What caused the Deepwater Horizons to sink?

Earlier AP investigations have shown that the doomed rig was allowed to operate without safety documentation required by MMS regulations for the exact disaster scenario that occurred; that the cutoff valve which failed has repeatedly broken down at other wells in the years since regulators weakened testing requirements; and that regulation is so lax that some key safety aspects on rigs are decided almost entirely by the companies doing the work.

http://news.yahoo.com/s/ap/us_gulf_oil_spill_inspections

L also informed me of a great "60 Minutes" interview with a Transocean (world's biggest offshore drilling company) Deepwater Horizons worker who was there during the explosion. That rig did the drilling, and another would handle the oil collection. Apparently the DH did already successfully drill the world's deepest well that I mentioned last week (4k feet water + 35k feet rock), and the current well was a "mere" 5k water + 13k rock. Much of the accident is due to BP's reckless, rushed, greedy timeline. In their haste to tap the huge Tiber reserve (since drilling cost BP $1M/day), they drilled too fast and destroyed their first well. That set them back 2 weeks (delaying the job from 21 to 42 days), showing that haste makes waste.

Regarding the blowout preventer (BOP), they ran a test 4 weeks before the accident. There is a giant rubber seal called the annular that closes around the drill pipe and essentially shuts off the well like the washer in your faucet. But during the test, a worker accidentally moved the drill pipe. So it rubbed on the annular while it was fully engaged (like driving with your parking brake engaged), which probably damaged it. Workers on the surface found chunks of rubber coming out of the well later. A damaged annular may skew pressure tests/measurements (and those tests are the main data for drillers to determine whether it's safe or not to let oil in the well), but the manager said it was "no big deal". Also there are 2 electrical control pods that connect the BOP to the surface (1 primary, 1 backup). They lost communication with 1 of them. A later investigation showed that the BOP was also leaking lubricant and had weak battery charge. But rig tests showed full functionality, so they went ahead as planned.

BP and Transocean managers were fighting over how fast to finish the job too. Drillers use a heavy metal-doped fluid nicknamed "mud" to fill the well and keep the pressurized oil from shooting up. But it needs to be removed before the oil can start to be pumped. Halliburton was paid to install a set of concrete plugs along the well to reduce the pressure without mud. One of the plugs wasn't finished yet, but BP ordered Transocean to remove the mud and get rolling. If the mud and third plug were in the well, it's quite possible that the blowout wouldn't have happened. Each of these mistakes may not have been critical, but the sum of the errors led to the catastrophe. Though the terrible part is that it was all human error and human decisions, not really technical failures. But no matter how infallible our tech gets in the future, how can you divorce human frailties from the offshore drilling equation? Laws can't be written to safeguard against every possible circumstance and bad judgment.

Despite all these concerns, they "stayed the course". Just as a crew was flown in to celebrate the rig's record of 7 years without accident, the explosion occurred. Flammable gases from the well probably leaked through the damaged annular and reached the rig, where it was sucked up by the rig's diesel electricity generators and blew them up. 11 people were incinerated and the rig sank soon after.

http://www.cbsnews.com/video/watch/?id=6490348n&tag=contentMain;contentAux
http://www.cbsnews.com/video/watch/?id=6490378n&tag=contentBody;housing