UK gas network combines 400 MHz IP-based transceivers with satellite technology for enterprise-wide connectivity.

Transco, the operator of Britain's natural gas pipeline and gas emergency service, is responsible for the safe and efficient flow of gas through 275,000 km of pipeline stretching across the UK. The pipeline carries 48% of Britain's energy needs and serves 20 million commercial, industrial and domestic consumers.

Proper management of this vast network requires rapid access to remote data such as gas pressures, flows, temperatures and calorific values from multiple points in the system. Operators must also be able to control pipeline facilities remotely, in response to system demands, or to deal with emergency situations.

In March 2000, Transco awarded a contract to Logica UK Ltd. to develop a new generation of telemetry and control for its gas network. The project, code-named Ulysses, was launched with the goals of enhancing the current level of telemetry and control, while providing flexibility to meet the future needs of Britain's gas market. Both satellite and terrestrial wireless would play a key role in the new system.

Telemetry Challenges - Picking up the Pieces

Under an old system of 12 area gas boards in the UK, there was little standardization between the telemetry methods used from one zone to another. This resulted in a patchwork of data speeds, protocols and communications media used across the country. Many zones employed early-generation analog radios that required frequent alignment and field maintenance to keep them on the air, and there was almost no commonality in spares or hardware equipment.

As the inheritor of these systems, Transco faced significant challenges in bringing the zones together for centralised control and monitoring at their Main Data Centre (Fig. 1). Finding a solution to the problem required a re-thinking of the way data communication had been carried out in the past, and had to take three factors into account:

• The long distances to be covered
• The large number of sites
• The rugged terrain at many installations

A careful review of the issues by industry experts concluded that a new network based on Internet Protocol (IP) should be developed. This would place all telemetry under one umbrella, and allow system-wide access to important data from a central location. In response to these needs, Logica UK Ltd. worked closely with GE MDS (MDS) to deploy MDS TransIt™ data transceivers at approximately 127 hilltop base stations and 1,100 Local Transmission Sites (LTS). TransIt equipment was chosen for its multi-protocol capability, long communications range and reliability in mission-critical applications.

Figure 1. Transco's main Data Centre in central England serves as the communications hub of the gas supply network.

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Communications - The Essential Link

A key element of any network is rapid and reliable communication. Without it delayed or inaccurate responses to system demands can result, and safety can be compromised. The Transco system uses a variety of communication technologies to ensure adequate coverage throughout its system, including dedicated landlines, very small aperture satellite terminals (VSAT), and 400 MHz point-to-multipoint radio. Each technology is suited to a particular task, as depicted in Figure 2.

Figure 2. Transco's telemetry network integrates several communications mediums, each well suited to a specific task.

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The GE MDS Solution

Because of the number of LTS sites and the criticality of uninterrupted gas flow, an economical, yet highly reliable solution was needed for terrestrial communications. Equally important to initial expense was the need to achieve a low cost of ownership over the life span of the system. With telephone installation costing approximately £3,000 per site (in addition to ongoing access fees) the use of wireless became a very attractive option to system designers.

While satellite communication was a possibility, this technology is best suited to very long-haul transmission paths, and would have been very expensive to install at all LTS sites. (Satellite does play a crucial role in other parts of the system, as shown in the illustration.) Designers focused on terrestrial radio solutions that were capable of carrying the Internet Protocol (IP)-based traffic required by the network. Industrial-grade radios capable of handling IP-based traffic are not common, however.

Utilities are adopting IP/Ethernet at a rapid pace today because of the distinct advantages it offers over serial communications. These benefits include hardware and software platform interoperability, collision avoidance, high transmission efficiency, and the ability to add new devices to a network without disrupting traffic flow. The use of IP also opens up new possibilities for integrating a utility-wide Intranet, voice-over-IP and/or video into a network control scheme.

The MDS TransIt Series of wireless transceivers (Fig. 3) provided the functionality required for the Ulysses project and were specified for use at approximately 1,100 LTS remote sites and 127 hilltop base stations. The TransIt system is designed to connect one or more central (host) computers to a large number of remote sites over a widely distributed area and offers an intelligent, multiprotocol, solution for mission-critical data. TransIt can replace traditional leased lines or switched telephone connections, thereby eliminating ongoing service charges.

Figure 3. The MDS TransIt NR-100 Remote Station (left) and NM-200 Base Station are key elements in the IP-based Transco network

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Network Overview

The terrestrial network used in the Transco application operates in a Point-to- Multipoint fashion. As such, a number of remote radios (outstations) exchange telemetry and control data with a central base station or hub over fixed 400 MHz frequencies. The transceivers operate at RF power levels up to 5 watts, and employ gain-type antennae to achieve reliable ranges of 40 km or more over average terrain.

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Data Flow

Operation of the system begins with the collection of data. Instruments installed on the pipeline acquire measurement information such as pressure, temperature, flow rate, etc. and convert this data into electrical signals suited for transmission over the radio channel. The remote sites are scanned (polled) once per minute by an associated hilltop base station, and send their data at that time. When the base station receives the data, it is relayed to Transco's main Data Centre via satellite for processing (see Fig. 2).

In addition to receiving remote data, the network can also be used to direct commands to a particular remote station. This might be done to effect changes in valve positions, activate or deactivate compressors, or conduct equipment diagnostics. All of these tasks can be accomplished without having to visit a site personally can obvious cost advantage for a widespread utility operation. Radio settings can also be changed remotely, using a Network Management System (NMS) supplied with the TransIt system.

Approximately 127 hilltop base stations are used in the Transco network. Each base station communicates with a subset of remotes, providing coverage to 1,100 LTS sites. Communications reliability is ensured through redundancy. Each hilltop site contains two TransIt master stations which provide automatic switchover to an alternate unit in the event of failure. For added protection, redundant antennae are also installed at many hilltop sites. This virtually eliminates the chance of interruption due to a radio problem.

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TransIt's "Cell" Concept

In its simplest configuration a TransIt system functions as a cellularised radio network (Fig. 4), with each cell consisting of two elements; a master station and a collection of remote outstations. The outstations communicate with the master over a narrow bandwidth radio channel. This building block is called a "cell" and multiple cells may be added to the network as needed to cover a desired region.

Figure 4. Single-Celled TransIt Wireless Network

The radio channel is time-shared by all of the remote stations assigned to a cell by means of the TransIt™s Wireless Access Radio Protocol (WARP). The WARP protocol efficiently allocates the RF spectrum among the active users for maximum efficiency and throughput.

In the Transco application, the basic network arrangement described here was expanded to cover the company's entire service territory. The master stations at the hilltop sites act as control hubs, coordinating all communications to and from the remote transceivers installed at LTS sites (see Fig. 5). Linkage to Transco's Main Data Centre is achieved through a VSAT terminal located at each hilltop site. Over this satellite link, any hilltop station can be accessed from the Data Centre, and communications can be established with any remote station.

Figure 5. Typical radio installation at an LTS. TransIt NR-100 radio is mounted at upper right of cabinet.

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Managing the Network

Besides the radio equipment itself, the TransIt system includes a secure Network Management System (NMS) tool. The tool provides administrators and other authorised personnel with a method of performing enterprise-wide management and maintenance functions from the convenience of an office setting. The NMS system can "see" and administer the system through a direct connection to any master station or remote transceiver in the TransIt network. Figure 6 shows a typical NMS installation running on a PC, and a close-up view of a configuration screen.

Figure 6. TransIt's NMS package provides authorised access to key network parameters from the convenience of an office setting

NMS functionality is important to Transco because it allows close monitoring and configuration of the network - including radio status at the outstations - without the need to personally visit sites to make minor changes. With over 1,300 radio locations to manage, frequent site visits would be time consuming and cost prohibitive.

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Conclusion 

This article has given an overview of how wireless is being used to improve the safety and efficiency of an extensive pipeline network. Industrial wireless solutions, such as the TransIt system described here, are now a practical reality and are being used by many organizations to replace older, less flexible systems. New developments in IPbased wireless continue to unfold, and will no doubt play a key role in the telemetry networks of the future.

Readers interested in more information about industrial wireless, including alternate frequency ranges, data rates and license-free solutions, can contact Microwave Data Systems, 175 Science Parkway, Rochester, NY 14620 (Tel. 585-242-9600). Additional information can also be found on the firms' website at www.microwavedata.com.

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