Key Events That Led To Deepwater Blowout

The Deepwater Horizon blowout preventer

This undated image provided by Deepwater Horizon Response Unified Command shows the Deepwater Horizon blowout preventer. The device was supposed to keep oil from spilling into the water. Deepwater Horizon Response Unified Command/AP hide caption

itoggle caption Deepwater Horizon Response Unified Command/AP

Three unrelated problems may have combined to cause a disastrous oil-well blowout last month in the Gulf of Mexico. The blast killed 11 workers and is releasing massive amounts of oil into the gulf.

NPR's Richard Harris and Peter Overby tell Michele Norris that an NPR investigation finds questions in these areas: an apparent but undetected leak in the well itself; a decision on the drilling rig that inadvertently let gas rise from the well and burst onto the deck of the rig; and the failure of the well's blowout preventer to stop the chain of events.

None of these issues was in evidence the morning of April 20 on the Deepwater Horizon, which floated almost one mile above a sea-floor wellhead that capped a 13,000-foot well. The rig was owned by Transocean, which leased it to BP for this job. At about 2 a.m., contract workers from Halliburton finished pumping cement down the production liner, a steel tube that reached the bottom of the well. The cement capped the bottom of the well and came back up to enclose the liner. After an interval to let the cement harden — "waiting on cement" in drillers' parlance — the well was pressure-tested. It passed the tests.

Through that afternoon, the Deepwater Horizon crew prepared for the last two steps: setting a concrete plug near the top of the well and dealing with the riser — the fat pipe connecting the rig to the wellhead.

The riser was filled with drilling mud — a heavy, toxic, oil-based concoction used during drilling to lubricate the drill bit and carry waste from the drilling back up the pipe. Once the well reaches the oil reservoir, the drilling mud has another critical purpose: It acts as a cork to hold down the oil and gas pressing up the pipe.

Before the Deepwater Horizon could leave the well, the crew members had to displace the mud with seawater, so they could lay the riser down on the seafloor without dumping toxic drilling mud. Displacing can be done either before or after the cement plug is put in. It saves time overall to do it before setting the plug, but it's also riskier — because seawater weighs less much less than drilling mud. The plug would provide another level of safety, a belt-and-suspenders approach.

Christopher Choy, a crewman on the Deepwater Horizon, told NPR's Joe Shapiro that the rig had encountered frequent problems with gas "kicking" up from this well.

"We'd taken little kicks previously," Choy said. "This well didn't want to be drilled. It just had problems from the start."

Displacement And The Blowout Preventer

But engineers concluded that with cement sealing the foot of the well — a seal that passed its tests — no gas or oil would flow up to cause a kick. The decision was made to displace the drilling mud before setting the plug.

Displacement began about 9 p.m. and proceeded smoothly for at least 30 minutes. But then there wasn't enough drilling mud to hold down oil and gas that had seeped into the well and mixed with the mud in the riser. The mud, made lighter by the gas, couldn't cork the well as it should have.

Another survivor of the Deepwater Horizon, speaking anonymously to radio host Mark Levin on his syndicated talk show, described the start of the eruption that followed: "It literally pushed the seawater all the way to the crown of the rig, which is about 240 feet in the air."

Drilling mud followed the water out of the pipe, covering the deck. Methane flowed out and apparently was sparked, setting off several explosions and the fire. Crew members who were near the pipe died. Others scrambled to lifeboats or leapt into the water.

Down on the seafloor was the last safeguard, the one device that could keep the reservoir from spilling its oil into the gulf: the blowout preventer, or BOP. It failed.

The BOP is a stack of several units, each one designed to stop blowouts in different ways. Here, the critical unit was the blind-shear ram. Simply put, its hydraulics would propel a big shear into the pipe, cutting the pipe in half and crimping it to stanch the flow. It appears that the shear jammed on a length of drill pipe that ran through it.

The shear may have lacked the power to cut through the drill pipe — especially if it happened to hit a joint where two sections of drill pipe were connected — or it may have been weakened by the counterpressure of the blowout itself.

It's also possible that stronger BOPs are needed to handle the superduty pipes used in modern deep-sea work. The federal Minerals Management Service, which overseas offshore drilling, hired a consultant to see if conventional BOPs could do the job. His answer: Maybe.

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