Blowout Preventer: (BOP) Here we can see 2 single set of ram (bonnets) and an annular stood back on the test stump, The lower  set are designed to hold  shear rams the upper set t0 hold  pipe rams

At some time or other you will bump into a set of BOPs. Providing that is,  you work in the oil field. I'm am sure the initials BOP mean something entity different in other industries. When people talk about the blowout preventers they will often refer to it as: a) the Stack  b) the BOP's c) the BOP all are the same but there are different type. d) the BOPE this stands for Blowout Preventer Equipment The BOP system are a set of  valves installed on the wellhead to prevent the escape of pressure either in the annular space between the casing and drill pipe or in open hole ( hole with no drill pipe) during drilling, completion and workover operations.   Here we can see the BOP installed on the well Most BOP stack are designed with a triple ram configuration which provides positive protection against blowouts and secures the well in emergencies. This permits work to be carried out under pressure on surface equipment while the pipe is still in the wellbore. The BOP is placed directly above the wellhead. The BOP closure design consists of an hydraulic actuator assembly connected to each of the two ram assemblies. This allows the rams to be hydraulically compressed around the line from opposite sides which effects a seal thus containing well pressure below the rams.  Older bops would need the the choke and kill lines on different   level or two kill lines so that pressure could be equalized in the event of stripping but now days bops are being designed with this in mind and the pressure across the rams is equalized, through an integral equalizer assembly located between each ram assembly. This takes place prior to hydraulic retraction of the rams and opening of the valve. This system enables the well bore pressure to be equalized from the bottom to the top of each ram assembly. Ram assemblies may be interchanged to create different configurations. Manual overrides which operate the rams in case of hydraulic failure are also part of the system with handles and . These overrides can also hold the rams mechanically locked in the closed position. An innovative attribute on the BOP is the plumbing feature. Single point open and close ports in each modular body permit operation of both hydraulic for each set of rams.  BOPs are available in nominal sizes from 3 1/16" to 30" , with working pressure ratings from 5000 PSI to 20000 PSI. The BOPs are useable in both standard and sour environments. Standard ram assembly configurations would normally be as such : !) Pipe seal rams - set of rams that serve to seal around the top string  isolating the well bore pressure below the rams 2) Shear/seal rams - top set of rams that serve to shear the tubing and seal off well pressure3) Pipe seal rams - set of rams that serve to seal around the top string  isolating the well bore pressure below the rams metal seal adapters, in conjunction with O-rings or rubber are used to make a metal to metal seal on the body to body connection. This configuration results in a highly effective seal at both low and high pressures.