The cap would be just a temporary solution, but it offers the best hope yet for cutting off the crude that has fouled the Gulf since the Deepwater Horizon rig leased by BP exploded April 20, killing 11 workers.
Engineers will slowly shut down three valves that let oil flow through the 75-ton capping device to see if it can withstand the pressure of the erupting crude and to watch if leaks spring up elsewhere in the well. National Incident Commander Thad Allen said the process of closing the valves, one by one, would start later Tuesday.
If pressure inside the cap stays in a target range for roughly six hours after the valves are closed, there will be more confidence the cap can contain the oil, Allen told a news briefing at BP's U.S. headquarters in Houston. That target range is 8,000 to 9,000 pounds per square inch, he said. Anything lower could indicate another leak in the well.
Allen and BP officials repeatedly cautioned there are no guarantees about the delicate work a mile below the sea. Allen urged Gulf Coast residents watching the possible fix evolve to be patient.
"They ought to be interested and concerned but if they hold their breath, they'll run out of oxygen. I won't be," Allen told The Associated Press after the briefing.
The tests could last anywhere from six to 48 hours, Allen and BP said.
Kent Wells, a senior vice president at the oil giant, declined to talk about BP's next steps until the test results are in hand.
"It's not simple stuff. What we don't want to do is speculate around it," Wells said in a BP news briefing.
The cap's installation after three days of undersea preparations was good news to weary residents of the coast from Texas to Florida, who have waited for BP to make good on its promise to clean up the mess. Still, even if the oil is stopped, the consequences are far from over.
"I ain't excited about it until it's closed off completely," said James Pelas, 41, a shrimper working on his boat at a marina in Venice, La. "Oil's scattered all over the place."
The cap will be tested by closing off three separate valves that fit together snugly, choking off the oil from entering the Gulf. BP expects no oil will be released into the ocean during the tests, but remained cautious about the success of the system.
Pipes can be hooked to the cap to funnel oil to collection ships if BP decides the cap can't take the pressure of the gusher, or if low pressure readings indicate oil is leaking from elsewhere in the well.
Even if the cap works, the blown-out well must still be plugged. A permanent fix will have to wait until one of two relief wells being drilled reaches the broken well, which will then be plugged up with drilling mud and cement. That may not happen until mid-August.
Even if the flow of oil is choked off while BP works on a permanent fix, the spill has already damaged everything from beach tourism to the fishing industry.
Tony Wood, director of the National Spill Control School at Texas A&M-Corpus Christi said the sloppiest of the oil -- mousse-like brown stuff that has not yet broken down -- will keep washing ashore for several months, with the volume slowly decreasing over time.
He added that hardened tar balls could keep hitting beaches and marshes each time a major storm rolls through for a year or more. Those tar balls are likely trapped for now in the surf zone, gathering behind sand bars just like sea shells.
"It will still be getting on people's feet on the beaches probably a year or two from now," Wood said.
But on Monday, the region absorbed a rare piece of good news in the placement of the 150,000-pound cap on top of the gushing leak responsible for so much misery.
Around 6:30 p.m. CDT, live video streams trained on the wellhead showed the cap being slowly lowered into place. BP officials said the device was attached around 7 p.m.
Residents skeptical if BP can deliver on its promise to control the spill greeted the news cautiously.
"There's no telling what those crazy suckers are going to do now," Ronnie Kenniar said when he heard the cap was placed on the well. The 49-year-old fishermen is now working in the Vessel of Opportunity program, a BP-run operation employing boat owners to lay boom, ferry coast guard officers and deliver supplies.
As of Tuesday, the 84th day of the disaster, between 90.4 and 178.6 million gallons of oil have spewed into the Gulf of Mexico
BP underwater video: http://bit.ly/bwCXmR
Weber reported from Houston. Associated Press writers Frederic J. Frommer in Washington, Matt Brown and Tom Breen in New Orleans and Holbrook Mohr in Belle Chasse, La., contributed to this report.