Structure of Gas Networks

Natural gas networks are complicated, starting from the wells to end users. Natural gas is found in some places underground, and there are many exploration methods to determine whether or not natural gas exists in a particular place. After investiga­tion, the act of drilling starts and a well is made, a process called extraction. After technically and economically ensuring that recovering the existing gas is feasible, the gas is lifted up. This gas cannot be used in its raw state and needs to be pro­cessed. Refining stations are usually close to the wells to separate parts of the gas and prepare it for customer use. For natural gas, pipes are the usual mode of trans­portation. The pipes that deliver natural gas from the wells to refinery stations are called gathering pipes-, they are usually low pressure and low diameter. If the gas extracted from a well has more than the standard amounts or levels of sulfur and carbon dioxide, special types of gathering pipes are required; the so-called sour gas is dangerous, and care should be taken in its transportation.

Because gas wells are usually located in places far from customers, a complex system is needed to deliver the gas. A transportation system called the mainline system of gas networks is used to deliver the gas through transmission pipes and compressor stations to customers. Transmission pipes are usually of high pressure, large diameter, and long distance. The task of compressor stations is to balance the gas pressure in the pipes. The mainline system needs large amount of investment, ^proximately 80% of total investment. The amount of investment depends on the Parameters of the system, including pipe diameter, thickness, pressure, length, and compression ratio. A large number of articles have tried to optimize this system from various aspects. Ruan et al. [1] presented a mathematical model that took into

4{iMlcs Operations and Management. DOI: 10.1016/ВЧ7Н-0-12-385202-1.00020-7

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account all of the parameters important to the amount of investment. Kabiriana and Hemmati |2) presented a strategic planning model to determine the type, location and installation schedule with a cost-minimization objective function. Cheboubaa et al. [3J proposed a metaheuristic algorithm called ant colony optimization (ACO) to determine the number of compressor stations and the discharge pressure for each.

In the next step in gas networks, which is called distribution, gas is delivered to the end user. Local distribution companies (LDCs) receive the gas in city gates transfer points from transmission pipes to LDCs, and deliver it to individual custom­ers. This delivery is done with the help of an extensive network of small-diameter distribution pipes throughout municipal areas. End users of natural gas from LDCs are residential, commercial, and industrial sectors and power-generation customers. Note, however, that some large commercial and industrial customers receive natural gas directly from the high-pressure pipelines. Literature is extensive on different aspects of gas distribution. Hamedi et al. |4] presented a six-level supply chain to minimize the cost of gas transmission and distribution. Generally, there are many articles related to the transmission or distribution of gas, among which Herran- Gonzalez et al. [5]; Martin et al. [6]; Rios-Mercado et al. [7]; Wong and Larson [8]; and Wu el al. 19] can be mentioned.

Natural gas is not always used when delivered, so it is usually stored under­ground. This storage capability can be very helpful, especially when shortages occur in the network. Gas networks also use physically manipulated and automated controls such as the supervisory control and data acquisition (SCADA) system to ensure appropriate communications between equipment and control center.

Figure 20.1 demonstrates the schematic view of gas networks with all key parts shown.