BP began a series of critical tests on a new cap placed over its leaking oil well in the Gulf of Mexico today, expressing hope that it may finally bring the disaster under control. But the company warned that the process was "not simple stuff."
Oil was still gushing from the well, as expected, as engineers prepared to gradually choke off the flow by closing three vents on a new device lowered on to the blown-out well on Monday.
Once the vents are closed, pressure readings will indicate whether the oil can be successfully shut in by the cap, pending a permanent "kill" operation that could be under way by the end of this month using a relief well that has been drilled close by.
If tests show that the flow is too powerful to hold back – or if the pressure falls below a level, suggesting a secondary leak – BP will reopen the well and allow the oil to gush again while the company brings a containment system online to divert it on to collection ships. It could be another 48 hours before the tests are complete.
Kent Wells, a senior vice-president at BP, said that although the installation of the cap had gone "incredibly well", it was "just one step in a multistep process" and held no guarantee of success.
"It's not simple stuff and what we don't want to do is speculate around it. We just need to see what the data tells us and make the right decisions going forward," he said.
A BP statement added: "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."
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Tens of thousands of people living in Louisiana, Mississippi, Alabama and the Florida have suffered financially from the spill, which began with an explosion on the Deepwater Horizon rig on 20 April that killed 11 workers.
Even if the oil can be temporarily shut in, or contained using surface ships, the only permanent end to the spill is through a relief well that is due to connect with the leaking well by the end of this month.
Once the two intersect, 2.5 miles below the seabed, heavy fluids will be poured via the relief well to "kill" the oil reservoir and seal it with cement. BP has said that this could take up to 14 days, but it aims to have the damaged well closed by 13 August.
Crude oil has been gushing out of the well, named the Macondo Prospect, at between 35,000 barrels (1.5m gallons) and 60,000 barrels (2.5 million gallons) a day, according to US government estimates, though doubts remain as to the accuracy of those figure.
If the tests show that the oil cannot be shut in safely, the new containment system can handle up to 80,000 barrels (3.4m gallons) a day, but will not reach that capacity until the end of this month. The federal incident commander, Admiral Thad Allen, said that the clean-up efforts would last well into the autumn.
US energy secretary, Steven Chu, joined scientists and technic experts overseeing the cap-testing operation from BP's headquarters in Houston, Texas