Blowout preventers (BOPs) are specialized valve assemblies that prevent uncontrolled flow of formation fluids during well drilling operations. Formation fluids begin to flow up the annulus when there is a shift in the balance of the hydrostatic pressure. The BOP is designed as a fail-safe mechanism that safe-guards the well until hydrostatic balance is regained, most commonly through circulation of fluids in the well.
Blowout preventer stacks or assemblies contain several BOPs stacked on top of each other. There are two basic type of BOPs; annulars and rams. Annulars are used to close the wellbore when a formation kick is first observed. It consists of an elastomeric packing unit and a piston that seals the annulus or openhole. Rams are used as a secondary measure to shut the wellbore when hydrostatic pressure remains unbalanced. Rams consist of opposing piston or plungers and operate analogous to a gate valve.