* BP starts four- to seven-day cap switch process
* Current cap removed, oil gushing between caps (Updates with new details throughout)
By Kristen Hays
HOUSTON, July 10 BP Plc (BP.N) (BP.L) removed a containment cap atop its gushing Gulf of Mexico oil leak on Saturday and began taking steps to replace it with a bigger cap and seal that could fully contain the crude.
"What I'd say to you at this point is we're on plan," Kent Wells, senior vice president of exploration and production, told reporters Saturday afternoon.
Wells said earlier on Saturday it would take four to seven days to install the new cap and seal. In the interim, oil will gush from the leak.
Retired Coast Guard Admiral Thad Allen, the top U.S. official overseeing the spill response, had said the cap switch could be finished by Monday. But BP's plan, which Allen approved late on Friday after he had publicly discussed the timeline, showed four to seven days.
Wells said the longer stretch allows for unexpected problems. He added that BP has another cap ready to install if the cap and seal doesn't work.
The old cap was removed at 12:37 a.m. CDT (1737 GMT), BP spokesman Mark Proegler said. A live video feed on BP's web site showed the cap hanging from a line used to lift it from the oil gusher.
A different feed showed an underwater robot starting the next phase of the switch, which Wells said aims to unbolt a flange and the jagged remnant of a pipe that the former cap covered so a new flange can be installed.
The new cap, which had yet to be lowered to the seabed, would later be connected to the new flange, Wells said.
Meanwhile, a rig installed June 16, the Q4000, continued to siphon and burn off an average of 8,000 barrels a day, he said. BP is doing final hookups and tests of another rig, the Helix Producer, that can collect up to another 25,000 barrels a day and hopes that it could begin operating on Sunday.
An eight-day window of good weather prompted BP to hook up the Helix Producer this week and begin the cap switch.
CALM WEATHER WINDOW
A team of U.S. scientists estimate up to 60,000 barrels of oil are leaking each day into the sea, including the oil that BP's systems collect and burn off.
Once the new cap is installed, it could capture the rest of the crude pouring from the seabed, Wells said, which would be funneled to vessels on the surface a mile (1.6 km) above.
The cap switch and the hookup of the Helix Producer are part of BP's overall effort to set up an upgraded oil-capture system with four vessels that can handle up to 80,000 barrels a day and disconnect and move quickly if a hurricane approaches.
All oil would gush unchecked in that instance, until the vessels return and reconnect, BP and Allen said.
The former cap system, which channeled oil to a drillship via a fixed pipe, needed five days' notice to disconnect and move from a storm, Allen said.
The new system will involve the Helix Producer, two drillships and a revamped well-testing ship.
That upgraded system could be in place "over the next two to three weeks," Wells said, later than the original mid-July target date. The delay stems in part from Hurricane Alex-related rough seas and high winds that held up the third vessel's June 30 hookup.
Also, the new cap and seal was originally scheduled to be installed by the end of June, but bad weather and protracted discussions among BP, Allen and government scientists on whether to switch the caps also delayed installation targets. (Reporting by Kristen Hays, editing by Vicki Allen and Todd Eastham)