Gulf well test starts after cap repair: BP exec

A new containment cap is lowered during capping stack operations at the site of the Deepwater Horizon oil spill in the Gulf of Mexico, in this frame grab captured from a BP live video feed July 12, 2010. BP said it had installed a cap meant to halt the flow of oil from its ruptured Gulf of Mexico well on Monday.

HOUSTON (Reuters) - BP Plc (BP.L)< BP.N> launched a critical pressure test on its stricken Gulf of Mexico oil well on Thursday after the company replaced a leaking hose that halted the operation a day earlier, the company said.

And retired U.S. Coast Guard Admiral Thad Allen, the top official overseeing the spill response, said the test will show whether the cap can shut off flow if oil-capture vessels at the surface must disconnect and move when a hurricane approaches, rather than leave the gusher flowing unchecked.

"All that is an interim step on containment, pending the finishing of the relief wells, killing the well with mud and putting cement in," Allen said.

BP said it shut down two oil-capture vessels and closed off valves and hoses in the cap to hem in crude and start the pressure test.

"Although it cannot be assured, it is expected that no oil will be released into the ocean during the test," BP said in a statement.

In other developments:

* The cap, installed Monday, is a crucial step toward a four-vessel oil-capture system to be in place by the end of July that is hurricane-ready and can collect up to 80,000 barrels a day.

* The first of two relief wells is expected to intercept and plug the leak by mid-August. The relief wells are considered the only permanent solution to the oil leak.

* The six- to 48-hour test is intended to show whether piping and cement in the well is intact after the April 20 blowout that caused the leak.

* BP and Allen said high pressure would indicate the well is stable, while low pressure would be a sign that oil and gas are leaking out the sides of a damaged well.

* If pressure remains too low, BP will stop the test and open the valves as the two oil-capture vessels on site ramp up and more get hooked up. (Editing by Doina Chiacu)