Tensions between oil giant BP and oil rig owner Transocean over which company is responsible for the oil spill in the Gulf of Mexico are expected to flare up Tuesday when two Senate panels hold hearings on the deadly incident. In prepared testimony before the Senate Energy and Natural Resources Committee, a top BP executive and the president of Transocean clashed over whether a piece of the machinery called the blowout preventer failed to prevent the well rupture, which led to the spill. In his opening statement made public by the committee Monday, Lamar McKay, president of BP North America, pointed to the blowout preventer as an essential "fail-safe" mechanism that closes valves when pressure causes fluid such as oil and gas to enter the well and threatens the rig. "Transocean's blowout preventer failed to operate," McKay stated in his written testimony.