IMO - When a corporation sub-contracts all or part of a project to ANOTHER company, that does NOT eleviate the primary corporation from ENSURING practices and standards are met.
IF BP did not have supervisory personnel on site at the time of construction, then BP is guilty of negligence, and as the OWNER/OPERATOR of the rig, responsible for any and all errors.
Pretty simple, really.
AFTER that, the internal squabbling about who did what will be handled in a never ending stream of civil suits.
Yes, BP has ultimate responsibility for the spill...period. But, the sub-contractors have responsibility to BP according to their contracts. A probable scenario might be:
1. BP, along with its partners, owned the lease...and ultimately the hole being drilled...and any oil or gas produced.
2. The rig and all its equipment was owned or leased by TransOcean. BP had no responsibility for the construction of or operation of this equipment. However their contract with each sub-contractor determined the necessary equipment and the operating perimeters of this equipment.
3. Service companies, such as Haliburton, were contracted to BP. They follow orders from the BP company man on site.
4. Any special downhole tools were probably leased by BP.
5. The casing was purchased by BP and the drill pipe was probably owned by Transocean, but might well have been owned by BP.
6. The BOP (blow out preventor) was supposedly mfg by Cameron. It was probably owned by Transocean. They will be responsible for its operation and any lack of maintenance. But, BP should have monitored these maintenance activities and tests.
7. Another person in question will be the Mud Engineer...who undoubtly was contracted to BP...but worked for whoever supplied the drilling fluid...and worked with the BP drilling engineers to decide the mud weight and viscosity.
Like I said, it will be an interesting law suit(s).