December 2009

Cover Story

All on one network

Wireless telemetry advances mean wider networks, more questions, advancement opportunities

Fast Forward

  • Telemetry technology pushes toward goal of one common radio network.
  • Wider networks add flexibility, but reliability still concern.
  • Ask right questions for peace of mind.
 
By Rory Lacy and Brent McAdams

This story refers to systems and equipment used in oil and gas production fields, but the ideas and implementations can see application in other field automation projects across industries.

 All1As automation processes and technologies evolve, the human imagination continues to find new ways to use this technology and push providers for more powerful tools.

Fifteen years ago, even the more advanced automation systems seldom used wireless telemetry. And if they did, the data throughput was low and rarely provided the coverage necessary to link all remote sites. As a result, the usefulness of the technology was limited when monitoring only a portion of the sites remotely was possible.

Driven by customer feedback, manufacturers have responded with key advances in telemetry technology. Now, it is commonplace for a network to provide complete coverage for all remote field locations and achieve data throughput from field sites of 9,600 bits per second up to 115.2 kilobits per second. High-speed backbone architecture is now seeing implementation in the megabits-per-second range.

The trend now is wireless instrumentation, or collecting analog and digital signals by a remote terminal unit (RTU), programmable logic controller (PLC), or electronic flow meter (EFM) from remote equipment and sensors without the constraints and expense of hardwiring. These signals include process variables, such as pressure, flow, temperature, level, plunger arrival, valve control, switch closures, and emergency shutdowns. Traditionally hardwired processes are now wireless with a radio.

However, the latest of these advances is the wireless network, in which one radio system communicates from the desktop PC, through the RTU or PLC, all the way to the field instrument (transducer, sensor, or valve) without any hardwired connections, other than what is local to the sensor. This system operates on one common radio network.

This allows the user to see the health and status not only of the controllers at remote locations, but also of the instrumentation attached to the controllers and the telemetry system. This diagnostics and troubleshooting capability is available to the user throughout the entire SCADA system, again available on one common radio network.

Several radio manufacturers can retrieve data from remote locations on the same wireless platform. However, there are only a few that manufacture wireless inputs and outputs (I/O), and an even smaller number that offer a single radio solution, giving you both technologies in one common communication network. These new wireless networks are challenging the conventional thinking in automation.

The earlier school of thought was based on the idea you could do the long haul for data from the remote site back to the host by telemetry, but you would need to hardwire the local area connections to the instruments to ensure reliability. For several reasons today's radio technology has proven more secure and reliable than the older hard-wired connections to sensors.

Networks

When most of us think of a network, we envision an office environment where various computers in the office connect to a server. We call this a local area network (LAN). Then the server connects to the Internet, a wide area network (WAN). In field automation, the same two network types exist. The WAN can cover a very wide area from just a few square miles to several hundred square miles. We also call this the backbone of the radio network. The construction of this backbone or WAN consists of a master radio and a series of repeater radios that connect the host computer to all remote locations, specifically all field RTUs, PLCs, or EFMs.

This repeater network, depending on the technology you use, offers high-speed throughput with the ability to bridge physical obstacles, such as hills, valleys, forests, and buildings. Repeaters allow the system operator to cover distances greater than any single radio link alone can cover. A single radio link may be only 20 to 30 miles, but by using multiple repeaters, you can rebroadcast the data and regain full signal strength at every repeater, thereby extending the network to several times greater than the single link, if required.

In some telemetry technologies, regardless of whether using unlicensed spread-spectrum radios or licensed frequency radios, the same slave radio seeing use as the RTUs can act as a slave to send data back to the SCADA host and as a repeater to other field devices or RTUs. We call this slave/repeater mode. This capability allows you to expand your WAN by using remote sites to act as a series of repeaters.

The ability for a single wireless I/O master radio to function as a slave for the RTU network and as a master to the I/O network adds more flexibility. The ability to poll the instrumentation creates a second network of instruments wirelessly reporting back to the RTU. This network is the equivalent of a LAN. It may be easiest to think of all the instrumentation on one well site, such as casing or tubing pressure, wirelessly talking to the RTU as the LAN, and the various well sites talking back to the field office as the WAN.

Now we have two interlacing networks in which the WAN and the LAN are working on one radio system and using a common platform. This common platform is available because the wireless I/O master is switching its functionality between a slave to the SCADA host, responding whenever the host requests data, and a master to the wireless I/O when the RTU requests data from the instruments.

Economics

The most intuitive of all the advantages of wireless I/O is the reduction of labor and material over conventional hardwired systems. For a typical oil and gas well location, the operator will want to bring measurements from multiple locations back to the RTU or EFM. If you hire a contractor (perhaps a licensed electrician) to install these hard-wired connections in field automation, the costs start at about $16.00 per foot and escalate from there. We derived the cost estimate from averaging prices in different areas of the country. Costs remain similar whether the install is direct burial cable or conduit and wire. Using this cost as a reference, the break-even point for wireless I/O is about 50 feet, and that is if we only consider the cost of wire and labor. The cost savings are far greater when there are two or more wire runs required at the same remote site. This is because the only additional cost is for the second slave radio located at the additional cluster of sensors. If an operator wants to gather casing and tubing pressure from a well head and also monitor tank levels at a different section of the remote site, for instance, he can use the one radio already in the RTU, and only one additional slave radio. This saves $16.00 a foot for both clusters of sensors.

In addition to the sheer costs associated with hardwiring instrumentation, one of the other advantages with respect to installation is the speed of deployment. It could take days or weeks to properly install, isolate, and commission wired systems. Wireless I/O networks usually require installing and configuring only the end points, saving substantial time for projects with aggressive schedules.

Reliability

It is common for people to question the reliability of wireless products. As in all changes or paradigm shifts, people take some time to adopt new ideas. Radio technology has proved itself in the oil and gas industry as a reliable data highway for remote data collection from RTUs and EFMs for over 20 years. Now with wireless I/O functionality of radio networks, the reliability question again is a concern and stumbling block for the advancement of this technology. Some wireless I/O equipment providers have built safe guards into their equipment and networks to address these concerns. Look at link alarms and the ability to program fail-safe conditions into the radio upon loss of communications with the master.

Communication link alarms let the user know the instrument is no longer receiving data due to a loss of signal between an I/O slave and the I/O master. Also, in the event of a communication failure, the I/O slave will control its outputs based on the fail-safe default condition that was pre-programmed in the radio during the configuration of the system. In other words, should the link between the wireless I/O master and slave be compromised, the outputs of the I/O slave will default to its pre-programmed fail-safe position of on, off, or remaining in the last position.

No system is completely immune to signal loss. Wired systems are prone to having wires cut during construction or even routine maintenance. Rust, corrosion, steam, dirt, dust, and water can all affect a wired instrumentation system. The difference is wire cannot alert a user of a problem.

Power requirements

The ability of some radios to operate as slave to the control network and master to the I/O network (in which the LAN and WAN networks use the radio in the RTU as the common link between the two systems) might be an elegant way to operate when installing new equipment. But quite a few end users and operators have legacy systems using older technologies that do not support this functionality. In these cases, having two radio systems is still a viable option. Use the legacy system as the long haul (WAN) back to the host computer, and then install a new LAN radio system at the wellhead to collect the local data wirelessly for the RTU or EFM.

This second method is still economically superior to running conduit or trenching at distances over about 50 feet. However, the two-radio answer does consume more power at the RTU. Typically at remote sites, this means larger batteries and larger solar panels. On the slave side, where the sensors and instrumentation are located, the power consumption remains constant. New wireless I/O radios draw as little as 6 mA of current when being polled continuously. Newer pressure transducers are low power and feature 1- to 5-volt output signals, while drawing only 7 mA per transducer.

An example of a typical well head gas field operation using wireless I/O will look like this: Two pressure transducers (one for casing and one for tubing pressure) at 7 mA each equals 14 mA continuous draw. One wireless I/O radio has a 6 mA continuous current draw. Total current draw for data collection and transmission for the RTU is 20 mA.

If we provide an 8 amp-hour battery, this site will have 12½ days of autonomy, and we can maintain the battery charge with a 5-watt solar panel. We can house the radio, charger, and battery in a 6"x8"x4" NEMA-4 enclosure. We can size the battery and solar panel required according to the load that each site will require. If the operator only wants one analog input, the power consumption drops by 7 mA, or about 1/3 of the previous calculation. We can then power the site by a 5-amp hour battery with nearly the same autonomy.

Wireless future in automation

Oil and gas companies are seeking to understand the future of automation, and their decisions today will affect them for years to come. Anadarko, BP, Chevron, Dominion, Kerr McGee, and others all have internal focus groups whose objective is to provide best practices and procedural guidelines. These steering committees then provide guidance through the maze of new products and technologies. Companies are also trying to achieve standardization in hardware and software across their entire operation for ease of support and maintenance.

In the early years, we gathered all gas flow and oil production data by hand. Today, we accept as the new standard the remote collection of data from instrumentation and transmission to a central location. The relevant question today is not, "Should we automate?" but rather, "Which types of automation are the best fit for our operation?" Before choosing equipment or solution provider, operators need to ask themselves the following questions:

  • What is the payback and total cost of ownership with this technology?
  • Will this technology help optimize production by giving us real-time information about our process and allow control for corrective action?
  • Will this technology save us manpower and time?
  • Does this technology allow us to share data between field offices and other locations?
  • Is this technology affordable?
  • Will this technology provider be here for the long haul?
  • Does the manufacturer support the end user before, during, and after the sale?
  • Does the factory have 24-hour telephone technical support? Is it free or fee-based?
  • Is local field support available?
  • Is local and factory training available for our personnel from the manufacturer?
  • Is the manufacturer's warranty program acceptable?

With the emergence of robust wireless field automation, the end user can easily find manufacturers where the answer to all of these questions is, "Yes."

Today's spread-spectrum and licensed radio technologies allow field operators to build robust WANs and LANs comparable to what has been available in the office and wired world for years. With the ability for serial radios, wireless I/O radios, and Ethernet radios to all work together in one seamless network, the future of field automation has never looked brighter.

ABOUT THE AUTHORS

Rory Lacy (rlacy@commercialradio.net) is a 29-year veteran in wireless systems integration and owns Commercial Radio Systems with offices in Midland, Tex., Weatherford, Tex, and Charleston, W.V. Brent E. McAdams (www.freewave.com/intech) is the director of major accounts at FreeWave Technologies, Inc., an RF design and manufacturing organization; former vice president of technology and business development for the U.S. Telemetry Corporation; and a former 10-year contract electrical and instrumentation engineer with Exxon Chemical in Baton Rouge, La.

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