Factbox: BP's efforts to stem oil flow at seabed

(Reuters) - BP is attempting to stop the Gulf of Mexico oil spill under 5,000 feet of water using some methods that are common and others that have never been done at such water depths.

Here are explanations of those efforts that include details from Bob Fryar, senior vice president of BP operations in offshore Angola, and Charlie Holt, head of drilling and well completion operations in the Gulf of Mexico.

* Relief Wells

Relief wells have worked in other leaks, such as the Montara wellhead in the Timor Sea last year. But they take time. BP began drilling the first of two relief wells on Sunday, not far from the leaking well, and will drill 18,000 feet vertically and horizontally to reach the flowing well to plug it with cement and stop the flow. A second relief well is slated to begin drilling within days. The process is expected to take 60 to 90 days. At the Montara spill, where a rig leased by PTT Exploration and Production spewed oil, five relief wells had to be drilled.

* Containment chambers

In hopes of bringing quicker relief, BP will place a new steel, 73-ton, rectangular-shaped "containment chamber" atop the main leak site, where oil is leaking from a broken pipe as well as the broken drillpipe within the larger pipe. The chamber, 40 feet tall, has a funnel on top that will be connected by pipe to a drillship. Officials hope the chamber will corral leaking oil to channel to the drillship until the leak can be stopped with the relief wells. Such chambers have been used at well and pipeline leaks in much shallower waters, but never before at such depths. The chamber is expected to be in place and connected to the drillship to begin operations in seven days.

Two more chambers are being built, one to place over a third, smaller leak from a bent part of the larger pipe within two to four days of getting the first chamber in place. The third chamber is a backup.

* Close failed valves on the blowout preventer

The blowout preventer sitting atop the well on the seabed has several valves designed to automatically close off the well. Those valves, or rams, run by hydraulic controls failed on April 20.

BP did not have an acoustic control shut-off system that may have allowed it to shut a valve remotely. Such a system is not required by U.S. law.

Within 24 hours of the blowout, BP used underwater robots to try to close the valves. BP official Doug Suttles said the valves have closed, but seals have not worked.

* Valve on drillpipe to close off one of three leaks

By Tuesday BP aims to place a valve at the end of the leaking drillpipe to cut off one of three leaks. Officials do not know whether that will increase or decrease the flow of oil from the larger pipe.

* Dispersants sprayed at seabed

Spraying chemicals that break oil into smaller droplets that can later dissipate. Currently being sprayed from a wand connected to tubing at the largest plume of oil escaping from the larger pipe that contains the drillpipe. Dispersants are commonly used on oil sheens at the surface, but they have not been used before in such depths. BP began spraying at the seabed on April 30.

* New pressure control equipment

The piece of equipment sits atop the blowout preventer, called a "lower marine riser package" or LMRP. It contains mechanisms that close around the pipe containing the drillpipe. The bent and broken pipes protrude from the top of that device. BP is considering removing the LMRP and placing a new blowout preventer atop the failed one. BP will use that option only if officials determine removal of the LMRP won't worsen the leak.

(Reporting by Kristen Hays; editing by Timothy Gardner and David Gregorio)