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

(Reuters) - BP Plc 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 the those efforts to date, which were compiled primarily from company briefings.

* 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 May 2, 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 at the end of this week. 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 placed a steel, 98-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, had a funnel on top that would have been connected by pipe to a drillship. Officials had hoped the chamber would corral leaking oil to channel to the drillship, but hydrates formed and blocked the chamber. Hydrates are crystals formed at high pressure and low temperatures where water and natural gases are present. To reduce the formation of hydrates, the company is readying a much smaller containment chamber to help contain the flow of oil. The so-called "top-hat," which is four feet in diameter and five feet high, is expected to be deployed in three days. Such chambers have been used at well and pipeline leaks in much shallower waters, but never before at such depths.

* 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 did not work. The company is now preparing a "junk shot," aimed at stuffing the failed blow-out preventer with materials including golf balls, shredded tires and knotted rope to stop the flow of crude. BP is also examining the possibility of replacing the existing blow-out preventer with a new one.

* Valve on drillpipe to close off one of three leaks - BP successfully placed a valve at the end of the leaking drillpipe to cut off one of three leaks on May 5. 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. BP said on Monday it received permission from the U.S. Environmental Protection Agency to use the dispersants, even though the agency said their environmental effects "are still widely unknown." BP's chief executive officer said the dispersants were having a "significant impact" containing the crude spilled from the leak.

* Tapping an undersea pipe - Called a "hot tap," the company could tap into the pipe or riser somewhere away from the well. If successful, the company would funnel the oil and gas off directly and bring it to the sea's surface.

(Reporting by Kristen Hays and Anna Driver; editing by Timothy Gardner and Marguerita Choy)