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ROBERT SIEGEL, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

MICHELE NORRIS, host:

And I'm Michele Norris.

Tomorrow, federal agencies, as well as Congress, will hold hearings to investigate what went wrong on April 20th on the Deepwater Horizon oil rig. Events that evening left 11 people dead and a massive stream of oil spilling into the Gulf of Mexico. Several of the companies involved with the sunken rig traded accusations today about who's to blame.

We turn to NPR correspondents Peter Overby and Richard Harris, who've been trying to unravel the events that led up to the blowout. And they join us now in the studio.

Obviously, there's a lot to learn, but can you say yet whether this was a single catastrophic failure or perhaps a series of problems that in the end added up to disaster?

Richard, I'm going to begin with you.

RICHARD HARRIS: Well, Michele, it's clearly a series of problems. First of all, something went terribly wrong inside the well down under the seabed. Secondly, there were decisions made on the rig and there are questions about whether those decisions hampered efforts to stop the blowout. And finally, the big question is why the fail safe device sitting on seafloor, called the blowout preventer, whether it failed.

NORRIS: So we're going to get back to the blowout preventer in just a minute. But first, Peter, let's go back and start at the beginning. Can you set a scene for us? April 20th, what happened that day?

PETER OVERBY: Okay. That morning, things were kind of winding down. They were finishing up this drilling operation. They were getting ready to move out. The Deepwater Horizon was sitting a mile above the wellhead. And the well itself went three and half miles down below that.

Now, one of the things that you have to know about oil drilling is that wherever there's oil, there's gas and the gas wants to get out. When it tries that, it was pushing back against the drill. This is called a kick. And kicks had been a problem all the way along.

One of the people who talked about this is Christopher Choy. He's a survivor from the Deepwater Horizon. And our colleague Joe Shapiro interviewed him this weekend.

Mr. CHRISTOPHER CHOY (Deepwater Horizon Survivor): We'd taken little kicks previously. I mean, this well didn't want to be drilled. It just had problems from the start.

OVERBY: So they'd had trouble. They kept going. And what they had now was this well. There was pipe in it and they had just put a steel liner inside the pipe. After they put the liner down, they pumped in cement. It went down to the bottom where there are a couple of valves in the liner, went out those valves and then flowed back up to enclose the liner. They waited for that to set. And by mid-morning, they were ready to go.

NORRIS: So if they've capped the bottom of this pipe, how do they get the oil out?

OVERBY: After the Deepwater Horizon left, another rig would come in and that crew would get the well ready for production. Part of that is putting explosives down the well and blowing holes in the side of the pipe so oil could seep out of the rock and into the well.

NORRIS: So it sounds like they had a plan and everything was moving along according to plan, but obviously something horrible happened.

HARRIS: Yeah, that's true. It looked like they were at the home stretch. They had - and they really had two tasks they needed to do. First of all, they needed to set one final concrete plug near the top of the well. They had the plug at the bottom, but they really wanted to be sure, since they were going to leave this well alone for a while, that they could have a second plug just in case something went bad underwater. So they were - had that to do.

The other thing they had to do was to clear out the pipe that runs between the ship and the seafloor. And that pipe is full of drilling mud, really heavy, gooey stuff. It basically serves a couple of purposes. One very important one is it serves as a cork, essentially to hold down any pressure that might be coming up.

And they - the question was whether to put in the concrete plug first or to remove that drilling mud first. And they decided, well, since the well seems to be fine, the plug in the bottom seems to be holding, we can take out that mud and it won't matter. So they started to take out that mud, and that turned out to be something that came back to haunt them because it turns out everything was not fine under the surface. And as they started to take out that plug, they realized very suddenly actually that things were going crazy.

NORRIS: In part, because mud is so much heavier than the water?

HARRIS: Right. They were replacing the mud with water. And the mud was holding down the gas. But when they took out the mud and put in 5,000 feet of seawater, they basically stopped pressing down on the well.

NORRIS: Changed the pressure gradient.

HARRIS: Yeah.

NORRIS: Peter, why don't you pick up the story from there?

OVERBY: Sure. They started swapping out the drilling mud for seawater about 9 p.m. And for the first half-hour or so, everything looked okay. And then they started to see that things were out of balance. And very quickly, it went completely out of balance. A geyser of water came up. Then mud came up and just covered the drilling floor. Then gas flowed out. It flowed down into the rig and it ignited and you had a series of explosions. There is a witness to this. He's a petroleum engineer. He was on the rig and he was on the Mark Levin syndicated radio show.

Unidentified Man: It literally pushed the seawater all the way to the crown of the rig, which is about 240 feet in the air.

HARRIS: And it's worth noting that if they'd left the mud in the pipe, they might have been able to control this. But since they made that decision, they basically had no way to push back. The gas came out, caused the fire, and we know the rest of the story.

NORRIS: So it sounds like that decision to remove the mud really came back to haunt them.

HARRIS: Very possibly so. But even at this point, they could have contained the damage because they had at the seafloor the blowout preventer. It's a device that when something like this happens is supposed to basically chomp down on this pipe, cut it in half and basically stop the flow. And unfortunately, that device didn't work either.

And why is going to be the subject of investigation, but one possibility is that the very thick drilling pipe that they had down that hole actually also has even thicker joints in it. And it's possible that this blowout preventer that was supposed to chop that pipe in half actually hit such a thick part of the pipe, it couldn't cut through it. That's one possibility. Another possibility is all the pressure built up in the well, as a result of the blowout, could have made the chopping action not strong enough to basically chop off this pipe.

NORRIS: Peter, Richard, thank you very much.

OVERBY: You're welcome.

NORRIS: That's NPR's Richard Harris and Peter Overby.

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