The factor called "time to first oil" is a key concept in drilling economics, in some way more important than how much cash is spent.
Two things related to blowout preventers have concerned people who are trying to reduce time to first oil:
1) the time spent in testing BOPs on station; and
2) what to do when a BOP fails a pressure test.
Because the BOP is on the critical path for deepwater drilling, and deepwater MODUs and drillships typically have no replacement subsea BOPs on board, a given project can lose the better part of a week tripping the BOP up and then back down from mile-deep waters. The following patent describes a response to the first concern, how to shorten the testing time with computer methods.
One item I look for when reading through the disclosed correspondence between Transocean, BP, and Cameron are concerns referring to the "MUX."
The Deepwater Horizon had a "MUX" style blowout preventer, MUX standing for multiplex control, meaning that the rams and shears were electronically controlled from the surface rather than relying on hydraulic lines. A typical MUX ram is dependent on stored energy down below (in the form of pressurized fluid inside the “accumulator bottles”).
Reading the industry literature circa 1997-2001 suggests to me that the operators like Transocean went to the MUX approach because it allowed faster disconnects than hydraulically controlled BOPs (fast disconnects can be important given that dynamically positioned ships sometimes drive off or drift off accidentally), and also it was supposed to cut maintenance costs. Also, MUX control lines are cheaper than hydraulic control lines.
While industry ads of the time talked much about the greater safety of the new MUX BOPs, according to the quality assurance paper on the House Energy committee website the MUX had its own set of failure modes, such as cable-connector failure. Ironically, the MUX controlled BOP was supposed to be a lot more robust than a hydraulically controlled one because it would instantly reveal failures down below, indicate the fix needed, and specify which parts were necessary.
From what I can tell the BOP on the Deepwater Horizon was a fairly early model in the MUX evolutionary tree, so it might have suffered more age effects than later models.
BOPs are custom built for a ship and so expensive that spares aren't kept ready to hand. Operators are reluctant to replace them, and will go to great lengths to recover BOPs accidentally dropped from a ship or floating platform, or from the end of the riser, and that landed in deep mud on deep seabeds. Even if it takes a month they have undertaken such efforts, at great expense -- one vessel that lost a BOP, and later found it, was the SAIPEM 10000.