"Blowout preventers are designed to shut in a well under pressure so that formation fluids that have moved into the wellbore can be contained and circulated out while continuous control of the well is maintained. Control Systems for the BOPs are of necessity highly efficient hydraulic systems. The objective is to operate functions, such as closing rams, on the BOP stack in as short a time as possible. Supplying enough volume of pressured hydraulic fluid to operate those emergency functions is essential. To have the necessary quantity of control fluid under pressure requires storing this fluid in accumulators. These accumulators operate by the expansion and compression of nitrogen gas that is separated from hydraulic fluid by either rubber bladders or pistons. Accumulators are used both on the surface and at the seafloor. As long as you use accumulators on the surface or in relatively shallow waters, you may not have a problem with the volume of hydraulic fluid capacity of gas charged accumulators. The problem may arise when the wellhead is at water depth of more than 3500 ft. In deep water drilling, the accumulators should be placed on the subsea blowout preventer stack to reduce hydraulic response times and provide a hydraulic power supply in case of interruption of surface communication. Accumulators are also used in subsea production control systems to provide local storage that allows smaller line sizes in control umbilicals. Hydraulic fluid capacity of an accumulator drops to 15% of its capacity on the surface and even less, depending on the water depth. A large number of accumulators are needed to perform BOP functions that could have been done by just a few of them on the surface or at relatively shallow water depth."
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