Virtual Flow Computing - A Case Study

In this article we will demonstrate how a Virtual Flow Computing system sometimes can be an attractive alternative for a traditional panel mounted flow computing and supervisory computer system - both costwise as well as on pure technical grounds.

Upgrade of a flow measurement system

An operator had to upgrade its flow measurement systems on an offshore platform that was more than 20 years old. The platform produced from multiple fields (with multiple and different ownerships), so utilities such as fuel, flare, gas lift, produced water and water injection had to be accounted and reported for allocation purposes in the upgrade project as well as field and platform exports. In total, 56 flow measurement points had to be accommodated.

The customer wanted to maintain 8 Gas Export and 8 Crude Export meter runs as dedicated flow computers but the remaining 40 measurement points were to be virtual flow computer (VFC) points.  Above all, space for the upgrade was to be minimized.

The metering system upgrade project comprised the following major elements: 

• 8 x Gas Export Flow Computers, orifice plates (Flow Computers)
• 1 x Gas Flow Computers, ultrasonic meter (VFC)
• 34 X Utility Flow Computers (VFC)
• 1 x Gas Flow Computers, vortex meter (VFC)
• 8 x Crude Oil Export  Flow Computers, turbine meters (Flow Computers)
• 3 x Produced Water Computer, electromagnetic meters (VFC)
• 2 x Produced Water Computer, orifice plate (VFC)
• 1 x Ancillary Flow Computer (VFC)
• 1 x Supervisory Computer System comprising dual (2) supervisory computers, with an integrated Graphical User Interface

Considerations

To implement the above using traditional panel mounted flow computers, this would have required at least 15 multi-stream flow computers and 4 or more 19 inch panels to install them + a separate panel for the supervisory computer.

Having used our VFC concept in combination with our Flow-X Rack mounted flow computer for the 16 Export flow meters, we were able to offer 16 hardware flow computers, 40 VFC’s and redundant VFC/HMI Servers and work station in just a single (1) 19 inch panel with wiring back to the exiting field terminals.

Design criteria

The function of the custody transfer gas metering system was to obtain to a high degree of accuracy and security for the quantities of gas and liquid transferred through each metering line.

In addition, a high degree of availability was to be achieved for the server computers.

For the targeted maximum security and availability, the configuration of the supervisory computers was designed on a Duty/Standby redundancy basis.

The supervisory / VFC computer system included a redundant server and dual redundant Ethernet network.

The VFC / supervisory computer system processed the information received from the different elements of the metering system, to optimize the validity and the accuracy of the measures.

The system also made a diagnosis of the measured values. The optimized and validated measurement values were to be sent to the central DCS.

Combined Flow-X & Virtual Flow Computer Solution

Our Virtual Flow Computer is highly efficient and accurate in terms of flexibility in calculations. In addition, the modular concept is most cost effective in comparison with hardware flow computers, especially with a higher number of metering streams (typically 4 or more).

Virtual flow computing functions usually operate on industrial hardware controllers to obtain real-time values for the connected 4-20mA analog, digital, and frequency inputs. The inputs from the hardware are transferred to a PC, which is running the VFC software.

For this system, we proposed Acromag 961EN-4006 devices, which are commonly used Ethernet to 4-20mA I/O devices. Each module is able to process 6x 4-20mA inputs. Each input having an accuracy of 0.05%. This accuracy is more than sufficient for the proposed meters, as these meters ONLY have one single dP input, and no live values for pressure and temperature.

The VFC application software processed each metering stream with meter-specific flow computations and presented the measured and calculated data on a number of worksheets, each containing a selected choice of the required VFC calculations, such as accurate eternal/period totalizers, and ISO-5167 gas flow calculations.

Since the software internally used double precision floating point formats throughout the entire application, high accuracy was ensured over the calculated flow results.

The Virtual Flow Computer allowed for a flexible and expandable solution which was to be used for measurements within the full station.

Performance Monitoring of ultrasonic meters

Besides pure flow computations, added functionality was made available to monitor the performance of the connected ultrasonic meter, and to present the available diagnostics from the meter integrated with the VFC.

Several display pages were made available for that purpose.

System architecture – VFC with Flow-X

The system architecture for the Flow Computing and Metering Control Supervisory system based on the Flow-X looked as following:

This alternative combined Flow computing and Metering Supervisory Control system consisted of two redundant servers, which were each connected to two Ethernet networks. The Flow-X flow computers and the Acromag 4-20mA I/O converters were connected to the redundant servers via the dual redundant LAN.

A redundant Ethernet network was used for the redundant link between the two server computers. A dual redundant network was used for high system availability as well as for network load sharing purposes.

In total, there were 17 Flow-X flow computers and 8 Acromag I/O converters used in the system.

How much more efficient can you become?