* Will take 60-90 days to drill relief well
* Other efforts to stop seabed untested
By Kristen Hays
HOUSTON, April 30 There is little hope that near-term efforts by London-based BP Plc (BP.L) to choke off a leaking underwater oil well will succeed, experts said on Friday, raising the prospect of an environmental disaster on the scale of the 1989 Exxon Valdez oil spill.
BP, the majority owner of an undersea well leaking 5,000 barrels (210,000 gallons or 955,000 litres) a day of oil into the Gulf of Mexico, is hoping two relatively quick fixes bear fruit before a pair of relief wells that could take up to three months to drill allow the company to plug the leak.
In the interim, the coastal states of Louisiana, Mississippi, Alabama and Florida could take a battering as oil flows unhindered.
"At 5,000 barrels a day, in two months' time it's going to be a bigger spill than the Exxon Valdez," said Tyler Priest, director of global studies at the University of Houston's Bauer College of Business. "You're looking at a huge disaster."
At that rate, it will take about 50 days for the spill to eclipse the 1989 Exxon Valdez spill in Alaska, the worst U.S. oil spill on record, which sent 10.8 million gallons (49 million litres) of crude oil into Prince William Sound.
Underwater robots have so far failed to activate a cutoff valve at the seabed 5,000 feet (1,524 metres) below the ocean surface to stop the leak left when a blowout preventer failed.
BP has ordered three "containment chambers," or giant 73-ton box-shaped inverted funnels, to cover the well and two other leaks and channel the oil to a drillship. But it will take two to four weeks to fabricate the piping necessary to connect the funnel to the vessel.
The containment chambers are 14 feet (4 metres) wide and 40 feet (12 metres) tall. Once lowered over a leak site, the oil would flow up the tapered funnel-like opening into a pipe that leads to ship that can store and transport it.
BP aims to drill two relief wells a half-mile (0.8 km) from the leaking well to stop the flow, but that will take 60 to 90 days. Unless the underwater robots find sudden success, the funnels are the main stopgaps at the seabed until a relief well can be drilled.
The seabed efforts are in addition to surface efforts that include containment booms at sea and at the shoreline, dispersants and controlled burns when weather conditions allow. The company also is using the robots to spray dispersants at the well on the seabed.
BP says it is spending $6 million a day.
But the funnel method is untested at such water depths, Priest said.
He said a similar containment chamber was used to try to stem the leak from a blowout in Mexico's Bay of Campeche in 1979, but that effort failed in 150 feet (46 metres) of water. The Ixtoc well leaked for nine months until two relief wells were drilled to relieve pressure so the spewing well could be capped.
BP spokesman Daren Beaudo said BP and other companies used similar containment systems on downed rigs in shallow Gulf waters after Hurricane Katrina hit in 2005.
"Now we're going to take what we know and expand it and push the edges of the technology for this never-tried-before deepwater application," Beaudo said.
Walter Chapman, a professor of chemical and biomolecular engineering at Rice University, said hope remained that the underwater robots could engage the blowout preventer, which failed last week when a Transocean drilling rig exploded and sank. Eleven workers are missing and presumed dead after the rig exploded.
"Of course they want to contain it as best they can and protect the shoreline with containment booms," he said. "But the options are becoming fewer." (Reporting by Kristen Hays; Editing by Peter Cooney)