BOP Accumulator Requirements & Inspections
The system used for closing the BOP's is a high-pressure hydraulic fluid accumulator unit. Hydraulic fluid is stored under pressure, the pressure being provided by stored nitrogen. When hydraulic oil is forced into the accumulator by a small volume, high-pressure pump, the nitrogen is compressed, storing potential energy. When the BOP's are activated the pressured oil is released, either opening or closing the BOP's.
https://www.drillingmanual.com/bop-accumulator-units-rig-operations-requirement/