Mix and match wireless tech for SCADA systems

Jim Gardner, FreeWave Technologies
29 Oct 2010

Companies with large, geographically dispersed networks, such as those in the oil and gas industry, can select one technology, one source, or one vendor to collect, retrieve and report data, and to assess the health of the network. Sometimes, this type of approach makes sense; other times the better solution is to incorporate a variety of technologies to form a single cohesive network.

The days of building large, single technology networks are likely behind us. Data and security demands at various levels of a large network are changing the game. Options now exist that allow us to achieve better manageability, expandability, cost, and speed. Hybrid networks can include landline phone, satellite, licensed radio, spread-spectrum radio, or cell-phone-based technologies. Data security, network speeds, infrastructure costs, and on-going costs are important factors to consider when selecting the right technology combination to meet these objectives.

If you are collecting data from multiple locations and delivering it to offices over a widespread area, no single technology can achieve your objectives. However, by combining technologies, you can create seamless data streams from several locations and share data over a LAN or WAN with multiple users. The end result is more effective and efficient management of the network and increased reliability through reduced downtime - all at a much more affordable price.

With the many new telemetry technologies available today, it is common to have communication capabilities available at all field locations, regardless of their location, and often involving several different technologies that are integrated into one hybrid system.

Planning it out

Planning a network is the critical first step.

For a start, how often do you need to poll? What are you trying to accomplish? If you need gas measurement, you can poll a few times a day. The EFM device will log hourly data on flow rate, pressure, temperature, etc., and you can poll to interrogate the EFM every four to six hours and bring back all the stored data at one time. If you are working on optimization (e.g., plunger lift) and need more granular data, you can again use the EFM to perform local control as well as to archive the data, or you can speed up the polling cycle in one of several ways, depending on the need.

First, you need to know how much data you are moving. For example, if the stored plunger lift data and the gas custody transfer data come to a total of 2,000 bytes and the EFM talks to the radio at a port speed of 19.2 kbps, you have about 5.22 s per EFM to retrieve the data.

Today's newer, license-free digital radios can talk over the air at speeds anywhere from 115 kbps to 1 Mbps. The great advantage of having a radio that talks so much faster than the port speed of the EFM is that you have a lot of open air time while the data is loaded from the EFM to the radio. This offers you such benefits as the ability to do real-time alarms in the middle of a polling cycle. The physical capability of the EFM to load the radio will likely be a limiting factor in how long it takes to poll the field. There are ways to speed up the system polling times.

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