Published: May 06, 2010 4:49 PM EDT

ON THE GULF OF MEXICO  - It's never been tried before, but crews hope to lower a 100-ton concrete-and-steel box a mile under the Gulf of Mexico on Thursday to cut off most of the hundreds of thousands of gallons of oil spewing from a blown-out well.

If it works, the system could collect as much as 85 percent of the oil that's been leaking from the ocean floor after the Deepwater Horizon rig exploded April 20, killing 11 workers.

"We're even more anxious," the Joe Griffin's first mate, Douglas Peake, told The Associated Press aboard the boat. The AP is the only news organization with access to the containment effort. "Hopefully, it will work better than they expect."

It won't solve the problem altogether. Crews are drilling a relief well to take the pressure off the blown-out well at the site, and that could take up to three months. Other possible solutions are also in the works.

More than 200,000 gallons of oil a day is pouring from the well, creating a massive sheen that's been floating on the Gulf for more than two weeks.

As it moved closer to land, crews were frantically laying boom and taking other steps to prevent it from oozing into delicate coastal wetlands. At sea, some boats were using skimmers to suck up oil while others were corralling and setting fire to it to burn it off the surface.

The Joe Griffin, the ship carrying the containment box that will be lowered to the seafloor, arrived Thursday morning at the leak site about 50 miles offshore. Workers hope to have the device down at the seabed by Thursday evening, but it will likely be Sunday or Monday before it's fully operational and they know if it's working.

The crew won't have to worry about dealing with the wreckage of the Deepwater Horizon, which sank two days after the explosion. It's not anywhere near where they're working. The waters were calm Thursday with some clouds in the sky, though visibility was good.

Roughly a dozen other ships either surrounded the site or could be seen in the distance. Thick, tar-like oil with a pungent scent surrounded the boat as far as the eye could see. Another BP-chartered boat will use a crane to lower the box - something that has never been tried before at such depths.

BP spokesman Bill Salvin said the drop is expected at about 1:00 p.m. EDT Thursday. The Coast Guard was keeping boats not involved in the effort out of a 500-meter zone around the site. A 20-foot pleasure boat that invaded the perimeter Thursday and pulled up right in the middle of the oil spill near the boat drilling the relief well was told to leave by a Coast Guard boat. It quickly turned around and left.

Boats were also being kept out of a wider area that included the rig drilling the relief well. Oil has been leaking in three places since the explosion. One small leak was capped Wednesday.

The containment box will be lowered over a much bigger leak in a pipe that's responsible for about 85 percent of the oil that's coming out. The rest of the oil is coming from the blowout preventer at the well, a heavy piece of machinery designed to prevent blowouts that failed in the April 20 explosion.

Crews have been trying to shut it off using robotic devices, but that hasn't worked. If the box being lowered Thursday is successful in containing the bigger leak, a second box being built may be used to stop the smaller leak at the blowout preventer.

The containment box has a dome-like structure at the top that's designed to act like a funnel and siphon the oil up through 5,000 feet of pipe and onto a tanker at the surface. First, crews need to properly position the four-story structure with the help of a remote-controlled robotic submarine.

A steel pipe will then be attached to a tanker at the surface and connected to the top of the dome to move the oil. That process presents several challenges because of the frigid water temperature - about 42 degrees Fahrenheit - and exceptionally high pressure at those depths. Those conditions could cause the pipe to clog with what are known in the drilling industry as "ice plugs."

To combat that problem, crews plan to continuously pump warm water and methanol down the pipe to dissolve the clogging. They are also worried about volatile cocktail of oil, gas and water when it arrives on the ship above.

Engineers believe the liquids can be safely separated without an explosion. Asked to handicap the odds of success, Bob Fryar, a senior executive vice president for BP's Deep Water Angola, offered up this assessment: "This has never been done before. Typically you would put odds on something that has been done before."

Fryar also said BP is exploring a technique in which crews would reconfigure the well that would allow them to plug the leak, but that effort is a couple weeks off. The cause of the rig explosion is still not known, but investigators from multiple federal agencies are looking into the matter.

The rig owner, Transocean Ltd., said in a filing with regulators Wednesday that it has received a request from the Justice Department to preserve information about the blast.

Panel probing oil rig explosion to start work

A board investigating a deadly oil rig explosion that caused a massive oil spill in the Gulf of Mexico will begin its work next week.

The six-member panel is split between representatives of the U.S. Coast Guard and the Minerals Management Service. MMS spokesman John Callahan said a more detailed schedule could be released Thursday.

The board is tasked with investigating the cause of an April 20 explosion that killed 11 workers at an oil rig operated by BP PLC.

BP is still trying to stop the leak spewing more than 200,000 gallons of oil a day. Authorities earlier said the investigative board will hear testimony from survivors of the explosion and technical experts.