The individual Blowout Preventers (BOPs) that constitute a system are stacked vertically atop the well. They are aligned to allow access through them into the wellbore while their various functions provide a variety of methods for sealing the well during drilling, completion or intervention operations. They are sized to fit the internal diameter of the wellhead and expected surface pressures. Blowout Preventers (BOPs) typically divided into 2 stacks, there are:
Annular BOPs were introduced to the industry in 1946 by Granville Sloan Knox. These devices force circular steel-reinforced rubber elements to close on and create a seal around drillpipe or other tools that may be in the wellbore at the time of shut-in. When annular BOPs are shut in around a pipe, the pipe may be moved up or down or rotated without breaking the seal. Designed to prevent flow up the casing-drillpipe annulus, annular BOPs are also able to seal a clear wellbore in which no obstruction is present, so this reduces the rated working pressure of the sealing element by 50%.
Ram-type BOPs include rubber-faced steel rams that are brought together to create a seal or form a seal around a tool in the wellhead. The rams are opened and closed by operating pistons on the sides of the BOP body. When open, the rams leave an unobstructed passage through the wellbore. When closed, the rams form a seal around the drillpipe. Shear rams are high-strength hydraulically powered rams that are able to sever drillpipe. They include casing shear rams and blind shear rams. Blind shear rams can completely seal the well. Casing shear rams, while not able to form a seal, typically are able to cut larger size tubulars than those that blind shear rams can cut