Blowout preventer is an important well control device that plays a crucial role in drilling, especially in the process of underbalanced drilling. The purpose of designing a blowout preventer is to close the wellbore under pressure, maintain continuous control of the well, and circulate the formation fluid entering the well.

Hydraulic blowout preventers mainly come in two forms: gate blowout preventers and annular blowout preventers. The gate blowout preventer is mainly composed of a housing, side doors, oil cylinders, cylinder heads, pistons, piston rods, locking shafts, seals, gate plates, etc. Its working process is to use hydraulic pressure to push the piston, drive the gate to close or open, thereby closing or opening the wellhead, achieving the purpose of sealing and opening the well.

The gate of the gate blowout preventer can be divided into four types: fully sealed, semi sealed, variable diameter, and shear gate. It is used to achieve different conditions of well sealing. The semi sealed gate can seal the corresponding drilling rods and oil pipes, and if necessary, it can also suspend the drilling tools. When there are no pipes in the well, the fully sealed gate can fully seal the wellhead. In special circumstances, the shear gate can cut off the drilling tools. In addition, the flange outlet on the side of the shell can be used for drilling fluid circulation and throttling well killing operations.

The types of annular blowout preventers can be divided into conical annular blowout preventers and spherical annular blowout preventers according to the shape of their sealing rubber cores. Their structure mainly consists of four major parts: shell, top cover, rubber core, and piston. When there are drilling tools, tubing or casing in the well, the annular blowout preventer can use a rubber core to seal various sizes of annular spaces. When there are no drilling tools in the well, it can fully seal the wellhead during drilling, coring, logging and other operations. It can also seal the annular space formed by square drill rods, coring tools, cables and steel wires and the wellbore. With the use of pressure reducing and regulating valves, the drilling tools can be forcibly lifted out through the drill rods.