Possible Alternatives for Deep-Water Gas Charged Accumulators

Abstract

Gas Charged Accumulators are widely used in Drilling operations; however, the current Accumulator design methods are inadequate for Deepwater Drilling. Gas-Charged Accumulators are used in subsea drilling as well as production operations. One important application of accumulators is in blowout preventers (BOPs). BOP’s are designed to shut in a well under pressure so that the well control procedures could be implemented. Control Systems for the BOPs should be highly efficient hydraulic systems and operate 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. Gas Charged Accumulators are the most commonly used accumulators 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 should be 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. We have to look for alternatives to Gas Charged Accumulators. It has to be something that is able to store energy, but unlike the nitrogen, its functionality should not be affected by the increasing hydrostatic pressure of water. The possibility of the use of springs and heavy weights as possible replacements for nitrogen in structure of accumulators will be discussed in this paper. High hydrostatic pressure of deepwater should 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 extent 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. 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. Key words : Drilling operations; Deep-water gas charged accumulators; BOPs