BP says Gulf spill well sealed despite surface oil sheen

London-based BP reported a sheen on September 16 in block 252 of the Mississippi Canyon, about 50 miles off the Louisiana coast. The U.S. Coast Guard said last week test samples indicated that the sheen matched the type of oil from the mile-deep Macondo well.

BP said an inspection of the sea floor around the Macondo well by remotely operated vehicles was "successful in identifying the cofferdam, a piece of containment equipment used during the Deepwater Horizon response, as the probable source of the surface sheen."

BP said no oil was leaking from the Macondo well itself, and the Coast Guard has said that the surface sheen poses no shoreline risks.

Swiss-based Transocean Ltd owned the Deepwater Horizon drilling rig and BP was the operator of the Macondo well, which ruptured on April 20, 2010, killing 11 workers and creating the worst U.S. offshore oil spill.

On October 17, a survey by undersea vehicles found "intermittent drops of oil" coming from an 86-ton metal cofferdam that BP attempted to lower onto the Macondo well in May 2010 to funnel the oil to the surface. BP engineers aborted the plan after methane hydrates, a flammable form of frozen natural gas, began collecting on the cofferdam. The cofferdam was abandoned on the ocean floor.

"Samples of the droplets have been collected from the opening at the top, known as the stovepipe, and will be analyzed to confirm a match with the sheen," BP said in a release. "Droplets were also observed coming out of a small connection port on one side of the cofferdam."

Two and a half years ago, the well spewed 4.9 million barrels of oil into the Gulf of Mexico for 87 straight days. The torrent of oil fouled the shorelines of four Gulf Coast states and eclipsed the 1989 Exxon Valdez spill in Alaska in severity.

The well was capped with cement on September 19, 2010, which U.S. officials said had "killed" the leaking well for good.

(Reporting By Chris Baltimore; Editing by Kenneth Barry)