Possible Alternatives for Gas-Charged Accumulators in Deep Water

Abstract

Abstract Gas Charged Accumulators are widely used in Drilling operations. These accumulators are not efficient at all in Deep waters, and there are not many alternatives for them. This paper looks into possible alternatives for Gas Charged Accumulators in Deep Waters. Supplying enough volume of pressurized hydraulic fluid to operate the BOPs for emergency situations is essential for Deep Water Drilling. This requires storing the pressurized hydraulic fluid in accumulators. A problem may arise when the wellhead is at water depth of more than 3500 ft. In deep water drilling, the accumulators are placed on the subsea BOP stack to reduce hydraulic response times and provide a hydraulic power supply in case of interruption of surface communication. Hydraulic fluid capacity of an accumulator may drop to 15% of its capacity on the surface and even less, depending on the water depth. The reason for this is that the nitrogen gas does not behave like an ideal gas as we go to very deep water, due to high hydrostatic pressure at that water depth. The possibility of the use of springs and heavy weights as possible replacements for nitrogen in the structure of accumulators will be discussed in this paper. High hydrostatic pressure of deepwater will not affect the functionality of these mechanical accumulators. Transferring bank of accumulators to the surface and connecting them to the BOP with properly sized and rigid pipes can decrease response time to an acceptable level to satisfy regulations and standards. This idea can be considered as an alternative solution too. We have to include the hydrostatic pressure of water in the usable fluid calculation. A low pressure tank located on the sea-floor can dismiss the negative effect of high hydrostatic pressure of seawater. This alternative idea is also discussed. Using a column of high density fluid as the accumulator system is another alternative method that is presented in this paper and should be investigated further for such application. Introduction Since the 1960's gas charged accumulators have been placed on subsea blowout preventers to reduce hydraulic response times and provide a local 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.1 Usable Fluid, which is declared as the amount of pressurized liquid that an accumulator can hold, noticeably decreases as drilling and subsea production moves to ever-deeper waters so that a large number of accumulator bottles is needed to store liquid required to do close and open functions in that depth of water. This issue of gas charged accumulators introduces itself as one of numerous obstacles to ultra-deepwater drilling technology. This behavior of accumulators is in part because of non-ideal behavior of compressed gas, usually nitrogen, in high ambient pressure at the sea floor where accumulators are located. Even if, nitrogen behaves like an ideal gas, the volume of usable fluid decreases, since the hydraulic fluid exhausts to the sea-water to reduce the length of umbilicals and pressure drop. So, the calculation of usable fluid should compensate for the hydrostatic pressure of water depth where hydraulic fluid is supposed to exhaust. Usable fluid volume of an accumulator decreases as the depth of operation of that accumulator increases. Fig. 1 shows how the volume of usable fluid decreases as water depth increases. This graph is plotted for a 15-gallon bladder accumulator (Vac = 13.7 gal.) with a maximum working pressure of 5,000 psi, minimum working pressure of 2,000 psi, and a precharged pressure of 1,800 psi.