A crane lowered the 100-ton dome onto the "Joe Griffin" before the barge embarked on the 12-hour journey from Port Fourchon on the Louisiana coast to the epicenter of the disaster some 50 miles (80 kilometers) offshore.

The structure -- a white silo with a dome-shaped top that stands five stories high -- carries with it the hopes of coastal communities from Texas to Florida whose way of life is threatened by the slick.

BP used the occasion to publicize the lengths to which it is going to contain the spill, bringing the barge and dome back to its starting point at one point so that media could get a second shot at its departure.

The company aims to place the structure over the main leak and trap the oil so it can be funneled up through a mile-long pipe to the Deepwater Enterprise, a ship that can process and store the crude-water mix.

The laborious task of transporting the dome, lowering it precisely over the leak and attaching it to the ship is expected to take five days, meaning officials hope it will be operational by Monday.

A nightmare scenario in which the dome could instead further rupture the well and let the oil stream out unchecked are barely being contemplated.

BP experts predict that scenario would lead to an estimated 12-fold increase in the rate of oil gushing out, or some 60,000 barrels or 2.5 million gallons a day, representing almost the entire volume emitted since the crisis began.

The operation to lower the tall structure over a leak 1,500 meters (5,000 feet) below the sea level is fraught with difficulty.

It "hasn't been done before, it's very complex and it will likely have challenges along the way," BP chief operating officer Doug Suttles cautioned.

Coast Guard Rear Admiral Mary Landry said it was important to manage everyone's expectations, and promised that even if the mission failed efforts would not relent until the well is sealed.

"We are all hoping that this... will work, but I want to remind everybody that this containment system is the first of its kind deployed in 5,000 feet of water," she said.

Meanwhile, response teams raced to stave off an environmental and economic disaster as a slick the size of a small country threatening to swamp fragile wetland nature reserves and vital fishing grounds.

BP managed to cap the smallest of three leaks hemorrhaging crude into the Gulf, and resumed burning operations later Wednesday of some of the heaviest parts of the slick.

The successful operation to place a valve over a ruptured pipe and shut off the flow using one of 10 remotely-controlled submarines has no impact on the volume of oil gushing into the sea, but allows efforts to focus on the two remaining leaks.

"Working with two leaks is going to be a lot easier than working with three leaks. Progress is being made," US Coast Guard Petty Officer Brandon Blackwell told AFP.

Fears are growing that sea life is already being affected in a region that contains vital spawning grounds for fish, shrimp and crabs, and is a major migratory stop for rare birds.

A sea turtle was spotted swimming through part of the slick about 15 miles (25 kilometers) off the coast by a National Wildlife Foundation boat, but no animal rescue experts were on board to treat the stricken animal.

More than two weeks after the Deepwater Horizon rig exploded, the full impact of the disaster is still unfolding.

If estimates are correct, more than 2.5 million gallons of crude have entered the sea since the BP-leased platform spectacularly sank on April 22, still ablaze more than two days after the initial blast that killed 11 workers.

The riser pipe that had connected the rig to the wellhead now lies fractured on the seabed a mile below spewing out oil at a rate at some 5,000 barrels, or 210,000 gallons, a day.

Though no official oil on shore has been confirmed, an armada of shrimp trawlers and response boats has been frantically laying orange inflatable boom around the Chandeleur Islands to protect them from the approaching slick.

The islands form the easternmost point of Louisiana and are part of the Breton National Wildlife Refuge -- the second oldest US refuge and home to endangered brown pelican, least tern and piping plover shorebirds.

BP began operations on a relief well Sunday, but the process is expected to take up to three months so the containment dome is seen as the best short-term fix.

In Washington, the White House said Wednesday it supported "significantly" raising the cap on damages energy firms that pollute the environment face.

Under a law introduced after the 1989 Exxon Valdez oil tanker disaster in Alaska, oil companies by law must pay for the full clean-up and containment costs of any oil seeping from their facilities after an accident.

But the law caps the damages for which the firm is liable at 75 million dollars, unless the company is guilty of "gross negligence." Bills introduced in the House and the Senate would fix the cap at 10 billion dollars.

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