A Method for Optimizing the Design of Gas Storage Systems

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Abstract The design of underground gas storage systems requires the selection of optimal values for several interacting variables. Some of these variables are related by nonlinear functions so that frequently used optimization techniques, such as linear programming, are less than precise and cumbersome to apply. precise and cumbersome to apply. A method of optimizing the variables, subject to the operating and physical constraints, has been developed and programmed for solution by digital computer. A Fibonacci Search routine is used for efficient convergence on the optimal solution. The relationships between the variables are preserved in original form, nonlinear where this is the case. The storage system includes the underground storage reservoir, the wells, the gathering system, the purification facilities, and the compressor station. purification facilities, and the compressor station. Because the gathering system is analyzed separately, only the average pressure loss from wellhead to compressor suction is entered into this system. The solution selects optimum horsepower for the withdrawal cycle. The reservoir is represented by the pressure-withdrawal relationship, and the producing ability of the wells by the back-pressure relationship. The variables subject to optimization are the volume of in storage, the producing rate of the wells, the number of wells, the size of the casing, the size of the purification system, and the size of the compression equipment used for withdrawal. The optimal solution is the set of values for the variables that will minimize investment. When the purification equipment is sized for the peak day volume, and the number and casing sizes of the peak day volume, and the number and casing sizes of the wells are temporarily considered fixed, the optimum-seeking computer program can be formulated as a two-variable, unimodal function that lends itself to efficient solution by the Fibonacci Search convergence technique. The program is designed to evaluate several casing sizes for any number of wells. By using the Fibonacci Search, the original range of uncertainty can be reduced to less than one percent in ten iterations. A tolerance of .001 percent can be achieved in 25 iterations. Introduction Variation in rates of gas consumption requires the use of some form of gas storage in conjunction with transmission and distribution facilities. Storage is used to supply additional volumes on peak demand days. it also permits the more efficient use of facilities during summer permits the more efficient use of facilities during summer months. Usually underground storage is the most practical means of gaining seasonal gas storage. An underground storage system contains several subsystems: the well casing, gathering, purification and compression facilities and the reservoir. The optimum design is the best combination of these subsystems. Gas transmission facilities should be operated at a high load factor. Interruptible sales are one method of accomplishing higher load factors.