Natural Gas Network Introducing

The aim of the natural gas network is to satisfying consumers' demands efficiently and at minimal cost. Therefore, some components should be used properly through the network, and some processes should be planned exactly. In this section, the main components of the natural gas network and its main processes are explained.

Natural Gas Network Components

Like other networks, natural gas network components can be divided into two physical categories: fixed entities and current entities.

Fixed physical entities include arcs, which correspond with pipelines, compres­sor stations, and valves and nodes that present physical interconnection points.

• Arc components

— Pipelines: The two types of pipelines of concern to researchers are passive pipelines, which correspond to regular pipelines, and active pipelines, which are regular pipe­lines with compressors [2J.

— Compressor stations: The transmission capacity of a gas pipeline is limited but can be arranged based on the supply-and-dcmand nodes by setting differences among input and output pressures of the pipeline. Compressors are locatcd at suitable locations through the network to enlarge the pressure differences between two nodes of pipe­lines to increase the network's transmission capacity [2J.

— Valves: To make the flow of natural gas stop for a certain section of pipelines in situa­tions such as maintenance or replacement, valves are used along the entire length of interstate pipelines.

• Node components. As shown in Figure 19.1, components belonging to nodes in the natu­ral gas network include the following:

— Supply nodes, which have only output flow

— Demand nodes, which have only input flow

— Intermediate nodes, which have both input and output flows

Current physical entities can be classified as financial, informational, and physi­cal flows. Usually, flows in the natural gas network are controlled by a dispatcher or dispatching organizations. These organizations obtain information about natural gas pressures and flows over pipeline systems and check warning signals from companies via simulation systems.

Natural Gas Network Processes

The natural gas is supplied through gas and oil wells and produced in refineries. In the natural gas network, some methods exist to move gas from producers to consu­mers in order to satisfy customers' demands, but the pipeline system is the most cost-effective way to transport gas over long distances. Consumers of natural gas are divided into three main groups: domestic and commercial subscribers, industrial consumers, and exports. Usually, the priority of natural gas networks is to serve domestic and commercial consumers.

(Щ) Gas and oil wells ^^ Compressor stations Gas flow

© Refineries City gate stations Data-Information

® ® О Types of consumers Figure 19.1 Natural gas network components.

Natural gas suppliers and natural gas consumers are connected through a com­plex and huge network in such a way that there is a long distance between them, and natural gas must flow by the use of suitable pressure. During long transporta­tion, pressures are lost because of friction between the natural gas and the inner walls of pipelines. In addition, the natural gas volume is reduced because of heat transmitted from the environment. Therefore, compressor stations are installed to set and hold pressure continuity along the network and to periodically determine the capacity of the transmitted gas.

Arcs: Pipelines < Compressor station City gate station

Demand nodes

Supply nodes

Generally, compressor stations are one of the most complex entities in a natural gas network because they consist of several compressor units (typically 15—20) that have teen connected in different configurations such as series, parallel, or a combination of both, and they have different types [3]. Two of the main types of compressor units are centrifugal and reciprocating units. Centrifugal units are more common in the industry and consequently in related research assumptions [4]. Without considering which type °f the compressor unit is used in the model, both types of unit have two options— turning on and turning off—which makes their behavior nonlinear. When the demand °f the customer increases, the pressure of the pipeline drops. Therefore, at least one compressor should be opened until the gas pressure resumes an acceptable level [5].

The whole process of the natural gas network can be concluded in four main parts: supply, transportation, storage, and sale of the natural gas in the market places.

Supply

Supply usually starts with development and exploration, extracting the gas reserves, and processing the extracted gas. In practice, the same company performs all three functions. Exploration is concerned with locating natural gas and petroleum depos­its. After a team of exploration geologists and geophysicists has located a potential natural gas deposit, in the extraction phase a team of drilling experts digs down to where the natural gas is assumed to be [30].

Transportation

Natural gas transportation is the most important process in the natural gas indus­try. It consists of a complex pipeline network that moves natural gas from various origins to consumers in order to satisfy their demands. The network is divided into two main networks, namely, transmission and distribution. Moving a large volume of gas at high pressure over long distances from a gas source to distribu­tion centers is done in transmission networks. Routing gas to individual consum­ers is done through distribution networks [6]. The efficiency of transportation is a suitable criterion to estimate the whole of a natural gas system's performance. Considering the aim of the network and consequently the physical characteristics of its components, the natural gas network can be split into the transmission net­work and distribution networks. The scope of both networks is presented in Figure 19.2.

Transmission network: Rios-Mercado et al. 16] have shown how a gas transmis­sion network, including pipelines, junction nodes, and compressor stations, is dif­ferent from conventional networks by two special characteristics. (1) Beside the flow variables, which present the mass flow rate for each arc, a pressure variable is

Transmission network Figure 19.2 The process of transportation during the natural gas network.

defined at every node. (2) Unlike most networks, which consider only mass flow balances, transmission networks take into account two other types of constraints:

. A nonlinear equality constraint on each pipe caused by the connection that exists between

flow and pressure drop.

. The feasible limits that are available to operate pressures and flows inside each compres­sor, which are represented by a nonlinear and nonconvex set.

Usually, the operating expenses of the natural gas transmission network are esti­mated through the operating costs of the compressor stations, which can be deter­mined based on the fuel consumed at each compressor station. For example, Borra/.-Sanchez and Rios-Mercado [7] estimated that 3—5% of transmitted gas is consumed by compressor stations. Because a huge volume of natural gas is being transported through the network, about 25—50% of the total operating budget of companies is spent on running the compressor stations. Therefore, minimizing the total fuel consumption of the compressor stations along the network is one of the main objectives for transmission networking because of its effects on overall gas operation costs [6].

Distribution network: Distribution networks are different from transmission net­works in several perspectives. They do not have valves, compressors, or nozzles, and pipes act under fewer pressures. Therefore, pipelines are smaller, and networks are simpler [8]. Most natural gas users, which are domestic and commercial consu­mers, receive the gas from local distribution companies. A smaller number of natu­ral gas users such as power-generation companies receive the natural gas directly from high-capacity interstate and intrastate pipelines. In large municipal areas, local gas companies usually deliver gas to users through stations called city gates. For the design of distribution networks, first the network topology should be defined by technical teams; second, required features must be determined so pipe­lines and pumps can meet the flow of their nodes and pressure requirements [8].

Storage

Because natural gas processes, including exploration, production, and transporta­tion, are time consuming and because all of the produced natural gas is not always needed at various destinations, a part of the extra gas is injected into storage units, which usually are located near market centers and are usable for unlimited periods. Gas storage is one of the new and critical steps of the natural gas network process that must respond to the demands of different periods of the year. Traditionally, during summer months, natural gas was stored to respond to increased demands during the coldest months, but nowadays natural gas demand in summer has •ncreased because special users such as power-generation companies must produce electricity for air conditioners during summer. In addition, natural gas storage plays ^a critical role in unexpected events such as natural disasters, which may affect pro­duction and transportation. In general, some of the main reasons behind using stor- ^aBe along the natural gas network are its capability to respond to cyclic fluctuations when temperatures vary and consumption is high, improving services ¹⁰ all customers, keeping market shares competitive with other sources of energy,
and achieving operations with high load factors. Natural gas storage can be done i_n different ways, but underground reservoirs are the most important method. The storage deals with pipelines, local distribution companies, producers, and pipeline shippers (US Department of Energy, US Energy Information Agency, March 1995).

Sales and Marketing

Marketing natural gas means selling natural gas or organizing its business from well to end users at various levels. At this stage, all of the required intermediate steps are considered: transportation arrangements, storage, accounting, and espe­cially sales. Marketers in the natural gas industry play a complex role and may be joint endeavors with producers, pipelines, or local utilities or may be an indepen­dent group concerned with selling natural gas to retailers or end users. Natural gas usually has three to four owners before reaching customers. Marketers utilize their skills to reduce their exposure to risks and increase throughput by forecasting the behavior of the natural gas market, finding buyers, and securing ways to deliver the natural gas to end users.