Factbox: How the oil spill containment dome works

(Reuters) - A 98-ton steel containment chamber, or cofferdam, reached the site of a massive oil leak 50 miles south of the Louisiana coastline on Thursday.

The device is intended to corral leaking oil almost a mile under the water's surface and channel it through a pipe to a ship above.

Such containment efforts have been tried before in shallow water depths, including after Hurricane Katrina, but the technology is untested in the high pressures and low temperatures 5,000 feet below the water's surface.

The chamber is BP's best hope so far of corralling the oil until a relief well is drilled within two to three months. The leak began after a blowout preventer at the seabed failed on April 20, allowing Transocean's Deepwater Horizon rig to explode and later sink.

Below are some details on the chamber and how it is intended to work, from BP and the Deepwater Horizon response team:

* At the leak site, the device will be lowered by cable to the site of the leak. Once at the seafloor, underwater robots will place it on top of the larger of two remaining leaks.

* The chamber, 40 feet tall, 24 feet wide and 14 feet deep has steel shelf-like "mud flaps" on the side to ensure it doesn't sink more than 15 feet into the mud.

* The funnel-like top of the chamber will be connected to a drill pipe inside a larger pipe, or riser. That mechanism will then be connected to Transocean's Deepwater Enterprise drillship, which is capable of processing 15,000 barrels of oil per day.

* The dual-pipe mechanism will allow warm water and a chemical, methanol, to be pumped into the space between the drill pipe and larger pipe. That is intended to counteract the possibility of ice- or sludge-like plugs in the larger pipe that could hinder or stop the flow of oil to the ship. Such plugs can form from natural gas, 3,000 cubic feet of which is in each leaking barrel of oil.

* When the fluids reach the drillship, they will go to a closed processing system designed for normal well testing. There, the oil, gas and water will be separated.

* The oil will be stored in a tank that can hold up to 139,000 barrels of oil; the gas will be flared; and the water will be dumped back into the sea.

* The oil collected aboard the drillship can be later offloaded onto a standby vessel, with the capacity to store 137,000 barrels of oil, to take to BP's 455,790 barrel-per-day (bpd) Texas City, Texas refinery to process.

* The larger leak is estimated to be releasing about 85 percent of the gushing oil from a riser that broke when the rig sank. A second leak stems from a bent pipe connected to the failed blowout preventer at the seabed.

* A second containment chamber is being built at Wild Well Control in Port Fourchon, Louisiana, to later be placed atop the second leak. In the meantime, underwater robots will continue trying to manipulate valves on the blowout preventer to stop both leaks at the main source.

(Editing by Philip Barbara)