TETRA in Control


Tim Guest looks at how TETRA offers a suitable technological bearer for critical data compiled by SCADA systems.

Talk of data in the word of critical communications often focuses on the growing video and image-capture requirements of first responders, and the capabilities to deliver such using TETRA, TEDS and, in the future, probably LTE. There are other sectors, however, whose demands for reliable and critical data transmission could be argued as almost as important – and, in some cases, more important for the ‘whole’ – than the public safety end-user segment. In the energy, oil & gas and general utility sectors, for example, real-time Supervisory Control and Data Acquisition (SCADA) systems are critical in monitoring and delivering data that helps maintain the equilibrium of some of the most sensitive installations around the globe.

First used almost 50 years ago, SCADA solutions are computer/IT systems, which gather, compile and analyse real-time information being created by the operational activities of both relatively simple – temperature monitoring of a building – and extremely complex industrial processes, such as those found in the nuclear and hydro-electric power plants/industry, as well as the oil & gas, and chemical sectors. If an oil pipeline were to fracture, for example, or a radiation leak occur at a nuclear plant, a SCADA system would pull together all the information it could about such an event and transfer it to a central site or control, setting out a logical series of priorities determined for that event and whether, or not, action needed was critical.

Some SCADA systems are used for telemetry purposes (the remote measurement and reporting of information from remote sensors and meters), and some for telecontrol (the remote control of equipment such as power switches and breakers, valves and actuators, and often including a telemetry function to enable status reporting). Other SCADA systems can be found supporting smart grid systems – intelligent integration of all the actions of all the users/participants connected to an electricity grid, including those who generate power, those who consume it, and those who do both, in order that secure, reliable, sustainable, and economically viable supplies are delivered.

SCADA scenarios

Typical network arrangements in a SCADA solution include polled systems where SCAD control stations link into master units, which, in turn, link to slave or remote units (RTUs) that are polled for information from critical infrastructure to which they will be connected. Groups of RTUs, known as scan groups, are controlled by the master units. In a high voltage (HV) power distribution scenario, for example, the RTUs will link with installations that are potentially failure critical or time critical items, such as a primary substation. Polling for data and latest status information from such infrastructure by the master unit will be conducted regularly and at high speed.

Another arrangement for SCADA operations is a ‘report by exception’ set-up where RTUs in the system will only report in the event of an incident. Such configurations are typically used for less critical infrastructure and scenarios than those monitored by the polling system, and include such scenarios as those used for monitoring at an oil and gas well. Standard procedures with such report-by-exception arrangements will involve regular checks to ensure the RTUs, which will lie silent between alarms, are still operational.  However, ‘events’ can often mean that many RTUs will suddenly ‘awaken’ and report anomalies and critical data all at the same time, and this ‘flooding’ of information must be handled by whatever data transmission technology is supporting the system.

The importance of each application can be seen in the response times expected of the SCADA systems supporting it. The power generation sector where HV substations are involved has a typical polling time of 250 m/s, because not only are they dealing with electricity, but problems arising will affect huge numbers of both domestic and corporate customers, so the system must keep one step ahead of events. When it comes to the high-pressure distribution of gas, on the other hand, such a network, (which already has automatic shut-off systems in kits infrastructure), would only need to be polled around every two minutes to keep on top of potential critical events; for an oil pipeline, where the changes in flow due to a leak will take quite a time to detect, polling would normally happen once every 24 hours.

TETRA in support

TETRA systems and networks are suited to support SCADA such applications as they are typically very resilient and secure with a high degree of availability. From TETRA 1’s IP-based short data service (SDS) – a packet data service delivering 3 kbit/s per slot messages in 1-2 seconds, as well as multi-slot messages – to TETRA Enhanced Data Services (TEDS), which can achieve even higher data rates and more comprehensive performance, TETRA is proving itself more than capable of handling the demands of SCADA – both polled and exception reporting scenarios.

According to Analysys Mason’s Nick Smye, who co-chairs the TCCA’s Smart Grid and Telemetry (SST) Group, it is a challenge to directly relate data rates to SCADA capacity per cell and many factors come into play. The required response time by the user/application and the amount of any non-SCADA TETRA traffic in the network will be key factors, as will be the architecture of the SCADA network itself, for instance, the number of RTUs in each cell. The complexity of the RTUs will also be a factor along with the number of measurement points and controls in the network. These stats are also determined by the maximum and minimum size of a scan group. In the case of report by exception, the maximum number of RTUs, which try to report at the same time and necessitate re-transmission, will also impact data rates.

Of the benefits of using TETRA for SCADA applications, Smye has indicated that along with its suitability to meet data requirements, end users will find they also have a voice capability at their disposal. Not only that, but TETRA includes authentication, air interface and end-to-end encryption, which in today’s dangerous world is essential to maintain security at the kinds of mission critical installations using SCADA applications. Smye also emphasises the fast and reliable nature of SDS and its group function, which can reach a number of devices quickly.  Of the greater capabilities of TEDS, as well as its higher speeds, he indicates that its ability to handle different data classes such as that generated from real-time, telemetry, or background information polling, together with different delay and reliability classes such as low, medium, high and unpredictable, are also factors supporting TETRA’s use in SCADA.

Demands of a sector

Staying with the oil & gas sector, a TETRA bearer will need to handle data of various types and from any number of data sources. From the remote measurement of physical values at the wellheads, involving temperature, pressure, flow rate and alarms, to monitoring voltage, current and alarms relating to Catholic Corrosion, to incident and leak-detection alarms requiring control room direction, the communications system will need to be robust.

A SCADA system’s aims in managing the activities at an oil well include maximising a well’s productivity and managing its reservoir to ensure the overall recovery from a site is as good as it can be. The system must also contribute to helping improve the recovery process but must also monitor the wider geographical area around every wellhead for any impact the drilling and recovery might have on the environment. And this must all be done in real time.

An oil field’s SCADA system will typically run, monitor and control three areas of the production process 1) data acquisition and upstream oil & gas field monitoring; 2) pipeline operation; 3) power distribution. Its major elements will include control centre servers, RTUs, wellhead components and, of course, a communication system.  For the latter to be suitable for supporting SCADA it needs to be of an open standard using tried and tested technologies and capable of supporting SCADA systems and their demanding capacity requirements. Such a communications system should also be extremely robust and reliable, as well as scalable to accommodate the need for growth. Yet in meeting all these needs, such a system must also be cost effective for the end user and easy to deploy and use.

With such parameters in mind, Saudi Aramco, one of the world’s largest oil & gas players has been operating a pilot TETRA system across more than 330,000 km2 of its operations in the Middle East. With the world’s largest proven oil reserves, the company’s network over this vast area of mainly land but also a large area of sea installations will eventually comprise more than 170 base stations, six TETRA switching and control nodes, at least 150 high-spec repeaters for in-building penetration, and equipment for over 14,000 end users. Saudi Aramco’s end users encompass a full gamut of disciplines from firemen requiring water-proof and ATEX standard equipment, refinery and bulk plant engineers requiring personal as well as vehicle-borne devices, security teams, and management personnel.

A company spokesperson recently said that the key reasons for selecting TETRA for this project were its ‘technology efficiency, where each frequency can provide up to four time slots’, which, he went on to say enabled ‘a cost avoidance of frequency rental fees of 75%.’ TETRA’s open platform nature offering greater choice for procurement between device and infrastructure manufacturers, as well as its reliability and security elements, were also key in persuading Saudi Aramco that TETRA was the right choice for its SCADA requirements.

LS Telecom has been instrumental in the deployment and has conducted much of the radio frequency planning for the project. In one region, the Berri offshore oil field in the Northern Arabian Gulf, the system connects the onshore SCADA host with scan groups of RTUs located on the oil producing platforms using the UHF narrowband operating in the 330-350 MHz range. One of the crucial early stage aims of the pilot was to determine how many land-based base stations would be required, and in what locations, to support and communicate with all the necessary offshore RTUs.

So far the deployment has shown TETRA, including TETRA 1, to be highly suitable to provide the reliability and data rates needed for all SCADA activities, with future SCADA requirements that need higher data rates likely catered for by TEDS.

Experience in detail

According to Funk-Electronic Piciorgros GmbH, a company with considerable experience in this sector, ‘Typically a complete polling cycle for gas and oil application should not last more than five minutes.’ Indeed, in the Saudi Aramco deployment, polling cycles range from 30 seconds for some of its RTUs to 300 seconds for the largest RTU scan groups.

In assessing the suitability of specific equipment to meet the polling requirements of a network, Piciorgros cites an assumed one-second polling cycle using the Modbus RTU over TETRA and SDS. This means that 60 RTU sites could be polled every minute and 300 sites every five minutes on just one of the four TETRA time slots. The company says that when using a TETRA data gateway like its own TGW-100, multiple polling nodes can use the same TETRA time slot in parallel without reducing the polling rate. If four polling nodes were to be used, the total number of sites that could be polled over a five-minute period would be 1,200 [i.e. 300 (the number of Polling Cycles in five minutes) x 4 (nodes used)]. In other words, if all time slots available were used the number of polling cycles that could be conducted in five minutes would increase to 5,000.

Recognising this sector’s importance for TETRA

In the autumn of 2012, the TCCA established the SCADA, Smart Grid and Telemetry (SST) Group to address a perceived need to promote TETRA for both traditional SCADA and telemetry applications, as well as for emerging opportunities in exciting new areas such as smart grid and smart cities. Appointed as one of the Co-Chairs of the SST Group along with Nick Smye was David Taylor, lead consultant at Analysys Mason. Speaking to TETRA Today he said that: “The SST Working Group’s main objectives are to promote TETRA as the bearer of choice in the SCADA, telemetry and smart grid areas. To that end we want to create a forum for users, manufacturers, system integrators and developers to share their experiences and requirements. This, in turn, will help drive relevant activities within the technology standards organisations to ensure evolution of critical communications standards that encompass the future needs of this market.”