BOP is used to close the annular space between casing strings, and completely close the wellhead. In closed cases, drilling fluid circulation, throttling and blowout, kill operation can be performed through the pipeline connected to the lower  four-way or shell side outlet. In special circumstances, the drilling tools can be cut off to achieve the purpose of sealing wells.

According to the structure, it can be divided into annular BOP, single ram BOP, double ram BOP.