* BP plans to install cap to funnel oil and gas
* Coast Guard: development "a significant step forward" (Adds comment from CEO, detail, expert quote)
By Kristen Hays
HOUSTON, June 3 (Reuters) - BP's (BP.L) (BP.N) underwater robots successfully sheared off a gushing oil pipe deep under the Gulf of Mexico on Thursday and prepared to cap the leaking well to try to funnel oil to the surface.
"We have cleared the riser from the top of the wellhead, and the team is working to complete the cleanup operation before we put the cap on top of the well," BP Chief Executive Tony Hayward said on Thursday afternoon.
Hayward did not say when the well would be capped but said the operation's success could be gauged within 12-24 hours, adding that BP would work to stabilize the flow of oil and gas over the next two to three days once the well was capped.
Earlier on Thursday, U.S. Coast Guard Admiral Thad Allen, the top U.S. official overseeing the operation, indicated at a news conference in Metairie, Louisiana, that BP would lower the cap onto what is left of the pipe, called a riser, on Thursday and "start to see if we can move gas and oil up the pipe."
BP did not respond to requests about the different timelines. The white well cap with the number "4" displayed on the side could be seen hanging from a pipe in the depths on BP's live underwater camera feed on Thursday.
Allen announced the pipe had been sheared from a lower marine riser package atop a failed blowout preventer. He called it "a significant step forward" in BP's so-far unsuccessful attempts to plug or contain the six-week-old leak that has gushed up to 19,000 barrels of oil a day into the basin.
BP turned to huge shears to cut through the pipe as well as a smaller drillpipe inside to make way for a containment cap and seal.
Earlier efforts to use a diamond saw to shear off the pipe with a clean cut failed, possibly because the drillpipe inside the riser put up too much resistance, Allen said.
BP's next move is to lower the cap on the jagged remnants of the pipe in hopes that the seal will contain most of the leaking oil and gas, Allen said. The cap will be placed over the leak to funnel oil and gas to a tanker vessel on the surface.
Because BP had to abandon efforts to slice through the pipe with the diamond-tipped saw, the ensuing cut is more jagged and irregular than initially planned, which means that BP will not be able to contain as much oil as it had earlier hoped.
The jagged pipe remnant also juts from the lower marine riser package, or LMRP, at a 10-degree angle, adding to the challenge, he said.
"This is an irregular cut," Allen said. "It will be a little bit more challenging to get the seal around."
BP's underwater strategy has turned to containment after its "top kill" plan to plug the well failed on Saturday. Oil is expected to flow from the ruptured well until BP completes a pair of relief wells, expected to be finished in August.
"They are slightly ahead of schedule right now, but we are not willing to declare victory" until one or both plug the leak, Allen said.
Tyler Priest, director of global studies at the University of Houston's Bauer College of Business and an expert in offshore oil and gas operations, said the containment cap plan is a stopgap until BP gets the relief wells drilled, which are a proven method of intercepting and plugging other wells.
"A lot of these efforts have an element of desperation to them, motivated by the need to show they are trying to do something," Priest said. "They are sort of buying time and reducing the flow until the relief wells get down there" to stop the leak. (Reporting by Kristen Hays, editing by Paul Simao)