What you see in this photo is an accumulator. It is one of the most important part of blow out preventers.
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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.
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Apart from energy storage, accumulators also serve as cushions to pressure fluctuations which is a common occurence with positive displacement pumps.
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The accumulator can be canisters filled with hydraulic fluid and a pressurized gas cap or it can be a piston type.
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Hydro-pneumatic accumulators (the ones with hydraulic fluid and pressurized gas cap) are most common in oil and gas installations.
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The gas in hydro-pneumatic accumulators is usually an innert gas with high compressibility like nitrogen.
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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.
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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).
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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.
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The accumulator thus can act as an auxillary power source whenever the main power source fails.
