Transocean, the owner and operator of the drilling rig that caught fire and sank in the Gulf of Mexico last year, has concluded in an internal investigation that oil well owner BP was primarily responsible for the massive oil spill that resulted. In a report issued Wednesday, Transocean blamed BP for “faulty” well design, failure to check the integrity of cement used to seal the well, and a plan to close up the well that “created unnecessary risk.” Transocean acknowledged some shared fault for its crew’s failure to detect the signs that hydrocarbons were breaching the cement and steel barriers in the Macondo well and making their way up the drill pipe.