What you see in this photo is an accumulator. It is one of the most important part of blow out preventers.
An accumulator is a device installed in hydraulic systems primarily to store energy which can be released quickly and transmitted to the rest of the system whenever this energy is needed to perform operations.
Apart from energy storage, accumulators also serve as cushions to pressure fluctuations which is a common occurence with positive displacement pumps.
The accumulator can be canisters filled with hydraulic fluid and a pressurized gas cap or it can be a piston type.
Hydro-pneumatic accumulators (the ones with hydraulic fluid and pressurized gas cap) are most common in oil and gas installations.
The gas in hydro-pneumatic accumulators is usually an innert gas with high compressibility like nitrogen.
The hydraulic fluid (could be light oil) is normally not as compressible as the gas but it has the potential to transmit power almost instantenously whenever the accumulator is activated.
The gas expands and pushes the hydraulic fluid which in turn runs through the system to either open or close the arms of the blow out preventer (BOP).
This is a critical aspect of a drilling rig system. The canisters of the hydro-pneumatic accumulator should always be checked for fluid leakage and pressure compliance. It is so charged that only a 2% reduction of total volume in the canisters can drastically reduce pressure.
The accumulator thus can act as an auxillary power source whenever the main power source fails.