BP spokesman Bill Salvin said Wednesday that the 2-ton box had reached the seabed overnight after being lowered late Tuesday by a crane from the deck of the Viking Poseidon.
But the box, dubbed a top hat, hasn't been positioned over the well because engineers want to make sure everything is hooked up correctly. Officials want to avoid the same icy, slushy buildup that thwarted their first attempt at using a much larger box that weighed about 100 tons.
This box will be connected to a ship on the surface by a pipe-within-a-pipe when it's lowered. Crews plan to pump in heated water and methanol so ice won't build up.
Coast Guard Petty Officer Connie Terrell said it is about 35 degrees Fahrenheit at the bottom of the sea floor where the oil is spilling.
Salvin said undersea robots will position the box over the gusher by Thursday. The blown well has spewed at least 4 million gallons of oil into the Gulf over three weeks.
Special Section: Gulf Coast Oil Disaster
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Gulf Blame Game
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BP Oil Spill: Who's to Blame?
BP is also planning a wrinkle on the technique it plans to try next to stem the flow of oil spewing from a sunken rig.
BP chief operating officer Doug Suttles said Wednesday that engineers hope to link a second pipe to the end of the pipe that was supposed to pump oil from the sea floor before the Deepwater Horizon oil rig exploded and sunk last month.
The pipe will funnel away the oil that's collected in the original piping, called the riser. Suttles says it could arrive Wednesday and be usable by Thursday or Friday.
In yet another sign the spill is getting worse, Louisiana wildlife officials said Wednesday that tar balls had washed ashore in South Pass in the state's southeastern tip. The marshy area is home to prime waters for shrimp and other seafood.
The latest developments in trying to contain the massive oil leak come as political patience is washing away for BP executives.
New disclosures Wednesday revealed a and procedural problems in the oil rig explosion and massive spill that is still fouling the waters of the Gulf of Mexico and threatening industries and wildlife near the coast and on shore.
The disclosures were described in internal corporate documents, marked confidential but provided to a House committee by BP PLC, the well's operator, and by the manufacturer of the safety device. Congressional investigators released them.
A senior BP executive, Lamar McKay, cautioned lawmakers, "It's inappropriate to draw any conclusions before all the facts are known." But the documents established the firmest evidence to date of the sequence of catastrophic events that led to the explosion and worsening spill, a series of failures more reminiscent of the loss of the space shuttle Challenger than the wreck of the Exxon Valdez.
Like the 1986 Challenger disaster, the investigation into the Gulf spill may well show that complex and seemingly failproof technical systems went wrong because of overlooked problems that interacted with each other in unexpected ways. In the 1989 Exxon Valdez disaster, a captain simply ran his ship onto a reef in Alaska's Prince William Sound, spilling some 11 millions barrels of oil.
The April 20 BP rig explosion killed 11 people. Since then, nearly 4 million barrels of oil have spewed from the broken well pipe 5,000 feet under water 40 miles off the Louisiana coast, threatening sensitive ecological marshes and wetlands and the region's fishing industry.
Congressional investigators revealed Wednesday that a key safety system, known as the blowout preventer, used in BP's oil-drilling rig in the Gulf had a hydraulic leak and a failed battery that probably prevented it from working as designed.
They said that BP documents and others also indicated conflicting pipe pressure tests should have warned those on the rig that poor pipe integrity may have been allowing explosive methane gas to leak into the well.
"Significant pressure discrepancies were observed in at least two of these tests, which were conducted just hours before the explosion," said Rep. Henry Waxman, D-Calif., at a House hearing on the rig fire and oil leak, citing documents his committee had received from BP.
Asked about the tests, Steven Newman, president of Transocean, which owned the drilling rig, and Lamar McKay, president of BP America told the committee the pressure readings were worrisome.
They indicated "that there was something happening in the well bore that shouldn't be happening," said Newman. McKay said the issue "is critical in the investigation" into the cause of the accident.
The well explosion unleashed a massive oil spill that after three weeks remains uncontained.
But Waxman said important elements of what went wrong were beginning to surface.
While "we have far more questions than answers," it appears clear that there were problems with the blowout preventers before the accident and confusion almost right up to the time of the explosion over the success of a process in which cement is injected into the well to temporarily close it in anticipation of future production.
In other developments Wednesday:
• The White House asked Congress to to cover damage from the spill beyond the $75 million cap now in law. It also wants oil companies to pay more into a federal oil spill cleanup fund.
BP president Lamar McKay said the company will pay any legitimate claim of damages beyond cleanup costs despite the federal cap.
• The Minerals Management Service told a government panel of investigators in Kenner, La., that inspections of deepwater drilling rigs has turned up only "a couple of minor issues."
• Gov. Charlie Crist asked BP PLC on Wednesday to pay nearly $35 million for an emergency ad campaign to assure the world that Florida's beaches and coastal waters are untainted by oil spill.
The House hearing into the spill was the third this week at which executives of BP and two other companies were questioned by lawmakers.
The committee produced one document from BP that provided the most detailed information to date on what led up to and may have caused the explosion and spill at the Deepwater Horizon rig, floating in mile-deep waters 40 miles off the coast of Louisiana, and why equipment designed to stop a spill failed to do the job.
Rep. Bart Stupak, D-Mich., said there were at least "four significant problems with the blowout preventer" - or BOP - including evidence that it had a significant hydraulic leak and a dead battery that was supposed to activate a so-called "deadman" trigger.
A 2001 report by Transocean, which made the BOP equipment, indicated there can be as many as 260 failure possibilities in the equipment, which is supposed to be the final safeguard against a well blowout by clamping down and sealing a gushing oil well, said Stupak, chairman of the panel's investigation's subcommittee.
"How can a device that has 260 failure modes be considered fail-safe?" asked Stupak.
Stupak said when an underwater remote vehicle tried to activate the blowout protector's devices designed to ram through the pipe and seal it, a loss of hydraulic pressure was discovered in the device's emergency power component.
When dye was injected "it showed a large leak coming from a loose fitting," said Stupak, citing BP documents. He said officials at Cameron, the company that made the preventer, had told the committee the leak was not believed to have been caused by the blowout because other fittings in the system were tight.
Stupak also questioned why the BOP had been modified.
Newman, the Transocean executive told the committee that, indeed, the BOP had been modified in 2005 at the request of BP and with approval of the Minerals Management Service.
Stupak said the committee had been told that one of the BOP's ram drivers had been changed so it could be used for routine testing and was no longer designed to activate in an emergency. He said after the spill BP "spent a day trying to use this ... useless test ram" which no longer was configured for emergency use.
Executives of the companies involved have sought to shift blame on one another at Senate and House hearings this week on the spill.
BP has cited the failure of the blowout preventer owned by Transocean, which in turn has raised questions about the cementing process conducted by Halliburton, a BP subcontractor.
At Senate hearings Tuesday and again before the House panel, Timothy Probert, an executive of Halliburton, said that its work had been completed except for the installation of a final cement cap and that it was done according to the BP drilling plan.