BP unleashes more oil in renewed bid to contain spill

Officials said there was no guarantee for such a delicate operation almost a mile below the water's surface and the permanent fix of plugging the well from the bottom remained set for mid-August.

Robotic submarines removed the cap that had been placed on top of the leak in early June to collect the oil and send it to surface ships for collection or burning. BP aims to have the new, tighter cap in place as early as today and said so far the work was going according to plan.

If tests show it can withstand the pressure of the oil and is working, the Gulf of Mexico region could get its most significant piece of good news since the April 20 explosion on the BP-leased Deepwater Horizon rig, which killed 11 workers.

"Over the next four to seven days, depending on how things go, we should get that sealing cap on. That's our plan," said Kent Wells, a BP senior vice president, of the round-the-clock operation.

But it would be only a temporary solution to the catastrophe that the US government estimates has poured between 87 million and 172 million gallons of oil into the Gulf .

Hope for permanently plugging the leak lies with two relief wells, the first of which should be finished by mid-August.

With the cap removed yesterday, oil flowed freely into the water, collected only by the Q4000 surface vessel, with a capacity of about 378,000 gallons. That vessel should be joined today by the Helix Producer, which has more than double the Q4000's capacity.

But the lag could be long enough for as much as five million gallons to gush into already fouled waters. Officials said a fleet of large skimmers was scraping oil from the surface above the well site.

The process has two major phases: removing equipment currently on top of the leak and installing new gear designed to fully contain the flow of oil.

Last night BP began trying to remove the bolted top flange that only partially completed the seal with the old cap.

Video images showed robotic arms working to unscrew its bolts. Mr Wells said that could last into tomorrow, depending on whether the flange can be pulled off from above, as BP hopes.

If not, a specially designed tool will be used to pry apart the top and bottom flanges.

Once the top flange is removed, BP has to bind together two sections of drill pipe that are in the gushing well head. Then a 12ft-long piece of equipment called a flange transition spool will be lowered and bolted over it.

The second piece of pipe inside the well head came as something of a surprise, and raises the possibility that one of the sections of pipe became jammed in the Deepwater Horizon's blowout preventer, through which the well pipes run. The failure of the blowout preventer, a massive piece of equipment designed to stop the unchecked flow of oil, is partly to blame for the size of the spill.