The BP oil leak could be completely contained as early as Monday if a new, tighter cap can be fitted over the blown-out well, the US government official in charge of the crisis said.
In some of the most encouraging news to come out of the Gulf in the two and a half months since the disaster struck, crews using remote-controlled submarines planned to swap out the cap over the weekend, taking advantage of a window of good weather following weeks of delays caused by choppy seas.
"I use the word 'contained'," said retired US Coast Guard Admiral Thad Allen. "'Stop' is when we put the plug in down below."
Two relief wells are still being drilled deep below the seabed to intercept the ruptured well and seal it up permanently with mud and cement, a job that may not be completed until mid-August.
The cap now in use was installed June 4 to capture oil gushing from the bottom of the sea, but because it had to be fitted over a jagged cut in the well pipe, it allows some crude to escape into the Gulf. The new cap - dubbed "Top Hat Number 10" - is designed to fit more snugly and help BP catch all the oil.
During the installation, the gusher will get worse before it gets better. Once the old cap is removed, oil will pour into the Gulf unhindered for about 48 hours while the new one is put in place, Allen said.
BP also worked yesterday to hook up another containment ship called the Helix Producer to a different part of the leaking well. The ship, which will be capable of sucking up more than one million gallons a day when it is fully operating, should be working by tomorrow, Admiral Allen said.
The US government estimates 1.5 million to 2.5 million gallons of oil a day are spewing from the well and the existing cap is collecting about one million gallons of that.
With the new cap and the new containment vessel, the system will be capable of capturing 2.5 million to 3.4 million gallons - essentially all the leaking oil, officials said.
The plan had originally been to hook up the Helix Producer and install the new cap separately, but the favourable weather convinced officials the time was right for both operations.
"Everybody agrees we got the weather to do what we need," Admiral Allen said. He said the calm weather was expected to last seven to 10 days.
In a response early today to Admiral Allen's request for detailed plans about the new cap, the Helix Producer and the relief wells, BP managing director Bob Dudley confirmed that the leak could be contained by Monday.
But he included plans for another scenario, which includes possible problems and mis-steps for the installation of the cap that would push the work back until Thursday.
The past 80 days have seen the failure of one technique after another to stop the leak. The latest approach is not a sure thing either, warned Louisiana State University environmental sciences professor Ed Overton.
"Everything done at that site is very much harder than anyone expects," he said.
Prof Overton said putting on the new cap carried risks: "Is replacing the cap going to do more damage than leaving it in place, or are you going to cause problems that you can't take care of?" he said.
Containing the leak will not end the crisis that began when the Deepwater Horizon drilling platform exploded on April 20, killing 11 workers. The relief wells are still being drilled, and a monumental cleanup and restoration project lies ahead.
Some people in Louisiana's oil-soaked Plaquemines Parish were sceptical that BP could contain the oil so soon.
"Too many lies from the beginning. I don't believe them any more," oyster fisherman Goyo Zupanovich said in Empire, Louisiana.