Fieldbus enables operators to attain control in the field

Aug. 1, 2011
The first major Foundation Fieldbus installation in the world was the Shell Malampaya offshore project in the Philippines. Shell selected Foundation Fieldbus for Health, Safety, Environment reasons – namely, its ability to help reduce on-platform staffing.

Ian Verhappen
Industrial Automation Networks Inc.

The first major Foundation Fieldbus installation in the world was the Shell Malampaya offshore project in the Philippines. Shell selected Foundation Fieldbus for Health, Safety, Environment reasons – namely, its ability to help reduce on-platform staffing. The bidirectional communications and diagnostics capability of Fieldbus meant 80% of maintenance “work” could be done remotely from the control room at the on shore gas processing facility. However, the benefits to offshore facilities do not stop here. As described below, in the event of loss of communications with the control system, control in the field allows safe operation of the process at the last set point. Alternately, the system can be configured to “fail gracefully” to a predetermined state.

Foundation Fieldbus is unique among fieldbus technologies in that it makes possible the return to “control in the field.” This means that as long as you have power to the device and motive force for the output device/actuator, you will continue to be able to control your process – regardless of what is happening in the control room. This is a return to the original days of closed loop control with pneumatic transmitters and actuators. The rules for control in the field with Fieldbus are somewhat the same as with pneumatics in that the input and output devices for the control loop must reside on the same wire. In pneumatic days they had to be on the same air line.

It is the PID function block that makes “control in the field” and (as a result) single loop integrity possible. Single loop integrity means Foundation Fieldbus is able to maintain control at the last set point without a host control system. The other effect of losing communications with the H1 network will be the loss of view/access to the device from the operator station. There are however a variety of local loop-powered indicators available that could be used for critical loops/variables; and of course all field devices also have the option of a local display.

In addition to the PID function block residing in one device on the network – typically the analog output device, to optimize bandwidth usage – every fieldbus segment in which you plan to implement “control in the field” requires that at least one device on the network be a “link master” device. Link master capability must be specified when ordering the device from the manufacturer, since it is supplied as part of the H1 communications stack. The link master device assumes control of all network traffic in the event the connection to the control system is lost. This could happen in a widely distributed system or one in a harsh environment (salt, corrosion, and vibration) as might be found offshore.

Implementing control in the field enables certain applications to be more efficient than is possible with conventional instruments or control in the host. This is because field-based control runs independent of the host control system and can execute as quickly as ¼ second loop times. The typical DCS scans its I/O on a 1 to 5 second cycle. Faster cycle times and more accurate measurements mean operating closer to the physical constraints of the system and hence increased throughput without the need to purchase larger equipment. There are of course specialized analog control systems as well that have very rapid scan rates for applications such as compressor surge control.

There are other buses that may have faster scan rates than Foundation Fieldbus. But, because they do not have the deterministic scheduling component unique to this bus implementation, they have to oversample the data to be able to achieve similar levels of control.

Fieldbus includes signal status information with every signal update. Thus, in addition to providing real-time control, fieldbus technology enables predictive maintenance practices. These practices result in higher reliability at lower cost than other related techniques. The technology can use the data pushed from the transmitter to the associated management software as the basis for determining remaining equipment life; and hence when to schedule needed work.

Fieldbus and “single loop integrity” provide increased reliability of the control loop, which is different than what is offered through redundancy. By definition, redundancy involves the installation of a similar, typically identical device with identical functionality. In the event of loss of communications with the H1 interface card, fieldbus will continue to control at the last set point as long as there is communications/power. And, as long as the input/output devices are functional, you will have reliable control without redundancy.

However, if the field devices are not functioning properly, you cannot expect to have good control. Bad input signals mean a bad process variable (PV, or in fieldbus terms AI) signal and failed output devices (MV, or in fieldbus terms AO) means you will not be able to change the process to maintain your set point. Good industry practice and safety system design recommendations suggest that a “redundant” input be of a different technology to avoid common cause failures. One example of the concept of conflicting redundant measurements might occur if two ultrasonic level transmitters in a vessel were to interfere with each other because of their operating frequencies.

One final additional advantage fieldbus has over pneumatic technology is that you can configure your output device to fail in different ways, depending on the type of failure in the field. It is possible to have the output device fail one way in the event of loss of fieldbus signal; another in the event of loss of communications with the PID function block telling the output device “where to move”; and a third as the mechanical mode of failure on loss of motive force. In most cases, all failure modes are configured the same way not only for ease of maintenance but also to effectively manage risk.

Even if you are not planning to implement “control in the field” when starting your project, it is recommended that all Foundation Fieldbus designs be based on the principle of all elements of a loop being on the same segment. This way, even if you do not wish to implement field control now, it will be possible to do so in the future without having to make any physical changes. With all the devices for a control loop on the same segment, migrating to “control in the field” is “just” software, engineering, and configuration. Once you have gained confidence in fieldbus technology, you too will see the benefits of “control in the field.” It allows for better control and also helps users manage risk in the event of an abnormal situation.

Offshore can be a high risk environment. Having “control in the field” not only provides better control with the associated benefits of higher throughput; it also has several HSE benefits which make a positive contribution to the bottom line.

The author

Ian Verhappen, P. Eng., is an ISA Fellow, ISA Certified Automation Professional, and a recognized authority on Foundation Fieldbus and industrial communications technologies. As director of Industrial Automation Networks Inc., Verhappen leads this global consultancy which specializes in industrial communications, process analytics and hydrocarbon facility automation.

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