BP starts deep-sea bid to plug gushing oil well

HOUSTON (Reuters) - BP Plc launched an ambitious deep sea operation to choke off a gushing oil leak in the Gulf of Mexico Wednesday, but President Barack Obama cautioned Americans there was no guarantee it would work.

BP is under intense pressure from Obama to bring a swift end to the five-week-old spill that threatens an environmental catastrophe and has ignited a political storm.

Undersea robots were being used to help inject heavy fluids and ultimately cement pumped down about a mile (1.6 km) to the sea-bed well. The so-called "top kill" procedure has never been attempted at such depths.

Obama said if successful, BP's plan to cap the well should greatly reduce or eliminate the flow of hundreds of thousands of gallons (litres) of crude billowing into the Gulf.

"We will not rest until this well is shut, the environment is repaired and the cleanup is complete," he said on a trip to California.

BP Chief Executive Tony Hayward told the NBC "Today" show it would take a day or two to determine whether the top kill procedure had worked.

A BP statement said the "top kill" action began at 7 p.m. British time. Interior Secretary Ken Salazar appeared to have jumped the gun when he told reporters at a congressional hearing about an hour earlier that the process was under way.

The gushing oil leak threatens some of the United States' richest fisheries and has already soiled more than 70 miles (113 km) of Louisiana's 400-mile (644 km) coastline.

The U.S. Coast Guard approved the top kill operation, the most ambitious effort to date to cap the well, after government scientists said it was safe to proceed, according to a statement by the oil spill response command centre.

Salazar said he did not know how long it would take BP workers to plug the well. "It depends on what kind of difficulties they encounter as they move forward," he said.

CRITICAL DAY FOR OBAMA, BP

It was a critical day both for the London-based energy giant and Obama. BP's reputation and its big presence in the United States is at stake and Obama faces growing criticism from lawmakers and Gulf coast residents over his handling of the crisis.

Investors, who have wiped $50 billion off BP's market value since the start of the spill, will watch closely to see whether the top kill procedure works.

BP shares seesawed in London trading, with hopeful investors boosting the share price about 2.6 percent before closing at 1.4 percent.

BP's announcement that it had launched the long-awaited operation came after London markets had closed.

If the effort ultimately fails, Obama's government may have no choice but to take central charge of the response to what threatens to become the worst oil spill in U.S. history. It has so far deflected calls for it to take a more direct role and said BP has legal responsibility for fixing the mess.

But, with no end in sight for the environmental disaster, analysts say the spill could be a major political liability for Obama ahead of November elections that are widely expected to erode his Democratic Party's control of the U.S. Congress.

Obama, who has told aides to "plug the damn hole," will head to Louisiana Friday for the second time since the April 20 rig blast that killed 11 and unleashed the oil.

Florida Democratic Senator Bill Nelson said if BP failed to plug the leak Wednesday, Obama must seize control of the effort immediately.

"If this thing is not fixed today, I think the president doesn't have any choice -- and he better go in, completely take over, perhaps with the military in charge...," Nelson said on CNN.

LIVE VIDEO OF LEAK

The company cautioned people watching its live webcam of the oil leak that it might not be a reliable indicator of whether the top kill procedure was working.

"Throughout the diagnostic process and top kill procedure very significant changes in the appearance of the flows at the seabed will be expected. These will not provide a reliable indicator of the overall progress, or success or failure, of the top kill operation as a whole," it said on its website.

If the procedure fails, Hayward said, "the next step of our plan is to move to a containment device that will be sealed on the top of the blowout preventer. It would be deployed within a three- or four-day time window."

So far, the company's attempts to plug the massive leak have been dogged by delays and failures, and government and public frustration with the company has risen. Residents of the U.S. Gulf coast region are particularly concerned about the impact of spreading oil on wildlife and area shorelines.

Commercial fishing, shrimping and oyster harvests have been shut down for weeks along much of the U.S. Gulf Coast, home to a $6.5 billion seafood industry.

Scientists and Gulf residents are most concerned about the encroachment of oil into environmentally fragile bayous and marshes teeming with shrimp, oysters, crabs, fish, birds and other wildlife.

BP has estimated that about 5,000 barrels (210,000 gallons/795,000 litres) have been leaking every day, although some scientists have given much higher numbers for the size of the leak -- up to 20 times more.

(Additional reporting by Kristin Hays and Chris Baltimore in Houston, Pascal Fletcher in Miami, Sarah Young in London and Susan Heavey and Tom Doggett in Washington; writing by Ross Colvin; editing by Will Dunham and David Storey)