Q+A: How does BP's new cap work?

HOUSTON (Reuters) - BP Plc has installed a tight-fitting cap over its blown-out Macondo well and will conduct a series of tests on Tuesday to determine if it can seal the runaway well completely.

How does the cap work, and what's at stake for the well test results?

HOW DOES THE NEW CAP WORK?

The new 160,000-pound (73-tonne) capping stack fits snugly over the blown-out Macondo well and allows BP to control the flow with a series of hydraulic rams that are designed to gradually "shut in" flow from the well. BP's previous containment cap used a series of vents which were unable to fully seal the well, but BP and the Coast Guard hope the new system will be water-tight and able to contain most, if not all, of the flow from the well, estimated at up to 60,000 barrels per day by U.S. scientists.

HOW WILL WE KNOW IF THE TESTS ARE A SUCCESS?

BP on Tuesday is expected to start a series of pressure tests on the well which are designed to tell if the Macondo well is strong enough to withstand the build-up of pressure that would be required to seal the well. The tests will last anywhere from six to 48 hours, and will measure the pressure deep beneath the ocean surface in the hydrocarbon reservoir. In this case, higher pressures are good news, because they mean that the well is strong enough and that the inner workings of the well are not damaged. If the well can withstand pressures of 9,000 pounds per square inch for 48 hours, odds are good that it could be "shut in" indefinitely, retired Coast Guard Admiral Thad Allen says. But if pressures linger in the 4,000-6,000 psi range for six hours, BP might have to suspend the test and switch to a plan to siphon the oil flow to ships on the surface.

WHAT HAPPENS IF THE WELL FAILS THE PRESSURE TEST?

If the Macondo well can't hold up under the pressure required for a shut-in, BP could contain the oil flow with a fleet of collection ships on the surface, which could siphon the oil and either burn it off or collect it. BP should have the ability to capture up to 80,000 barrels per day of oil by July 17-19, Allen has said. And regardless of the well tests, BP is in the process of drilling two relief wells, which are expected to permanently kill the well by mid-August with heavy mud and cement.

(Reporting by Chris Baltimore and Kristen Hays)