Blowout preventer (BOP)

One of several valves installed at the wellhead to prevent the escape of pressure either in the annular space between the casing and drill pipe or in open hole (i.e., hole with no drill pipe) during drilling or completion operations. Blowout preventers on land rigs are located beneath the rig at the land’s surface; on jackup or platform rigs, at the water’s surface; and on floating offshore rigs, on the sea floor. See annular blowout preventer, inside blowout preventer, ram blowout preventer.