outer edges of the 135-mile slick are now being found.

Word also came yesterday that the oil spill may be five times worse than previously thought. Ian MacDonald, a biological oceanographer at Florida State University, said he believed, after studying Nasa data, that about one million gallons a day were leeching into the sea, and that the volume discharged may have already exceeded the 11 million gallons of the 1989 Exxon Valdez disaster, widely regarded as the world's worst marine pollution incident. Mr MacDonald said there was, as of Friday, possibly as much as 6,178 square miles of oil-covered water in the Gulf.

Meanwhile, at the site of the ill-fated well, a mile beneath the surface, a massive metal chamber had been positioned over the rupture so it could contain and then capture the bulk of the leaking oil. The operation, which uses undersea robots, and has never before been attempted at this depth and pressure. But last night, the formation of ice crystals meant the dome had to be moved away from the leak.

The interviews with rig workers, described to the Associated Press by Robert Bea, a University of California Berkeley engineering professor, recall the chain reaction of events that led to the disaster. They said that on 20 April a group of BP executives were on board the Deepwater Horizon rig celebrating the project's safety record. Far below, the rig was being converted from an exploration well to a production well.

The workers set and then tested a cement seal at the bottom of the well, reduced the pressure in the drill column and attempted to set a second seal below the sea floor. But a chemical reaction caused by the setting cement created heat and a gas bubble which destroyed the seal.

As the bubble rose up the drill column from the high-pressure environs of the deep to the less pressurised shallows, it intensified and grew, breaking through various safety barriers. "A small bubble becomes a really big bubble," Professor 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 sea water in the drill column suddenly shooting back at them, rocketing 240ft in the air. Then, gas surfaced, followed by oil. "What we had learned when I worked as a drill rig labourer was swoosh, boom, run," he 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 Professor Bea, who worked for Shell Oil in the 1960s during the last big northern Gulf of Mexico oil well blow-out. "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."

According to one interview transcript, a gas cloud covered the rig, causing giant engines on the drill floor to run too fast and explode. The engines blew off the rig and set "everything on fire". Another explosion below blew more equipment overboard. The BP executives were injured but nine crew on the rig floor and two engineers died. "The furniture and walls trapped some and broke some bones, but they managed to get in the lifeboats with assistance from others," said the transcript. The workers' accounts are likely to be presented in some form to the hearings held by the US Coastguard and Minerals Management Service, which begin next week.

By then, the success of the dome-lowering, if it is resumed, will be known. On Friday, a BP-chartered vessel lowered a 100-ton concrete and steel vault on to the ruptured well in an attempt to stop most of the gushing crude from fouling the sea. "We are essentially taking a four-storey building and lowering it 5,000ft and setting it on the head of a pin," said BP spokesman Bill Salvin. With the contraption on the seafloor, workers needed at least 12 hours to let it settle and stabilise before the robots could hook up a pipe and hose that will funnel the oil up to a tanker. By today, the box the size of a house could be capturing up to 85 per cent of the oil.

The task became urgent as toxic oil crept deeper into the bays and marshes of the Mississippi Delta. A sheen of oil began arriving on land last week, and crews have been laying booms, spraying chemical dispersants and setting fire to the slick to try to keep it from coming ashore. But now the thicker, stickier goo is drawing closer to Louisiana's coastal communities.

There are still untold risks and unknowns with the containment box. The approach has never been tried at such depths, where the water pressure is enough to crush a submarine, and any wrong move could damage the leaking pipe and make the problem worse. The seafloor is pitch black and the water murky, though lights on the robots illuminate the area where they are working. If the box works, another one will be dropped on to a second, smaller leak at the bottom of the Gulf. At the same time, crews are drilling sideways into the well in the hope of plugging it up with mud and concrete, and they are working on other ways to cap it.

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