API 16A Drilling Annular BOP

Blowout Preventers (BOPs) are safety devices used to "prevent" the uncontrolled flow of liquids and gases during well drilling operations.
Blowout preventers are large, high-pressure valves that are capable of being remotely controlled. When the driller closes the valve, a pressure-tight seal is formed at the top of the well, preventing the fluids from escaping. There are two major types of Blowout Preventers, Annular (also known as Spherical) and RAM. Annular BOPs are usually mounted to the very top of the BOP stack. The drilling crew then mounts two, three, or four RAM BOPs below the Annular preventer. If a kick is detected, the annular is usually closed first and then the RAM is used as a backup if the Annular should fail.

Annular blowout preventer

An annular-type blowout preventer can close around the drill string, casing or a non-cylindrical object, such as the kelly. Drill pipe including the larger-diameter tool joints (threaded connectors) can be "stripped" (i.e., moved vertically while pressure is contained below) through an annular preventer by careful control of the hydraulic closing pressure. Annular blowout preventers are also effective at maintaining a seal around the drillpipe even as it rotates during drilling. Regulations typically require that an annular preventer 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.