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May 8, 2010 10:31 AM by Letitia Walker

Methane Bubble Triggered Explosion

ON THE GULF OF MEXICO (AP) - 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.

While the cause of the explosion is still under investigation,

the sequence of events described in the interviews provides the

most detailed account of 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

described in detail 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.

A group of 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.

Based on the interviews, Bea believes that the workers set and

then tested a cement seal at the bottom of the well. Then they

reduced the pressure in the drill column and attempted to set a

second seal below the sea floor. A chemical reaction caused by the

setting cement created heat and a gas bubble which destroyed the

seal.

Deep beneath the seafloor, methane is in a slushy, crystalline

form. Deep sea oil drillers often encounter pockets of methane

crystals as they dig into the earth.

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 various safety barriers, Bea

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 sea water

in the drill column suddenly shooting back at them, rocketing 240

feet in the air, he said. Then, gas surfaced. Then 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."

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," the account said. Another explosion below blew more

equipment overboard.

BP spokesman John Curry would not comment Friday night on

whether methane gas or the series of events described in the

internal documents caused the accident.

"Clearly, what happened on the Deepwater Horizon was a tragic

accident," said Curry, who is based at an oil spill command center

in Robert, La. "We anticipate all the facts will come out in a

full investigation."

The BP executives were injured but survived, according to one

account. Nine rig 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 life boats with assistance from

others," said the transcript.

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

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

and anger."

On Friday, a BP-chartered vessel lowered a 100-ton

concrete-and-steel vault onto the ruptured well, an important step

in a delicate and unprecedented attempt to stop most of the gushing

crude fouling the sea.

"We are essentially taking a four-story building and lowering

it 5,000 feet and setting it on the head of a pin," BP spokesman

Bill Salvin told The Associated Press.

Underwater robots guided the 40-foot-tall box into place in a

slow-moving drama. Now that the contraption is on the seafloor,

workers will need at least 12 hours to let it settle and make sure

it's stable before the robots can hook up a pipe and hose that will

funnel the oil up to a tanker.

"It appears to be going exactly as we hoped," Salvin said on

Friday afternoon, shortly after the four-story device hit the

seafloor. "Still lots of challenges ahead, but this is very good

progress."

By Sunday, the box the size of a house could be capturing up to

85 percent of the oil.

The task became increasingly 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 - arrayed in vivid, brick-colored ribbons -

is drawing ever 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 onto a second,

smaller leak at the bottom of the Gulf.

At the same time, crews are drilling sideways into the well in

hopes of plugging it up with mud and concrete, and they are working

on other ways to cap it.

Investigators looking into the cause of the explosion have been

focusing on the so-called blowout preventer. Federal regulators

told The Associated Press Friday that they are going to examine

whether these last-resort cutoff valves on offshore oil wells are

reliable.

Blowouts are infrequent, because well holes are blocked by

piping and pumped-in materials like synthetic mud, cement and even

sea water. The pipes are plugged with cement, so fluid and gas

can't typically push up inside the pipes.

Instead, a typical blowout surges up a channel around the

piping. The narrow space between the well walls and the piping is

usually filled with cement, so there is no pathway for a blowout.

But if the cement or broken piping leaves enough space, a surge can

rise to the surface.

There, at the wellhead of exploratory wells, sits the massive

steel contraption known as a blowout preventer. It can snuff a

blowout by squeezing rubber seals tightly around the pipes with up

to 1 million pounds of force. If the seals fail, the blowout

preventer deploys a last line of defense: a set of rams that can

slice right through the pipes and cap the blowout.

Deepwater Horizon was also equipped with an automated backup

system called a Deadman. It should have activated the blowout

preventer even if workers could not.

Based on the interviews with rig workers, none of those

safeguards worked.

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