The gulf oil spill has offered the nation an often agonizing study course in the complexities of oil engineering. BP's errors have prompted anger and accusations of ineptitude.

As the drilling of the relief wells progresses, Coast Guard Adm. Thad Allen Friday took time at the opening of his briefing to explain how the operation to stop the leak is progressing, and, in the process, what just might go wrong.

Unless storms and hurricanes or engineering glitches stop the progress, one of the wells could intersect with BP's leaking well 18,000 feet below the gulf's surface and manage to cap it as early as August.

"The relief wells continue to go at pace," Allen said. "The Development Driller III, which is the primary relief well, is now at 10,968 feet below the mud line. Development Driller II is 4,697 feet below the mud line.

"In regards to Development Driller III, two points to be made there. They are starting what they call ranging activity. This is where they withdraw the drill pipe and put down an electrical cable into the end of the wellbore, and they put out an electrical signal, and they actually could pick up the magnetic field around the wellbore. This tells them how close they are getting.

"They have made contact with this electromagnetic field. What they will do is continue to drill down in short intervals, withdraw the pipe, put that sensing device down, and slowly close on the wellbore to the point where they're ready to do the intercept drilling.

"This last part takes some time, because they only do several hundred feet at a time, withdraw the drill pipe, and then put the sensor down to figure out how close they're coming. After a series of these readings, they can have a very precise idea of how close they are to the wellbore and then how to actually turn the drill in and make the intercept. But then we'll get much slower, because they have to basically drill, withdraw the drill pipe and put the sensor down.

"They also have a vessel standing by that's full of mud on the top, in the event they were -- they get really close to potentially nick the wellbore, they can actually put mud down to control any hydrocarbons that might come out."

-- Margot Roosevelt