NEW ORLEANS — After a new setback because of a hydraulic leak, BP prepared Thursday to pick up where it left off the day before: with a crucial test that could stop the rush of oil from its well in the Gulf of Mexico for the first time since the disaster began three months ago. Overnight, engineers using remotely operated submersibles replaced equipment on the tight-sealing cap at the top of well, 5,000 feet under water, said Kent Wells, a senior vice president of the company.
The equipment, part of a choke line that will be the last valve to be closed during the test, had been found to be leaking late Wednesday.
With the repair made, “we’re looking to start this test as soon as we possibly can,” Mr. Wells said Thursday morning in a conference call with reporters.
While the faulty equipment was being swapped out, Mr. Wells said, containment systems that had been shut down Wednesday afternoon were restarted. That meant collection of oil began again, as did flaring of oil and gas from booms on two surface ships. The flames had been extinguished Wednesday in preparation for the test.
The test involves closing valves on the new cap, which was installed earlier in the week, to increase pressure in the well so that BP can assess its condition over the length of the well bore, which extends 13,000 feet below the seabed.
The delay for repairs was the second in as many days. On Tuesday, the government had asked BP to postpone the test for 24 hours while scientists reviewed the procedures. Officials were concerned about the possibility that the test itself might damage the well.
The decision was made to allow the test to begin Wednesday, with some modifications.
On Wednesday, Thad W. Allen, the retired Coast Guard admiral who is leading the response, said that the test results would provide crucial information about the well’s condition, and “would determine our confidence to shut the well in and understand we’re not harming the well bore.”
That knowledge would be particularly useful during a hurricane, he said, because it would prevent oil from leaking into the gulf after collection ships left for safe waters.
At the White House on Wednesday, Robert Gibbs, the press secretary, said that Energy Secretary Stephen Chu and others had been involved in the review, asking BP about the possible effect of the test on the well’s condition.
Mr. Gibbs described the review as “a series of steps” that were being taken “in order to ensure that what we’re doing is being done out of an abundance of caution to do no harm.”
Among the concerns was that if the well was damaged during the test, oil and gas might leak from the seafloor around the well rather than up through the well bore as it is now.
While the test is being conducted, drilling of a relief well — considered the ultimate solution to stopping the gusher at its source — will be halted as a precaution, BP said.
The suspension of work would set the task back by a couple of days.
Earlier in the week, BP officials had said that if the test showed that the well could hold pressure, the valves might remain closed. That would end the gusher that began shortly after the Deepwater Horizon drilling rig exploded April 20, killing 11 workers.
If the test shows the well is damaged, the flow of oil into the sea could still be stopped by increased collection of oil, a process that could continue for weeks, awaiting completion of a relief well.
Admiral Allen said the government had asked BP for more information on the structural strength of the well. And in allowing the test to proceed, the government stipulated that pressure be allowed to build up in intervals, with acoustic tests to gauge the well’s condition every six hours. That would most likely lengthen the duration of the test, which had been expected to last from 6 to 48 hours.
But the admiral said that the well would be monitored closely during the first three hours and that the test could be even shorter if the pressure stays low.
The test brings to an end, at least temporarily, a period when more oil had been spewing from the well after a loose-fitting cap was removed to begin work on the new one. The old cap was diverting about 15,000 barrels of oil a day.
If the pressure test shows that the well is damaged and the valves have to be reopened, full containment of the oil would probably not occur for several weeks, until other collection vessels could be brought in to handle more of the flow. That would raise total collection capacity to more than 60,000 barrels a day, the current high-end estimate of the well’s flow rate. Halting the gusher would then await the completion of the first relief well.
Viewed from a Coast Guard cutter about a mile away, the well site was a floating city on Wednesday, with scores of vessels scattered across the calm area. The activity was centered on the spot where the rig burned and then sank two days later.
A huge drill rig, the Development Driller III, was about a mile away, working on the first relief well. A nearly identical rig, the Development Driller II, was drilling a backup that is not as far along. Supply boats attended both rigs, long sections of casing pipe on their decks.
At about 3:30 p.m. Wednesday, the tongues of fire from the collection ships that had marked the site for weeks went out, only to be restarted later when the leak was discovered.
