page_title: Blowout preventer
section_title: Annular blowout preventer
hierarchical_section_title: Blowout preventer / Types / Annular blowout preventer
context_page_description: A blowout preventer is a specialized valve or similar mechanical device, used to seal, control and monitor oil and gas wells to prevent blowouts, the uncontrolled release of crude oil or natural gas from a well. They are usually installed in stacks of other valves.
  Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic event known as a blowout. In addition to controlling the downhole pressure and the flow of oil and gas, blowout preventers are intended to prevent tubing, tools and drilling fluid from being blown out of the wellbore when a blowout threatens. Blowout preventers are critical to the safety of crew, rig and environment, and to the monitoring and maintenance of well integrity; thus blowout preventers are intended to provide fail-safety to the systems that include them.
  The term BOP is used in oilfield vernacular to refer to blowout preventers. The abbreviated term preventer, usually prefaced by a type, is used to refer to a single blowout preventer unit. A blowout preventer may also simply be referred to by its type.
context_section_description: The annular blowout preventer was invented by Granville Sloan Knox in 1946; a U.S. patent for it was awarded in 1952.  Often around the rig it is called the "Hydril", after the name of one of the manufacturers of such devices.
  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.
  An annular blowout preventer uses the principle of a wedge to shut in the wellbore.  It has a donut-like rubber seal, known as an elastomeric packing unit, reinforced with steel ribs.  The packing unit is situated in the BOP housing between the head and hydraulic piston.  When the piston is actuated, its upward thrust forces the packing unit to constrict, like a sphincter, sealing the annulus or openhole.  Annular preventers have only two moving parts, piston and packing unit, making them simple and easy to maintain relative to ram preventers.
  The original type of annular blowout preventer uses a “wedge-faced” (conical-faced) piston.   As the piston rises, vertical movement of the packing unit is restricted by the head and the sloped face of the piston squeezes the packing unit inward, toward the center of the wellbore.
  In 1972, Ado N. Vujasinovic was awarded a patent for a variation on the annular preventer known as a spherical blowout preventer, so-named because of its spherical-faced head. As the piston rises the packing unit is thrust upward against the curved head, which constricts the packing unit inward.  Both types of annular preventer are in common use.
caption: Diagram of an Annular blowout preventer in open and fully closed configurations. The flexible annulus (donut) in blue is forced into the drillpipe cavity by the hydraulic pistons.