The opening and closing of the BOP’s is controlled from the rig floor. The control panel is connected to an accumulator system which supplies the energy required to operate all the elements of the BOP stack. The accumulator consists of cylinders which store hydraulic oil at high pressure under a compressed inert gas (nitrogen). When the BOPs have to be closed the hydraulic oil is released (the system is designed to operate in less than 5 seconds). Hydraulic pumps replenish the accumulator with the same amount of fluid used to operate the preventers (Figure 26).
The accumulator must be equipped with pressure regulators since different BOP elements require different closing pressures (e.g. annulus preventers require 1500 psi while some pipe rams may require 3000 psi). Another function of the accumulator system is to maintain constant pressure while the pipe is being stripped through the BOPs.