width=315BP engineers prepared to start lowering a 98-ton metal chamber over a ruptured undersea oil well in the Gulf of Mexico on Thursday as officials confirmed the first oil landfall from the spill on a Louisiana island beach.

A sheen of oil washed ashore on uninhabited Freemason Island in the Chandeleur chain, a spokeswoman for the U.S. response team said. It was the first confirmed report of oil reaching a U.S. shoreline from this Gulf spill.

We have teams that have confirmed oil on the beach, Coast Guard Petty Officer Connie Terrell told Reuters from the Unified Command operation in Robert, Louisiana.

It's largely just sheen. There is no evidence of medium or heavy oil, she said.

The barge carrying the massive white box arrived at the spill site where a BP-owned well blew out two weeks ago 40 miles off the Louisiana coast, causing the explosion and sinking of the Deepwater Horizon rig.

The cofferdam (containment dome) is at the site and is getting ready to be offloaded into the water, BP said in a message sent over Twitter.

Oil workers, volunteers and the military have battled desperately to shut off the gushing leak and stop the huge spreading oil slick from reaching major ports, tourist beaches, wildlife refuges and fishing grounds on the Gulf Coast.

Once the massive riveted metal dome is lowered to the seabed in an operation that could take two days, it is supposed to capture leaking oil and channel it to a drilling ship on the surface. BP said the dome, the best short-term option for containing the leak, could begin operations by Monday.

But company officials, facing enormous pressure to limit the ecological and economic damage from the accident, cautioned the device had never been used in water nearly 1 mile deep.

Several more days of calm weather were forecast for the area, giving crews a window of opportunity to fight the leak. More than 250 boats deployed protective booms on Thursday to block the slick and dispersants to break up the thick oil.

The weather allowed a series of controlled burns of the massive slick on Wednesday, burning nearly 1,000 gallons of fuel. It was the first burn since a 28-minute blaze on April 28 that removed thousands of gallons of fuel.

(Additional reporting by Matt Daily in New York and Tom Bergin in London; Anna Driver and Chris Baltimore in Houston; Tom Brown and Pascal Fletcher in Miami; Michael Peltier in Pensacola; Steve Gorman and Brian Snyder in Mobile, and Richard Cowan in Washington; Writing by John Whitesides; Editing by Philip Barbara)