Workers are preparing to replace the "containment cap" that covers the mouth of the oil well, about 1,500 meters below the surface.
A loose-fitting attachment that has been funneling some of the leaking oil to the surface is to be replaced with a tight cap attached firmly to the wellhead.
If the effort is successful, the top U.S. official overseeing work on the oil spill says the leak can at last be declared "contained." Retired Coast Guard Admiral Thad Allen says containment would be only a temporary measure, however, until the well is permanently plugged in the future.
The work is delicate and time-consuming, and could last up to 10 days. During that time oil will gush out of the well at full volume, 60,000 barrels or more a day.
The new "containment cap" will have a pipeline allowing oil to flow up to a fourth large ship, the Helix Producer, on the surface. If it works, the new system will be able to capture up to 80,000 barrels of oil a day, more than three times the amount (25,000 barrels) currently being collected, and more than the amount (60,000 barrels a day) the government estimates is flowing into the Gulf.
The environmental disaster in the Gulf began April 20, when an explosion on an oil rig BP was operating killed 11 workers and caused the leak. The spill, by far the largest ever in U.S. waters, has fouled huge expanses of the U.S. Gulf coast, wreaked havoc on birds and sea creatures, and caused vast damage to fishing, tourism and other industries.
Admiral Allen said the spill cannot be considered under control until the well is plugged permanently. BP is drilling two shafts intended to intercept the leaking well far below the sea floor, at a point where engineers say cement can be pumped into the original well. Final stages of that effort are still weeks away.