Supposedly BP had the engineering know how to determine if they were buying a pig in a poke. Caveat emptor and all that.
Moreover, a top to bottom inspection is supposedly ongoing in a facility of the complexity of a refinery. If BP had kept the plant in good condition, felony fines would not have been imposed on them.

As noted by Dick Greenhouse, BP has a terrible operating record.

And no report more damning than the one on the Gulf blowout aired on "60 Minutes" and linked in another thread:
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60 Minutes