BP removed a containment cap from its Gulf of Mexico oil well in the first step toward installing a bigger cap to contain all the crude gushing into the sea and fouling the coast.
The removal released a torrent of oil that will spew unrestrained into the Gulf for four to seven days - the time BP says it will take to put in place a bigger cap and seal.
Officials say the new cap would capture all the oil leaking from the well and funnel it 1.6km upward to vessels on the water's surface.
The new solution, 82 days into the worst oil spill in US history, would not allow crude to billow out the bottom and the top, as the current cap does, said Kent Wells, senior vice president of exploration and production for BP.
Retired Coast Guard Admiral Thad Allen, who is overseeing the US response to the spill, had said the cap switch could be finished by late tomorrow or Monday.
BP's plan, which Adm Allen approved late Friday, showed a four- to seven-day process.
Mr Wells said the longer stretch allows for unexpected problems. He also said BP has another cap ready to install if the new, bolted-on cap and seal do not work.
BP was also hooking up and testing a third vessel in hopes that it could begin siphoning crude late tomorrow.
The two procedures are part of BP's effort to upgrade its oil-capture system with four vessels that can handle up to 80,000 barrels of oil a day and disconnect and move quickly if a hurricane approaches.
The operation is the latest attempt to contain the spill that was caused by the 20 April explosion on the BP-leased Deepwater Horizon oil rig, which killed 11 workers.
