Comment

1 December 2003

In the pipeline

What Users Need When They Select, Design, Implement a SCADA System.

By Rao Kalapatapu

Supervisory control and data acquisition (SCADA) system advances over the past few years make it possible to not only monitor processes, but also to provide features including advanced reporting and control, instant status and alarm reporting, and easy interfaces to Internet, intranet, and local area networks. Advances also allow connection to the distributed control system (DCS) and programmable logic controller (PLC) systems of the process. Current SCADA reports have an incredible amount of detail that manufacturers can use for accounting and custody transfer applications.

The remote communications capability is quite advanced in SCADA systems, where fiber-optic cabling, satellite communications, bridges, and routers are routinely used for backup, redundancy, and capturing data from hundreds of remote terminal units and PLCs scattered all over the world to a desired central location. In one location, a manufacturer can capture 10,000 miles of pipeline system process data and distribute it daily to pipeline managers and corporate management. In addition you can make the data available through a company’s intranet as well as through secure Internet.

Also the distinction between SCADA and DCS/PLC systems is diminishing, because vendors are now offering I/O systems with communications and network capabilities. In addition, the present SCADA system engineer needs to be familiar with information technology and communication systems by taking appropriate courses in these fields. In one system design and installation, the user added telephones, local and wide area networks, public exchanges for telephones (PBX), and radio systems for operation and maintenance of 1,000 kilometers of pipeline through fiber-optic-cable-based telecommunication systems. In another system, a manufacturer used a satellite-based communication system to collect data from 100 remote terminal units.

SCADA systems are leading the way for better control and monitoring of remote processes and plants in a cost-effective way with off-the-shelf software and hardware.

Better control and monitoring is a real asset for oil and gas pipelines, which typically consist of aboveground and/or underground piping, block valve and scraper trap stations, pumping (for oil) and compressor stations (for gas), storage, and oil and gas distribution facilities to the end users.

The onshore and offshore production facilities have oil, gas, and water well sites, piping, and gas oil separation plants/production platform (GOSP) sites. The plant process and utility facilities, as well as the operator controls and displays, are at the GOSP.

So when an end user decides to select a (SCADA) system for pipelines and production facilities there are three main items that become part of the system:

• Remote terminal units (RTUs)—field I/O units with network interface units
• SCADA control room equipment with host computers, operator consoles, and data-gathering units
• Communication/network equipment to connect the field RTUs with control room SCADA servers as well as other control systems in the control room

The pipeline facilities for which SCADA RTUs are provided include:

Departure terminal—The departure terminal is the start of the pipeline. It contains gas storage and valve facilities, including scraper launch and outlet flowmeter systems, to send the material (oil/gas) through the pipeline system.

Block valve stations—These stations consist of pipeline block valves at 15 to 20 miles along the pipeline (to isolate the pipeline in case of accidents) and a secondary pipe system (bypass) designed to balance pressures when reopening the valve.

Scraper trap stations—These stations have a scraper (for pipeline cleaning through a pig cleaner) launcher and receiver, valve assemblies, and piping. The sampling stations as well as the injection stations are inside a station (pipeline valve station [PS], PC, or compressor station) and are not separate in this specification.

Compressor/pumping stations—These stations are essentially composed of large compressors/pumps, turbogenerators/power supply drivers, valve and piping assemblies, field flow pressure and temperature instrumentation, scraper launching and receiving traps, and a metering system. The main controls and monitoring for these stations come from the compressor supplier in coordination with the DCS vendor. The SCADA system will monitor the incoming and outgoing main line valves and piping.

Cathodic protection stations—These stations should maintain the potential of the gas pipeline with respect to the ground at a value below –850 millivolts to protect against corrosion using a grounding system and protection power generation. You will need a small-size RTU for this purpose.

Arrival terminal—This terminal is the end of the pipeline. Similar to the departure terminal it has a scraper receiver, inlet and outlet valves, and flowmeter and communication subsystems.

The remote-control facilities are generally located in the arrival terminal control room for pipelines, including PC or minicomputer-based SCADA hosts, operator consoles, RTU polling, and data-gathering hardware and software units.

The pipeline operating philosophy comes from requirements for remote control and supervision of the individual stations of the pipelines. The functions handled by SCADA differ slightly for each type of site. The compressor stations usually have their own compressor designer/vendor–provided equipment and personnel. Hence the role of SCADA is remote monitoring and supervision of the compressor station relative to the overall pipeline operation.

Generally, the block valve stations, scraper trap stations, and departure terminal will have no permanent personnel. Therefore the role of SCADA is to control the pipeline and monitor the status of station utilities remotely to obtain information for maintenance.

The compressor stations at a few sites may be located next to the scraper trap stations. For scraper trap stations at these compressor stations, the RTU I/O signals will come to the RTUs located in the control room. To transfer real-time operating data at each compressor station, you will need hardware and software to implement a direct communication interface between the compressor station controls and the SCADA system via local area network. The data transfer shall be user configurable. The SCADA system at the central control room at the arrival terminal will provide displays and reports of the data received from the compressor station controls.

The data transmission support requirements between the block valves, scraper trap RTUs, and the arrival terminal control center come from a private fiber-optic cable, public switched networks, or a satellite-based communication system. Usually the communication/ transmission equipment goes in the RTU cabinets at all stations.

The transmission system facilities undergo installation along the pipelines to connect all these stations from the departure terminal to the arrival terminal for pipeline operations.

The system communicates with the multiplexing and demultiplexing links to voice and data facilities at all stations (block valve stations, compressor stations, arrival terminal, and departure terminal). The system shall have the following interface capabilities:

• Two-wire and four-wire voice interfaces including analog/digital feature telephones and PBX systems
• Data channels suitable for RS 232, RS 422 protocols for RTU links
• Local area network (LAN) interfaces for token ring and Ethernet network protocols for compressor station and offices along the pipelines
• Interfaces to other miscellaneous facilities such as transmitting video in digital format along the network for the pipeline security system

The telephone channels link all related facilities (departure terminal, compressor stations, block valve stations, and arrival terminal) to the dedicated telephone system located at the arrival terminal control room. Various data transmission channels capable of transmitting synchronous and asynchronous data shall be provided—such as 64/128 kilobytes per second for SCADA RTU data transmission and 10 megabit/100 megabit Ethernet/other local area networks for transmission of data between compressor stations and office PCs at the various sites along the pipeline.

In addition, you may also need a 2-megabytes-per-second (E1) link for an interface between various PBX systems or for other voice, data, or video needed for the pipeline operation.

A dedicated telephone system is also supplied for the pipeline operation with PBX with two operator stations, telephones connected to an external speaker for each of the block valve stations (PS), scraper trap stations, and compressor stations (SC), and the arrival and departure terminals. The telephones installed in the vaults of the secondary stations should be suitable for table or wall mount.

The OMNI central unit (CU) will be at the arrival terminal. Its function is to manage the switching of the various lines for remote supervision and control of the pipeline facilities. Connections to all ancillary station sites will be by digital transmission networks made of flexible pulse code modulation (PCM). Dedicated links will connect the CU to the two operator stations. The product selected to make up the CU must have a proven track record in the field of security telephone technology.

Towers for the very high frequency antennas shall be self-supporting. The appropriate interfaces for the interconnection will come from fiber optics. If you design the transmission system based on fiber-optic cable, you can bury a single-mode, dual-window 1310/1550 nanometer fiber-optic cable containing ten or more fibers along the gas pipeline right-of-way, from departure to arrival terminal, with voice and data facilities at all pipeline stations. The geometrical optical and mechanical specifications will comply with the relevant international standards.

The system shall include automatic switching for vital communications to provide redundancy in case of a cable break. The vital communications provided with automatic switching are:

• Four fibers support either a 150 megabit or 600 megabit transmission system in a redundant configuration. The remaining fibers are for backup and future use.
• These communication links connect at the arrival terminal and departure terminal to a 2-megabit multiplexer that interfaces with the postal, telephone and telegraph (PTT) system. A similar multiplexer in the arrival terminal will break out the channels for connection to subsystems.

The transmission system will provide support for a wide variety of voice, data, and local area networks based on the latest communication standards.

PRODUCTION SIDE

Production facilities consist of oil/gas well sites with water well drill sites for onshore and collection platforms for offshore sites, piping, pumping/compressing equipment, safety and corrosion equipment piping, and GOSP for onshore and production/floating production/storage platforms for offshore sites.

The typical drill site instrumentation consists of pressure, flow and temperature instrumentation, as well as safety shutdown (SSD) system PLC units. In addition to the above, the drill site subsystems also include scrappers, down hole monitoring systems, injection water supply and flushing systems, and a multiphase flowmeter unit.

A user may install a down hole monitoring system, which monitors the flow and pressure of the wells, at selected drill sites. Corrosion monitoring equipment and a data collection system for piping around the drill sites and to the production header connect to the GOSP. The H₂S gas detection system monitors for the gas at all sites.

Each oil/gas well will come with a safety selector valve (SSV) shutdown at the wellhead in case of emergency. A remote command can come through the SCADA system for each SSV for shutdown. In case of H₂S leakage at the wellhead the SSV will shut down. H₂S detection and the shutdown command will come from a stand-alone PLC system. In addition, the system will monitor the pressure downstream from the choke valve. It can also shut down the SSV just in case. The PLC-based system will provide safety shutdown for drill sites and will connect to the H₂S gas monitoring system.

The RTUs collect digital and analog measurement signals from the instrumentation located at the wellhead sites, remote production headers, and production manifolds. The RTUs will store control functions/schemes and provide output signals to final control elements. The RTU will be capable of digital sequential control and standard analog control.

The RTUs will communicate to the host SCADA system at the GOSP control room or production/storage platforms on a multiplexed fiber-optic or satellite network. The communication system will operate on an industry standard protocol such as E1/T1. This will enable individual autonomous links to the RTU at a speed of 9.6 kilobauds or better. Some of these RTUs in the future may contain a unique IP address or universal code locator (URL). This in turn enables access of the SCADA system on an intranet or the Internet.

CONTROL ROOM HOST

The SCADA system at the control room consists of SCADA servers (host systems), operator consoles, communication equipment, and network equipment.

The associated RTUs will communicate through the fiber-optic transmission/communication interface equipment located in each control room.
In addition, the SCADA system will interface with the DCS and the plant information data collection and archiving system to allow remote data to be available at any process control system (PCS) display or for action by any PCS-based control scheme. Thus the wide area distributed SCADA system will have various levels of automation and communication networks including:

• Peer-to-peer monitoring and control between local RTUs and local control systems
• Information availability for real-time applications such as daily and hourly reports
• Data availability for the central data management system for corporate networks
• Gateway to other controls such as DCS and PLC systems
  

The design and implementation of these systems consist of engineering, procurement, installation, and commissioning of all SCADA and associated voice and data communications.

The scope of works for the communication system includes design, supply, and transmission facilities, such as termination equipment for communication between RTUs and the SCADA system.

• Fiber-optic line driver/satellite or other multiplexer (MUX) system racks and/or cabinet and associated ancillary equipment at drill sites and remote sites in RTU/RTU shelter
• Power and grounding to the MUX rack/cabinet
• Telephone sets in the RTU shelter

Remote sites and control room facilities need smoke detectors and alarms in the shelter. In addition, the RTUs in the shelter need temperature monitoring and alarming in case of alternating current power failure, RTU battery charger failure, battery charger voltage Hi/Lo, and common trouble alarm, etc., for fire protection of the field sites and remote facilities.

The SCADA system interfaces in the control room include connections with communication systems and PCS. The communication system interfaces include connections between field SCADA, RTU, MUX systems, and SCADA terminal servers.

Thus the hardware and software interfaces between SCADA and other systems in the control room include the following:

• Ethernet LAN interfaces between the PCS and SCADA systems
• Ethernet LAN interfaces between the SCADA and leak detection systems
• Wide area network interface between SCADA in the control room and the RTU
• Tie in with fiber-optic MUX system and SCADA terminal server equipment from communication room to SCADA termination equipment room
• Tie in between field pumping/compressor DCS and SCADA networks in the control room

You should base the selection of a SCADA system on process equipment, operational and remote control requirements, and available and cost-effective telecommunication transmission systems. You need to take into consideration the safety and reliability of operations and long-range system goals during the design and implementation of the SCADA system.  IT

Behind the byline

Rao Kalapatapu, P.E., is a senior SCADA/control system engineer and consultant. He has twenty-five years of experience in SCADA, DCS, PLC, and basic instrumentation and control systems design, procurement, installation, and commissioning for oil & gas, pipeline, chemical, and petrochemical plant projects. He is a senior member of ISA, and his e-mail is vrkalapatapu@hotmail.com.