BP tries to thread tube into leaking pipe to stop flow
Forecasters predict the slick will begin washing ashore in more states soon
BP is using undersea robots in a bid to jam a tube into a broken pipe that is gushing oil into the Gulf of Mexico.
The attempt to divert the oil flow came as workers readied a so-called "top hat" containment dome to be lowered over the leak.
Some scientists have significantly raised their estimate of the rate of oil flow, as the surface slick has grown to 3,650 sq miles (9,500 sq km).
Several US senators have proposed a ban on drilling off the Pacific coast.
Five thousand feet (1,500m) below the surface on Friday, remote-controlled robots were attempting to thread a six-inch (15cm) tube surrounded by a stopper into the 21-inch pipe spewing crude oil and natural gas at a high rate into the water.
If the attempt is successful, the tube could siphon leaking oil and gas into a tanker on the surface, allowing oil workers to concentrate on cleaning up the already-spilled crude.
In addition, BP was preparing to deploy a metal box, or top hat, to cap the leak. A previous attempt to lower a bigger box on to the well site failed.
President Barack Obama has stressed that BP must pay for the clean-up from the disaster. Senators from coastal states have proposed a ban on oil drilling on the Pacific coast.
On Friday, Mr Obama was due to meet advisers to discuss the spill response, and he is expected to discuss the latest efforts to plug the leak in remarks later.
As of late Thursday, more than 500 vessels and 13,000 people were involved in the spill containment and clean-up effort.
Workers had deployed more than 1.4 million feet of boom to arrest the spread, and had dumped more than 475,000 gallons of chemical dispersant into the gulf waters.
Meanwhile, oil has begun washing ashore on the barrier islands in the gulf in the form of tar balls and patties.
US officials forecast the slick could make landfall on Saturday in Florida, Mississippi and Alabama.
The spill is poised to become the worst environmental disaster in US history.
Some scientists have begun to cast doubt on official estimates of the rate of oil flow, saying the widely repeated figure of 5,000 barrels of oil per day dramatically understates the amount.
Florida State University oceanographer Ian MacDonald has estimated the rate is four or five times the government figure, the New York Times reported.
Other researchers who have analysed underwater video of the leak released by BP told US news media outlets the leak was spewing about 70,000 barrels per day, with a margin of error of plus or minus 20%.
The spill began on 20 April, when the Deepwater Horizon oil rig, leased by BP from Swiss drilling giant Transocean, exploded into flames and sank two days later, killing 11 workers.
In testimony before US House and Senate committees this week, BP, Transocean and US oil services company Halliburton each pointed the finger of blame at the others.
Underwater efforts to cap oil leak
Initially, BP tried to lower a 125-tonne, 18-metre (59 feet) high container dome over the main leak on the sea floor. However, this failed when gas leaking from the pipe mixed with water to form hydrates, ice-like crystals, that blocked up the steel canopy.
Instead, engineers have lowered a smaller device onto the site. Dubbed the Top hat, it will sit over the tear in the pipe and partially stop the leak. To prevent the build up of hydrates, methanol is pumped into the top hat to disperse the water and gas.
The top hat is 1.5m (5 feet) high and 1.2m in diameter. Two special side lines are used to pump methanol into the top hat to displace water and gas leaking from the broken oil pipe. This should prevent the build-up of hydrates. Once in place, oil can be pumped up to the surface.
BP plan to lower the original subsea containment dome over the top hat to provide a better seal over the leaking site and pump oil up to the surface. This time, it will be attached to a pipe that can pump warm water into the dome to prevent the build-up of hydrates.
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