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With BP's know-how and U.S. authority, the Macondo well was plugged

BP, the government and an army of volunteers are fighting to contain and clean the millions of gallons of oil spewing from the site of the Deepwater Horizon explosion in the Gulf of Mexico.

The government urged BP to go to the more elaborate containment system. A government-supervised team of scientists said the well was producing 35,000 to 60,000 barrels a day, an extraordinary flow. In a testy exchange of letters in early June, the on-scene coordinator for the government, Rear Adm. James Watson, demanded that BP ramp up its containment.

BP had to be inventive, on the fly.

"They had to take two technologies that did not pre-exist in the Gulf of Mexico," Allen said.

A production tanker with satellite-guided positioning raced to the gulf from the North Sea. Engineers also borrowed technology used off the coast of Africa. That system used floating riser pipes, which allow ships to attach and detach via flexible lines.

"The North Sea meets Angola in the Gulf of Mexico," Allen said.

This expanded containment capacity included a new, 75-ton structure atop the well, known as the sealing cap, or 3-ram capping stack. The cap had been envisioned as early as April 24, just four days after the blowout, according to Wells.

The new cap changed everything. Although it was designed to siphon oil to the surface, the new system created another possibility: Engineers could simply shut in the well.

Chu said the BP engineers had assumed, after the "top kill" failed, that the well had a loss of "integrity" somewhere down below the wellhead, with breaches that let the mud from the operation surge into the rock formation instead of straight down the well.

"I said, 'No, I don't think so, there's another scenario,' " Chu said. The well, he said, might have integrity after all. That opened the possibility, he said, for the "integrity test." They could close the well and see what happened.

This led to the most anxious period of the response. The danger was that, by choking the flow at the top, the pressures could build to such high levels that the oil and gas could explode laterally, through the well casing. The hydrocarbons might flow into the surrounding formations and then work their way up, into the gulf.

"The worst-case scenario is you create a fissure that doesn't heal, and the entire reservoir empties," Chu said.

The entire reservoir contains upward of 50 million barrels of oil, according to BP.

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