Hydril type, or Tapered type BOP was the first annular preventer to incorporate into one unit the ability to pass tools and drill bits and to close on any shape present in the well bore. It used both externally applied hydraulic pressure and the well bore pressure to effect a closure.
Sanjack Tapered annular blowout preventer is the most common annular preventer in use in drilling rig, particularly in surface stack installations, and is unique in its response to well pressures. Like most other annular preventers, the preventer is closed with about 5,520 kPa (800 psi) pressure. The Sanjack Tapered annular blowout preventer with a working pressure of less than 69,000 kPa (10,000 psi) is however also energized by the well pressure so that when the well pressure increases, the closing pressure must be reduced to avoid damaging the packing unit. This wellhead pressure assist is derived from its piston configuration and applies also to the GL and MSP types of annular BOP preventers.