An annular-type blowout preventer can close around the drill string, casing, or a non-cylindrical object, such as the kelly. Drill pipes, including the larger-diameter tool joints (threaded connectors), can be “stripped” (moved vertically while pressure is contained below) through an annular preventer with careful control of the hydraulic closing pressure.
Annular blowout preventers are also effective at maintaining a seal around the drill pipe, even as it rotates during drilling. Regulations typically require an annular preventer to be able to completely close a wellbore, but annular preventers are generally not as effective as ram preventers in maintaining a seal on an open hole. Annular BOPs are typically located at the top of a BOP stack, with one or two annular preventers positioned above a series of several ram preventers.