MIAMI — The steel containment dome that crews prepared to lower over the Deepwater Horizon site on Thursday represents the best immediate chance to slow the oil spilling into the Gulf of Mexico from the blown-out well. But even if it works — a big if that may not play out for days — it's still a temporary measure subject to weather and other conditions.
"A dome might slow the leak, but it can't stop it," said Dr. Philip Johnson, a professor of petroleum engineering at the University of Alabama.
The only permanent solution is to drill relief wells to shut off the flow, Johnson and other experts say. And BP says that will take three months. Because of that, a half-dozen other methods — from burning the oil to dispersing it with chemicals, continue at full speed.
Workers hoped to have the four-story dome on the seabed by late Thursday but said it will be Sunday or Monday before they will know if it's working.
Oil has been leaking in three places since the April 20 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.
"This kind of system worked very effectively after Hurricane Katrina," said Greg McCormack, director of the Petroleum Extension Service at the University of Texas. "But it was in much shallower waters, mostly less than 200 feet deep."
This leak is at 5,000 feet.
"It's pitch black down there. There are no divers. And there are all kinds of currents," McCormack said.
The box must be accurately positioned over the well, or it could damage the leaking pipe and make the problem worse.
Other risks include ice clogs in the pipes, and the danger of explosion when separating the mix of oil, gas and water that is brought to the surface.
"I'm worried about every part, as you can imagine," said David Clarkson, BP vice president of engineering projects.
The rest of the oil is coming from the blowout preventer. Crews have been trying to shut it off using robotic devices, but that hasn't worked.
Even with two domes in place, the method depends on piping the oil up to a ship, which will siphon it into smaller ships to be carried away. But, Johnson notes, "if a hurricane comes, you're in trouble."
BP engineers are examining whether the leak could be shut off by sealing it from the top. The technique, called a "top kill," would use a tube to shoot mud and concrete directly into the well's blowout preventer, spokesman Bill Salvin said. The process would take two to three weeks.
Relief wells are the best solution, the experts say.
BP began drilling the first of two planned relief wells near the broken well on Sunday.
In drilling a relief well, a new well is bored near the broken one, at least 13,000 feet below the seabed. A horizontal shaft is then drilled to tap into the broken well, and a "heavy fluid" solution of water and finely ground barium sulfite is pumped into the broken well to stop the flow of oil, Johnson said.
Three months to drill a relief well is "an optimistic estimate," says Dr. Don Van Nieuwenhuise, geology professor at University of Houston who helped drill two relief wells for an earlier gulf oil well blowout.
Information from the Associated Press was used in this report.