In a potentially pivotal moment in the Gulf crisis, BP planned to begin closing valves Tuesday in a slow and methodical process that could finally choke off the geyser of crude at the bottom of the sea after three gloomy months and up to 180 million gallons spilled.
A new, tighter-fitting cap was lowered over the blown-out well Monday night. The next phase was to shut the openings in the 75-ton metal stack of pipes and valves gradually, one at a time, while watching pressure gauges to see if the cap would hold or any new leaks erupted.
The operation was expected to begin late Tuesday, and officials said it could last anywhere from six to 48 hours.
BP and the government's point man on the crisis -- the biggest offshore oil spill in U.S. history and one of the nation's worst environmental disasters -- stressed there were no guarantees, and they urged patience from Gulf residents.
"They ought to be interested and concerned, but if they hold their breath, they'll run out of oxygen," retired Coast Guard Adm. Thad Allen told The Associated Press.
If the cap works, it will enable BP to stop the oil from gushing into the sea, either by holding all the oil inside the well machinery like a stopper or, if the pressure is too great, channeling some though pipes to as many as four collection ships.
Along the Gulf Coast, where the spill has heavily damaged the region's vital tourism and fishing industries, people anxiously awaited the outcome of the painstakingly slow work.
"I don't know what's taking them so long. I just hope they take care of it," said Lanette Eder, a vacationing school nutritionist from Hoschton, Ga., who was walking on the white sand at Pensacola Beach, Fla.
The cap is just a stopgap measure. The best hope for stopping the leak altogether rests with a relief well being drilled through the seafloor. BP hopes to use it by mid-August to plug up the broken well permanently with heavy drilling mud and cement. After that, the Gulf Coast faces a long cleanup.
In Washington, White House spokesman Robert Gibbs said the effort to put the containment cap into operation "represents the best news that we've had in the preceding 85 days."
"We are approaching what we hope is the next phase in the Gulf -- understanding that that next phase is likely to take many years," he added.
On Tuesday, engineers planned to shut off pipes that are already funneling some oil to two ships, to see how the cap handled the pressure of the crude coming up from the ground. Then they planned to close, one by one, three valves that let oil pass through the cap.
Experts said stopping the oil too quickly could blow the cap off or further damage the well.
Scientists will be looking for pressure readings of 8,000 to 9,000 pounds per square inch. Anything lower than 6,000 might indicate previously unidentified leaks in the well.
"What we can't tell is the current condition of the wellbore below the seafloor," Allen said. "That is the purpose of the well integrity test."
If the cap cannot handle the pressure, or leaks are discovered, BP will have to reopen the valves and let some of the oil out. In that case, BP is ready to collect the crude by piping it to as many as four vessels on the surface.
The leak began after the Deepwater Horizon offshore drilling platform exploded on April 20, killing 11 workers. As of Tuesday, the 84th day of the disaster, between 90.4 and 178.6 million gallons of oil had spewed into the Gulf.
BP underwater video: http://bit.ly/bwCXmR
Weber reported from Houston. Associated Press writer Colleen Long in New Orleans contributed to this report.