Factbox: BP's next steps on killing Gulf leak

HOUSTON/LONDON (Reuters) - BP Plc fixed a leak on the cap atop failed blowout preventer equipment at the Gulf of Mexico on Thursday, allowing a critical pressure test on the blown-out Macondo well to proceed.

The test is intended to reveal whether the well is intact after the April 20 blowout that caused the leak. If so, the cap could possibly shut in all oil flow if oil-capture vessels must disconnect and move when a hurricane approaches or until a relief well intercepts and plugs the leak by mid-August.

Here is an explanation of the six- to 48-hour test, BP's oil-capture plans and the relief wells:

THE INTEGRITY TEST

* The test involves closing vents and valves on a new cap atop failed blowout preventer equipment to gauge pressure in the blown-out Macondo well.

* High pressure up to 9,000 pounds per square inch would indicate the pipe and cement in the well remains intact after the April 20 blowout.

* A high-pressure result would help relief wells kill the leak, as all heavy drilling mud and cement pumped in would stay in the stricken well.

* Lower pressure at or below 6,000 psi would be a sign oil and gas were leaking from sides of the piping and cement holding the well open and possibly breach the sea floor.

* A low-pressure result could complicate or lengthen the relief-well effort as cracks and leaks are plugged along with the well itself.

* No matter what the test result, the cap is part of a larger, four-vessel oil-capture system to be in place by late July that can handle up to 80,000 barrels and day.

OIL-CAPTURE VESSELS CONTINUING

* On July 12 BP started up a new oil-capture vessel, the Helix Producer, that can collect up to 25,000 barrels a day.

* Another rig -- the Q4000, installed on June 16 -- can collect and burn off up to 10,000 barrels of oil per day, although it sometimes fails to reach that total.

*The vessels have a combined oil-capture capacity of about 35,000 barrels a day.

* Both were shut down for the duration of the pressure test.

MORE VESSELS COMING

* Along with the new cap, the Helix Producer is part of the upcoming four-vessel oil-capture system that is hurricane ready.

* By the end of July, the Toisa Pisces, a well-testing ship revamped to process up to 25,000 barrels a day, will replace the Q4000. Like the Helix, the Toisa Pisces will be hooked up to a seabed blowout preventer atop the well via a hose and pipe.

* Transocean Ltd's Discoverer Enterprise, which had collected crude via the containment cap removed on July 10, and a second drillship, Transocean's Discoverer Clear Leader, will each be connected to the new cap via drill pipes.

* The four vessels will have a combined capacity of up to 80,000 barrels a day and can disconnect quickly to move out of a hurricane's path.

* If the well test has a high-pressure result, the new cap could shut off the leak if vessels must disconnect. Without that assurance, crude will spew unchecked during a storm until the vessels return and reconnect.

THE RELIEF WELLS

* To prevent it from interfering with the integrity test, BP on July 13 suspended drilling on the first of two relief wells at 12,840 feet beneath the seabed on July 13, or 160 feet from the bottom of the blown-out well.

* Drilling will resume on the relief well, which remains on target to intercept the Macondo well far beneath the seabed in late July.

* Plugging the leak could take until mid-August, depending on how deep the relief well must bore into the stricken well and how many times BP must pump in heavy drilling fluid and cement.

* The second relief well, a backup to the first relief well, also suspended drilling, having bored 10,961 feet beneath the seabed by July 13. It suspended drilling to avoid disturbing the first relief well's use of sensors to find the right intercept target.

(Reporting by Kristen Hays in Houston; editing by Mohammad Zargham and Alan Elsner)