The deep water gas charged accumulator and its possible replacements
Mir Rajabi, Mehdi
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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. Gas inside gas charged accumulators does not behave like an ideal gas as we go to very deep water, due to high hydrostatic pressure at that water depth. The higher the ambient pressure, the more the gas behaves like a real gas rather than an ideal gas and the lower the fluid capacity of the accumulators. Compressed gas has energy in it, and can release this energy at the time desired, that??s why it is used in accumulators. Now, we have to look for something that is able to store energy, but unlike the nitrogen, its functionality must not be affected by the increasing hydrostatic pressure of water as a function of water depth. Springs and heavy weights will be discussed as two options to replace nitrogen in accumulators. Efficient deep water accumulators would reduce the number of accumulators required in deepwater and cut the cost of the project. With the advent of such efficient accumulators, we can hope that one of the numerous problems of deepwater drilling has been solved and we can think of drilling in even deeper waters.