Transocean's semi-submersible drilling vessel Deepwater Horizon was finishing work on a wellbore that had found oil 18,000 feet beneath the seafloor, in mile-deep water fifty miles off the Louisiana coast. Supervisors in the control cabin overlooking the drilling operations area were directing routine procedures to cement, plug and seal the borehole, replace heavy drilling fluids with seawater and extract the drill stem and bit through the riser (outer containment pipe) that connected the vessel to the blowout preventer (BOP) on the seafloor.
