HOUSTON/PORT FOURCHON, Louisiana (Reuters) - BP Plc on Thursday weighed a quick fix to contain a gusher of oil from its blown-out undersea well as a giant oil slick crept west across the Louisiana coast.
BP said it would deploy a small containment dome, known as a "top hat," to trap the oil at the site of the leak a mile beneath the Gulf of Mexico.
"The top hat deployment is (in) the next couple of days," BP spokesman Jon Pack said. "We don't have a definitive date."
If short-term efforts fail, it could take 90 days for BP to drill a relief well to cap the ruptured well.
Oil has been gushing from the well for three weeks at the rate of 5,000 barrels (210,000 gallons/795,000 liters) per day, threatening to eclipse the 1989 Exxon Valdez disaster as the biggest U.S. oil spill, and possibly the worst environmental disaster in history.
BP, whose shares have tumbled and wiped out $30 billion of market value since the rig fire on April 20, said on Thursday the oil spill had cost it $450 million so far. [ID:nLDE64C0I9] BP stock closed up 1.1 percent in London after weeks of steep declines.
"All of the techniques being attempted or evaluated to contain the flow of oil on the seabed involve significant uncertainties because they have not been tested in these conditions before," BP cautioned in a news release.
The company is trying to stop the flow through the underwater blowout preventer -- which did not work as it was supposed to after the rig explosion -- and also by placing the top hat dome over the main leak.
Nearly 100 lawsuits have already been filed across the Gulf region and the disaster, which lawyers see becoming one of the biggest class actions in U.S. history, involves billions of dollars in potential liabilities.
A buildup of slushy gas hydrates stymied BP's first attempt last week at covering the leak with a much-larger dome and there is no certainty that the next stab at it will succeed.
London-based BP, Transocean Ltd and Halliburton Co are all under scrutiny for their roles in the April 20 Deepwater Horizon rig explosion that killed 11 workers and caused the leak.
Transocean, the world's largest offshore drilling contractor which owned the Deepwater Horizon, filed papers in a Houston court on Thursday seeking to limit its legal liability to $27 million.
Hundreds of soldiers and prison inmates have scrambled to shore up Louisiana's coastline with sandbags and thousands of feet (meters) of boom meant to hold the oil slick from the most vulnerable coastal regions.
West of the Mississippi Delta in Louisiana, oil debris have been reported on East Timbalier Island near Port Fourchon, Whiskey Island further to the west near Terrebonne Bay, and Racoon Island.
At Port Fourchon, the tip of southeastern Louisiana's La Fourche Parish and the main supply harbor for the Gulf's deepwater oil and gas industry, gooey, rust-colored globules were found washed up on a beach around sunset on Wednesday with more seen early on Thursday.
Scientists say coastal wetlands threatened by the spill, which provide critical habitat for bird life and serve as rich nurseries for the region's valuable shrimp and oyster stocks, are already dwindling from erosion and development.
Oil pollution would accelerate the process by killing the vegetation that holds the marshes together. It threatens regional economic mainstays including fishing and tourism as well as wildlife throughout the region.
"If we allow that oil to come in and touch our marshlands, that'll shut us down for about five to six years," said Rodney Dufrene, 23, a new shrimp boat owner from the hamlet of Cut Off, north of Port Fourchon, Louisiana.
Scientists are examining samples from seven dolphins and over 100 sea turtles found dead along the U.S. Gulf Coast in the past two weeks to see if they were victims of the spill, wildlife officials said on Thursday.
Global scrutiny of the offshore oil industry could intensify after another drilling rig sank off Venezuela, although no fatalities were reported and authorities said there was no leak from the natural gas well.
The U.S. government response to the BP oil spill has frustrated environmental groups and Gulf Coast conservationists, who say they're getting scant information about the disaster's potential ecological effects.
"There's a lot of concern now about the marine impact and we're not getting a truly transparent response from NOAA (the National Oceanic and Atmospheric Administration)," Aaron Viles of the Gulf Restoration Network said on Thursday.
(Additional reporting by Deborah Zabarenko in Washington, Kelli Dugan in Mobile, Alabama, Tom Brown and Pascal Fletcher in Miami, Victoria Bryan in London and Anna Driver in Houston; Writing by Ed Stoddard and Deborah Charles; Editing by Eric Beech and Sandra Maler)