Over the past few decades, there has been an increase in the size and complexity of pipeline systems. This has given rise to environmental and safety concerns that require a more sophisticated level of monitoring and control, necessitating the centralised monitoring of pipelines and system automation. At the core of such an arrangement is supervisory control and data acquisition (SCADA) technology.
SCADA accomplishes the task of monitoring and controlling the flow of natural gas through a pipeline by supplying gas flow data to centralised gas control stations. The gas stations then assimilate and manage the data provided by SCADA and the compressor stations. This information, which mainly comprises the pipeline’s flow rate, operational status, pressure and temperature readings, is further used to assess the status of the entire pipeline network at any point of time. The application of SCADA in city gas networks has become an inevitable trend as it plays a significant role in detecting leakages and preventing environmental hazards. SCADA also enables city gas distribution (CGD) companies to modernise the management of gas transmission and distribution scheduling.
Evolution of SCADA
The first SCADA system involved the development of simple local logic controllers that used electromechanical relays to execute the logic. Over the years, SCADA systems have evolved with the development of pipeline automation technology. The first field control systems in the pipeline industry were confined to a particular plant facility without any remote or centralised controls, which meant that operators at control centres had to contact local operators via telephone or radio to actuate or halt equipment like pumps or compressors. The first step towards centralised automation was the introduction of remote telemetry, which enabled the monitoring of key pipeline parameters on a remote meter.
The next step in the evolution of control systems was the development of personal computers and the introduction of local and wide area networks that could kick-start the pipeline automation system. This not only obviated the need for electromechanical relays but also made business operations affordable. The deployment of IT in almost all areas of the pipeline industry led to the emergence of more advanced communications infrastructure. Consequently, the cornerstone components of a modern SCADA system were established, including field instruments and devices, station control using programmable logic controllers (PLCs) and remote terminal units (RTUs), master terminal units and a control centre.
The use of wireless instruments in pipelines and gas production operations has been gaining momentum in the past few years. Wireless instruments eliminate the need for expensive trenching and cabling and also provide access to hard-to-reach areas using self-contained battery-powered instruments. A web-enabled SCADA system allows a plant to be accessed and controlled from anywhere in the world via the internet. It can connect to multiple servers and clients in an organisation as well as at remote locations to control and monitor processes in a secure manner by authorised users only. The web-enabled system is capable of sending maintenance reports and breakdown reports via automatic emails.
Functioning of SCADA
A SCADA system usually consists of the following subsystems:
- Human-machine interface (HMI): The appar-atus that presents industrial process data to the operator who further monitors and controls it.
- Supervisory (computer) system: Acquires process data and sends commands.
- RTUs: Microprocessor-controlled electronic devices that are connected to sensors and can convert sensor signals to digital data, which is then sent to the supervisory system.
- PLCs: These are used as field devices because they are more economical, versatile, flexible and configurable than special-purpose RTUs.
- Communications infrastructure: Connects the supervisory system to RTUs.
A SCADA system is connected with the pipeline network through various components. The pipeline network is further connected to the digital/ analog sensors that gather data on gas flow, temperature and pressure. These sensors are, in turn, connected to RTUs or PLCs situated at various input locations (this could be the pipeline, compressor station or a gas storage facility). Each PLC and RTU is operated by a supervisor who controls these units and takes corrective steps in case of any disruption in gas flow.
Data from these remote devices then flows through the communication network to the SCADA master station, which displays it through the HMI. The SCADA master station continuously monitors all sensors and alerts the operator in case of any aberration or unusual activity like a higher- or lower-than-optimum gas flow pressure or temperature. The operator at the master station can access both historical and real-time data on the system, thereby making for more effective control.
The SCADA network model requires some form of communication medium for connecting the supervisory system to RTUs. The most popular kinds include microwave, radio, dedicated land lines or intranet networks using a mix of copper, fibre optics and wireless technology. The choice of medium depends on factors like cost, reliabi-lity, bandwidth and geography, among others.
In case there is a signal via the SCADA system in emergency situations, an emergency shutdown system ensures that the pipeline is shut down.
Benefits and drawbacks
A centralised SCADA system has a number of advantages. It provides accurate and real-time information so that gas pipeline operators can perform their tasks remotely. They can monitor and control product movement accurately and allow for the safe operations of a pipeline system, including the pump and compressor stations.
Use of SCADA by key CGD players
Many CGD companies have installed SCADA technology and are using it extensively for their gas operations. Reliance Gas Transportation Infrastructure Limited (RGTIL), GAIL (India) Limited, Adani Gas Limited, Gujarat State Petronet Limited (GSPL), Haryana City Gas Distribution Limited, Indraprastha Gas Limited (IGL), Mahanagar Gas Limited (MGL) and Assam Gas Company Limited are the major players in the CGD segment.
RGTIL uses a state-of-the-art SCADA system on the East West Gas Pipeline, which transports gas from Kakinada (Andhra Pradesh) to Bharuch (Gujarat). The company has deployed an optical fibre cable-based telecommunication system for effective long distance communication. Meanwhile, GSPL’s gas grid is equipped with the latest bidirectional gas transmission technology to enable a two-way gas flow. The network is continuously monitored through SCADA systems integrated with geographic information system technology. GAIL has also implemented a centralised SCADA system using the FAST/TOOLS data communication package for all its natural gas pipeline networks. It relies on dual redundant front-end processor servers for uninterrupted monitoring and control.
In addition, Adani Gas Limited’s network is completely SCADA controlled, and Assam Gas Company Limited has been employing SCADA systems in its operations since 2005. SCADA has proved helpful for utilities like IGL and MGL by enabling them to combine the monitored data with demand, environmental, physical and gas composition data across the enterprise for business decision support.
SCADA is even being used globally by major gas companies. For instance, the supervision of more than 200,000 km of Russia’s pipeline network is done through a SCADA system. The lessons gained by Turkey’s largest gas distribution firm, iGDAS, from implementing a SCADA system have helped detect gas failures in real time and enabled their immediate rectification.
Internet-enabled applications have been proliferating with the advances in communication networks and internet technologies. Even though the pipeline industry has not yet fully unlocked the potential of web-based technology, a closer integration between field and office information systems can now be seen. As gas utilities increase their adoption of technology solutions, the efficiency of the CGD segment is expected to improve. Economic, safety and regulatory pressures, along with the emergence of new market conditions in the oil and gas industry, are driving the development of next-generation SCADA platforms that will help ensure critical infrastructure remains online by providing backups and fail-safes.