(Reuters) - A larger, tighter-fitting containment cap was installed on Monday atop BP's ruptured wellhead on the floor of the Gulf of Mexico in a move the British energy giant said it hopes will fully contain the deep-sea oil gusher.
Crude oil continued to spew into the sea for the time being, but BP Plc said it planned to begin testing the new cap, and the internal pressure of the well, on Tuesday by closing off valves on the device to constrict the flow of oil.
If the test goes as intended, it would mark the first time since the April 20 explosion and blowout of the Deepwater Horizon drilling rig that the flow of oil from the crippled well has been halted, at least temporarily.
But BP, in a statement announcing the installation of the new cap, warned that success was not certain.
"It is expected, although cannot be assured, that no oil will be released to the ocean for the duration of the test," the statement said. "This will not, however, be an indication that flow from the wellbore has been permanently stopped."
BP said it does not expect to plug the undersea geyser for good before mid-August, after intercepting the rupture point with one of two relief wells now being drilled.
Former U.S. Coast Guard Admiral Thad Allen, overseeing the U.S. government's spill response, said that if the cap works it will be used to resume the siphoning of oil to ships on the surface until the ruptured well can be permanently plugged.
The new cap-and-seal stack, which is larger than the one removed on Friday and bolted over the top of the wellhead rather than clamped loosely over it, is designed to capture three times more leaking oil, or virtually the entire flow.
Next will come the critical "integrity test." It's really a pressure test. How the well performs in the test will shape everything that follows.
Before the test begins, BP will stop collecting oil from the well. Although the top hat, which funneled oil to a surface ship, is no longer in the picture, there is still oil flowing to a surface rig called the Q4000 via a line attached to the blowout preventer. The Q4000 has been burning about 8,000 barrels (336,000 gallons) of oil a day. On Monday, a new ship, the Helix Producer, began siphoning oil through a different line connected to the blowout preventer. That ship potentially could capture up to 25,000 barrels a day if ramped to full production.
To test the well's integrity, BP will gradually shut down the flow of oil and gas until the flow stops -- nothing out the top, nothing to surface ships. BP engineers and government scientists will scrutinize the pressure building in the well.
"Higher pressures are good news. They indicate that the well bore has integrity," said Doug Suttles, BP's chief operating officer.
If the pressure doesn't rise as expected, that will suggest that the well has been damaged below the seafloor. Suttles played down the possibility that the test could damage the well and cause leaks into the surrounding rock formation.