A BOP booster assembly is provided for use with BOPs that utilize hydraulic actuators mounted in BOP end caps to open the BOP for replacement of the shearing members. The booster assembly has three main components comprising a booster housing, piston, and end plate. An extension in the booster housing is sufficiently long to position the booster hydraulic cylinder axially outside of the ends of the bonnet end caps of the BOP with respect to the wellbore. The booster piston is equal in diameter or larger than the operating piston of the BOP. The operating pistons and booster pistons move simultaneously, in sync, and the same distance for closing and cutting. In one embodiment, an internal hydraulic line is provided in the booster cylinder wall.

A high power, compact BOP booster and bonnet assembly for rams and, more particularly. Solid ram change pistons or bonnet pistons are in bonnet end caps that are hydraulically controlled. The bonnet pistons move from either an open position or a closed position to urge the booster and bonnet assembly replacement and installing the BOP shear members.