The image above, taken from the forensic examination report on the Deepwater Horizon blowout preventer (pdf), is the heart of what went wrong and what remains unaddressed in the blowout preventer design.
That dark stripe is the drill pipe. The green stripe is the wider hole the drill pipe runs through called the wellbore.
What happened with the Deepwater Horizon blowout preventer is that the drill pipe moved up -- or at least the bottom of the drill pipe moved up. The top of the pipe was held secure. So, like the straw in your Snapple bottle when you screw the cap back on, the drill pipe bowed.
What's supposed to happen in the case of an emergency is that a powerful pair of blades called Blind Shear Rams (BSR) chops the drill pipe, crimps it closed and seals the whole well shut. (see page 157 & 158 of the report) That apparently works when the drill pipe is in the center of the wellbore. But when the drill pipe is buckled to the side, as it was in the Deepwater Horizon, the blades don't cut all the way, they don't crimp all the way, and they don't seal the well.
And the bad news is that "the BSRs are the only set of rams designed to cut drill pipe and seal the well in the event of a blowout." (page 16) So if those don't work, as they didn't, then game over.
The final section of the forensic report is not a single conclusion about the way the blowout preventer is designed. Rather, it is a series of unanswered questions - matters that need further study in order to devise a safety system that actually works. Everything from understanding the buckling of the drill pipe to how the parts of the blowout preventer are operated remotely. It's hard to fathom how the Bureau of Ocean Energy Management, Regulation and Enforcement could be satisfied with continued deepwater drilling without the assurance that the exact same thing won't happen again, especially when the most painful irony of all is that what bent the drill pipe in the first place is likely the very blowout the blowout preventer is meant to deal with. (page 4)
Rachel also points out in the exclusive TRMS Investigates report below that the emergency response plan filed with the application for the newly issued drilling permit for Noble Energy is dated September 2009 (pdf), before the lessons of the Deepwater Horizon disaster.