Control systems help manage weight of offshore facilities

Two key considerations when designing offshore facilities are weight and volume or space, both of which are at a premium.

Ian Verhappen
Industrial Automation Networks Inc.

Two key considerations when designing offshore facilities are weight and volume or space, both of which are at a premium. Advances in automation and control systems have not only made these systems "smarter" but also smaller and more capable. As a result, these changes if combined with a design firm familiar with the resulting possibilities can have an impact on weight and space while also increasing functionality of the installed system.

For those facilities that are reticent to migrate from the "traditional" control system migration the first step in reducing weight and space is the removal of field junction boxes with the required multi-conductor cables to a central or even distributed interface room containing the control system input/output (I/O) cards and associated marshalling cabinets and controllers. Field junction boxes can easily be replaced with remote I/O cabinets of approximately the same dimensions and slightly more weight, which will now be distributed around the platform rather than concentrated in one area. In addition, rather than having a multi-conductor cable running from this new field terminal box to the control center, a single Ethernet or fiber-optic cable is all that will be required going out and a power cable going in. To get rid of the communications cable, as demonstrated by the FF ROM activities, use a wireless backhaul instead. However, this requires a greater leap of faith for control loops, especially in a tightly congested steel environment offshore where signals are not so easily transmitted.

Another recent development in I/O cards has been the removal of the need for dedicated I/O cards for each signal type. Traditional systems required separate cards for analog input and output signals, temperature signals, as well as discrete (on/off) input and output signals. Several control system manufacturers now supply what the author calls "configurable I/O" because the card is able to handle a variety of signals on a single card, with the definition of what type of device is connected to the wire pair being done in the control system itself.

Fieldbus systems also have the advantage of only having a single I/O card and in most cases – at least for the fieldbus networks typically used in the process industries – the additional benefit of combining both signal and power on the same wire pair. The individual transmitter signals are then combined at a field device coupler from which a single twisted pair runs to the fieldbus interface card in the interface room, where because all signals are of a single type a marshalling cabinet is not required. As a "rule of thumb" fieldbus systems result in an order of magnitude increase in signal density versus traditional analog installations.

When comparing different types of control systems installations, the assumption has always been a one transmitter one signal approach. However, the majority of today's transmitters are able to provide multiple measurements from a single process connection. The most commonly considered multi-variable transmitter is the Coriolis flow meter, which is able to provide mass flow, volumetric flow, density, and process temperature from a single device. However, for about the past 10 years vortex flow meters have been able to measure flow and in most cases process and body temperature, while also using the process temperature to offer a temperature compensated flow measurement as the devices primary signal output. Pressure transmitters used in flow applications can provide both the differential pressure on which the flow rate is based as well as the bulk process line pressure and the transmitter body temperature. More importantly, the final control elements/valve assemblies are able to provide feedback to the control system of the actual valve position at in real time.

The same communications protocols that provide these secondary and tertiary signals are also able to communicate device status and "health" diagnostics to the control system then to asset management and computerize maintenance/ERP tools. When integrated into a system, this diagnostic information provides predictive maintenance alerts.

Not every measurement or piece of information required to run a platform reliably is economic or available. However, it is possible to obtain some measurements without adding weight to a system, because these measurements are done in the control computer's sensors. Virtual sensors can create models to provide measurements such as the "health" of a heat exchanger as indicated by the heat transfer effectiveness or fouling factor, or the emulsion layer in a vessel from an estimated density profile, or the health of a pump system.

There are many ways to "manage" the weight of the control system on offshore facilities while at the same time increasing functionality and hence better control, safety, and reliability. All that is required is a bit of ingenuity, the discipline to insist on effective use of what you have, as well as the development and implementation of a strategic plan to optimize the use of a control system. Sort of the same thing required of any weight management program – a clear goal, a plan to get there, and the discipline to see it through.

The author

Ian Verhappen, P.Eng., is an ISA Fellow, ISA Certified Automation Professional (CAP), Automation Hall of Fame member, and a recognized authority on process analyzer sample systems, Foundation Fieldbus, and industrial communications technologies. Verhappen provides consulting services in the areas of field level industrial communications, process analytics, and hydrocarbon facility automation. Feedback is always welcome via e-mail

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