Watching your weight

Though the automation system is typically only a small part of the total weight of an offshore platform, we all know every little bit of weight reduction counts.

Digital technologies lighten offshore load, enhance HSE, and cut costs

Ian Verhappen
Industrial Automation Networks Inc.

Though the automation system is typically only a small part of the total weight of an offshore platform, we all know every little bit of weight reduction counts. So how can end users take steps to reduce both the footprint and weight of the control system on a platform? The answer should be no surprise: digital technologies.

Fortunately, most transmitters used today are all "smart" in one way or another, and support some form of digital communication. One of the enhancements possible through digital communication is the ability to share status and diagnostic information useful for improved maintenance of the devices. By effectively using this diagnostic information and integrating with an asset management and enterprise resource planning (ERP) system, it is possible to optimally manage maintenance scheduling and staffing. As a result, exposure hours are reduced, with a commensurate impact on an operator's health, safety, and environmental program. As we will see below, there are additional environmental returns from digital technologies.

Though not commonly used, smart transmitters are also able to communicate more than a single variable back to the control system. Pressure measurement in its various forms is the basis for as much as 80% of the input signals in a typical facility. All pressure measurements are in reality differential pressure signals; therefore, it is possible to use a differential pressure meter to measure both the flow and the bulk line pressure to calculate a pressure-compensated flow, or a vortex meter having integral thermocouple to provide a temperature-compensated flow. In both cases, the accuracy of the measurement increases, enabling tighter control of the process. There are also integrated differential pressure meters with the orifice directly connected to the pressure sensors, and a vortex meter with embedded pressure and temperature compensation sensors allowing the calculation of mass flow of vapor or steam from one device, at a much lower cost than a Coriolis meter.

We are now able to obtain additional information from a single transmitter. In the above differential pressure transmitter example, we no longer need to purchase and install a second device to inform us of the line pressure. We can also use the temperature measurement integrated in every smart meter's electronics to tell us the ambient temperature in a weatherproof enclosure. Each device that can be removed from the work scope means fewer hours to design the necessary process connection, instrument specification and loop drawing, as well as the electrical work to connect it back to the control system. The net result is one less process connection, one less device, one less cable run, and one less input-output (I/O) termination, which all adds up to fewer design engineering hours. More importantly, it means less weight and fewer components on board, which should also increase overall reliability.

Additional operating expenses savings and environmental benefits will result because fewer nozzles/process connections also mean there will be fewer possible leak points requiring EPA monitoring.

Once the plant is operating, the benefits of using smart field devices continue. For example, the ability to communicate to field devices as part of the network means a change in the field device, such as a range change, is now propagated through to the control system, and vice versa. As a result, there will be less risk of a range mismatch between the field and control system. It will also be possible to perform some maintenance practices without having to physically be present at the device, again reducing overall exposure hours.

The discussion so far has assumed that we are using a communications protocol such as HART that uses a single wire connection to each device. However, if we include fieldbus systems or electronic marshalling technology, the benefits continue to expand, because every fieldbus home run cable from the field junction box to the interface card now carries at least ten signals per wire pair. That means that instead of installing a 12-pair cable, a single-pair cable will suffice. When the home run fieldbus cable arrives at the control system, rather than having to separate the signals to the appropriate I/O card by type, there is only one type of signal. The number of I/O cards is reduced by a minimum of a factor of four, and in most cases, closer to or better than the order of magnitude factor already obtained by the cable count.

The next step in cable management is to eliminate the cable by using wireless technology. Wireless field sensor network technology such as WirelessHART and ISA100.11a eliminate the spur cables between the field devices and junction box, replacing the junction box with an access point. The access point requires reliable power, but because of the density of an offshore facility, this is likely to be less of a challenge than for widely distributed onshore operations. Because field sensor network technology is still relatively new, most implementations are not being used for close loop control, but rather are restricted to process monitoring only. Fortunately, approximately 80% of measurements are monitor only, so wireless is a viable alternative to cable - even if you occasionally lose communications, which is a rare event, as these systems have been designed from the ground up to be both secure and very robust.

Many of us make weight management a New Year's resolution. Unfortunately, it is one we often do not keep. Weight management through digital technology, however, is one you can keep, and reap the benefits for years to come.

The author

Ian Verhappen operates the global consultancy Industrial Automation Networks Inc. He can be reached at iverhappen@gmail.com.

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