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BP crews began slowly lowering a 114-tonne steel and concrete dome over a blown-out well on the sea floor of the Gulf of Mexico in the hopes of containing the oil gushing from it daily.

A crane lifted the box from one boat and crews from another  boat began to drop it down to the bottom, a process expected to take hours.

Then workers will have to install a riser pipe, which will funnel the leaking fuel from the cofferdam to a barge on the surface.

It will likely be Sunday or Monday before the cofferdam is fully operational and engineers know if it's working.

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A worker in Port Fourchon, La., watches construction of a 114-tonne cofferdam that BP will use to contain oil gushing from an undersea well in the Gulf of Mexico. ((BP))

The containment dome, or cofferdam, arrived Thursday morning on a ship in calm waters at the site of the leak about 80 kilometres off the coast of Louisiana.

The U.S. Coast Guard was keeping boats not involved in the effort out of a 40-kilometre perimeter around the site.

Meanwhile, The Associated Press reported that a pinkish oil substance was washing up on the shores of New Harbor Island and reaching the marshland off the coast of Louisiana. 

'Never been done before': BP

The cofferdam is designed to trap the oil gushing out of one of two leaks from the undersea well and suck it up — like a vacuum — to a tanker on the water's surface.

The structure itself is really more of a box containing an inner dome, seven metres wide and 12 metres tall, with a dome-like roof inside of it.

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The inside of the containment system that will be used to try to contain the Deepwater Horizon oil is shown in Port Fourchon, La. ((Gerald Herbert/Associated Press))

First, crews need to properly position the structure with the help of a remote-controlled robotic submarine.

Installing the riser pipe will present several challenges because of the frigid water temperature — about 5 C — and exceptionally high pressure at those depths. These conditions could cause the pipe to clog with what are known in the drilling industry as "ice plugs." To combat that problem, crews will have to pump warm water and methanol continuously down the pipe to dissolve the clogging.

Crews are also worried about a volatile cocktail of oil, gas and water when it arrives on the ship above. Engineers believe the liquids can be safely separated without an explosion.

"This has never been done before," Bob Fryar, BP senior executive vice-president, said when asked to handicap the odds of success. "Typically, you would put odds on something that has been done before."

Workers at Wild Well Control Inc. in Port Fourchon, La., put the finishing touches on the cofferdam on Tuesday.

BP hopes it will collect as much as 85 per cent of the almost 800,000 litres of crude that have been gushing out of the well daily for the last two weeks.

The leak is one of three that sprung after the Deepwater Horizon oil rig leased by the multinational petroleum company exploded on April 20, killing 11 people. The rig sank two days later.

BP managed to cap another of the leaks on Tuesday night, the U.S. Coast Guard said Wednesday, although this wasn't expected to reduce the overall flow of oil into the Gulf of Mexico.

Cofferdams were used during the repair of oil rigs damaged by Hurricane Katrina in 2005.

With files from The Associated Press