Blowout preventers (BOPs) are used to control blowout. The crew usually install several blowout preventers (BOP stack) on top of the well, with an annular blowout preventer at the top and at least one pipe ram and one blind ram blowout preventer below. Also, some well control techniques require both the annular and the ram blowout preventers.

An annular blowout preventer has a rubber sealing element that, when activated, seals the annulus between the kelly, the drill pipe, or the drill collar. If no part of the drill stem is in the hole, the annular blowout preventer closes on the open hole.

Ram blowout preventers are large steel rams, that have sealing elements. One type of ram blowout preventer is called a pipe ram because it closes on the drill pipe; it cannot seal on open hole. Blind ram blowout preventers are straight-edged rams used to close an open hole.