Top oil executives face a second day of grilling by U.S. lawmakers on Wednesday over a deadly well rupture that unleashed a huge oil slick and the specter of environmental disaster in the Gulf of Mexico.

BP Plc, Transocean Ltd and Halliburton Co are all in the hot seat for their roles in what could be the worst oil spill in U.S. history. A desperate race is on to contain the catastrophe, with BP readying a potential subsea fix and troops and prisoners rushing to limit damage to the coast, where crude has started to reach shorelines.

Investors have driven the value of BP shares down more than $30 billion, far exceeding even the worst estimates of the spill's cost, reflecting uncertainty about how the calamity will play out, with an unprecedented and shifting situation.

BP shares were up about 0.3 percent in early trading on the London stock exchange.

With oil gushing unchecked from the sea floor at an estimated daily rate of least 5,000 barrels (210,000 gallons/795,000 liters), and the expanding slick oozing across the surface, Wednesday was shaping up to be another rough one for the oil industry and its soiled reputation.

The scheduled U.S. House of Representatives hearing and a series of panels in coming weeks could spawn new legislation on offshore drilling, an issue which has been thrust onto the crowded domestic agenda of President Barack Obama.

If Tuesday's hearings are anything to go by, Wednesday's congressional grilling is likely to be harsh.

In hearings before two Senate committees, lawmakers accused executives from BP America Inc, Transocean, and Halliburton of trying to shift the blame to each other, and subjected them to tough questions about safety and how the well was sealed.

Senator Ron Wyden, a Democrat, at one point interrupted BP America's president, saying, The culture of this company has been one accident after another. BP had been trying to repair its image since a 2005 explosion at its Texas City refinery killed 15.

Also on Wednesday, an activist group called Seize BP plans protests at the company's offices and other sites across the United States to demand the government freeze its assets to ensure payment for the cleanup and compensation for those hurt by the spill.

Eleven workers were killed in the April 20 explosion that sank the rig. Fisheries and tourism, two of the Gulf's economic mainstays, and birds, sea turtles and other wildlife, are all threatened by the unfolding fiasco that could next month exceed the Titanic-sized Exxon Valdez disaster in Alaska in 1989.

Cathy Norman of the Edward Wisner Donation, a land trust that owns the property that makes up the Port of Fourchon, the principal supply harbor for the Gulf's deepwater oil and gas industry, said the area's shoreline already is disappearing at an astronomical rate.

The land is holding on by its fingernails. If oil gets in there and the plants all die off, we're going to have just all water, she said.

BP spokesman Daren Beaudo, who took reporters on a boat tour, said oil had already washed ashore at three locations: Dauphin Island, Alabama; the Chandeleur Islands off Louisiana; and the South Pass-Port Eads area on a remote stretch of Louisiana's mainland.

Louisiana Governor Bobby Jindal on Tuesday urged the National Guard to expand efforts to reinforce the state's storm-battered shoreline as a buffer against the slick.

Jindal's call for additional helicopter sand-bagging operations along stretches of beach, and crews to fill in other shoreline breaches with sand hauled in with dump trucks, came just as the state ran short of oil containment booms for newly menaced coastal areas west of the Mississippi River.

The Unified Command for spill response operations said the U.S. Air Force was flying in additional boom to Louisiana from Alaska. But Jindal said time was growing short.

Besides an expansion of emergency coastal restoration work, Jindal sought approval from federal authorities for plans to dredge sand from the Gulf floor to build artificial barrier islands in three zones off southeastern Louisiana. He said such an operation could start to produce new land within 10 days.

The U.S. government is also concerned about whether enough protective booms are being provided to adequately defend the U.S. Gulf Coast shoreline, Homeland Security Secretary Janet Napolitano said on Tuesday.

We have some concerns about getting adequate boom, she told reporters during a visit to Mobile, Alabama, referring to the plastic barriers that are being strung along the coast to keep the oil off the shore.

BP will try covering the leak with a much smaller funnel than the 98-tonne dome it tried in vain to put in place over the weekend. The so-called top hat dome is expected to be placed over the relentless leak on Thursday.

In Port Fourchon, Louisiana, fatigue-clad Army National Guard troops from the 769th Engineer Battalion of Louisiana sweated alongside prisoners in scarlet red pants and white T-shirts with Inmate Labor on the back as they filled giant 1,000-pound (450-kg) sandbags.

Black Hawk helicopters dropped the sandbags to plug gaps in coastal beaches through which the oil could seep.

U.S. Interior Secretary Ken Salazar and U.S. Energy Secretary Steven Chu were to meet with scientists and engineers from both industry and the federal government at the BP Command Center in Houston to discuss ways to stem the disaster.

(Additional reporting by Tom Doggett in Washington, Greg Savoy in Venice, Louisiana, Verna Gates in Mobile, Alabama, and Tom Bergin in London; Writing by Ed Stoddard; Editing by Eric Walsh.)

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