The operation could last anywhere from six to 48 hours. BP first targeted a midday Tuesday start but later said that was overly optimistic and pushed expectations back.
A series of methodical, preliminary steps were completed, including mapping the seafloor. Later Tuesday, National Incident Commander Thad Allen met with the federal energy secretary and the head of the U.S. geological survey and other scientists and geologists.
"As a result of these discussions, we decided that the process may benefit from additional analysis that will be performed tonight and tomorrow," Allen said.
The solution is only temporary, but it offers the best hope yet for cutting off the gush of billowing brown oil.
Kent Wells, a senior vice president at BP, said at a briefing that they cannot make any promises that the cap will work.
"We need to wait and see what the test actually tells us," Wells said. "It's not simple stuff. What we don't want to do is speculate around it."
The oil giant expects no oil will be released into the ocean during the tests, but remained cautious about the success of the system.
As a safety measure, 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.
"The sealing cap system never before has been deployed at these depths or under these conditions, and its efficiency and ability to contain the oil and gas cannot be assured," the company said in a statement.
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.
- The cap's installation was good news to weary Gulf Coast residents who have warily waited for BP to make good on its promise to clean up the mess. Still, they warned that even if the oil is stopped, the consequences are far from over.
- 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.
- Rebuffed twice by the courts, the President Barack Obama administration is taking another crack at a moratorium on deep-water drilling, stressing new evidence of safety concerns and no longer basing the moratorium on water depth. But those who challenge the latest ban question whether it complies with a judge's ruling tossing out the first one. The new order does not appear to deviate much from the original moratorium. It still targets deep-water drilling operators but defines them in a different way.
- The first public hearing by a presidential oil spill panel Monday zeroed in on the relationship between BP and the company it hired to drill the now exploded rig. In an effort to fight a new drilling moratorium, a rival drilling executive and a Louisiana congressman said other oil operators shouldn't be tarred because of one bad apple: BP's Deepwater Horizon rig. Larry Dickerson, president of a rival drilling company, told commissioners the April 20 explosion and resulting oil spill were "the result of reckless operating mistakes." He said errors were likely made in monitoring drilling mud, in decisions on when to use the blowout preventer and about whether BP PLC or its contractor, Transocean Ltd., was in charge of safety.
The Associated Press contributed to this report.