In 2005, the Sultanate of Oman issued a Royal Decree to develop the giant Mukhaizna oil field covering a vast expanse of desert in the center and south of the country.  A major worldwide producer of oil, natural gas and chemicals was given responsibility for developing the Mukhaizna field, and from 2005 to 2008, oil recovery rates were increased by more than 600 percent through the use of a steam-assisted gravity drainage process. As each year goes by, the company makes every effort to continuously upgrade technology and improve productivity of the field.

Eight separate production facilities in the Mukhaizna oil field are using programmable logic controllers (PLCs) from automation supplier Rockwell Automation Inc., of Milwaukee, linked to human-machine interface/supervisory control and data acquisition (HMI/SCADA) systems supplied by software vendor Iconics Inc., in Foxborough, Mass., for data visualization and operator control. This data collection and distribution mechanism worked well when first implemented, but as the number of data points increased over time, it became clear to the project engineers that they needed a way to improve performance. So they began to look for a way to streamline the data flow. The solution they found not only performed well, but it created other, significant opportunities for real-time data integration.

At each of the eight locations, Rockwell PLCs are connected to an Iconics Genesis32 HMI through an OPC server, using the OPC open communication standard. The TOP Server OPC server from Software Toolbox Inc., Matthews, N.C., gathers data from as many as 20 PLCs, and feeds that to the HMI. As new equipment was brought online, the number of tags in the system approached 30,000, which is normally not a problem for TOP Server. But something was clearly different with this system, and it became apparent that some sort of optimization was necessary.

Too many reads

The problem was that the HMI was forcing the TOP Server to make device reads, which bypassed TOP Server's optimization at the device level. Device reads by an OPC client are intended to cause the OPC server to get the information and reply back to the OPC client before doing anything else. While these types of calls are useful in critical situations, all communication optimization has to be done through full system design. The HMI was also requesting updates on groups of OPC tags as it needed them, but these groups were often in a different logical order than how the data points were represented on the PLC. The combined effect was forcing the TOP Server to make more requests for smaller amounts of data, slowing the data-gathering process.

"The OPC server seemed to be dying under the load,"  says Juan Munoz, project manager for the Mukhaizna oil field project. "Even at rates as low as once per second, it was difficult to scan 30,000 tags, and get the critical data changes that we needed." Based on his experience with the TOP Server in other projects, Munoz knew the server itself was not the issue, so he searched the Software Toolbox Web site for a solution and found the OPC DataHub.

The OPC DataHub, developed by Cogent Real-Time Systems Inc., of Georgetown, Ontario, Canada, is a highly optimized data integration tool. It is a memory resident real-time database that provides quick, reliable and secure access to valuable process data and makes it available to other production and management systems, database archives and remote clients.

Once he started configuring the OPC DataHub, Munoz soon realized how it could solve his data flow problem. Acting as an OPC client to TOP Server, the OPC DataHub can request data based on tag value changes (referred to as asynchronous advise). This means that instead of 30,000 tags per second, TOP Server only sends data for a tag when it changes value. It is free to poll the devices in the most efficient way, always keeping the OPC DataHub up to date with the latest data values. The OPC DataHub keeps all of the latest tag values in memory, and can efficiently send them to the HMI on each poll.

"The OPC DataHub effectively decouples the OPC server from the client," says Munoz. "All the load is on the DataHub's shoulders now, and the performance is much better. The TOP Server is now free to optimize the communications to the device while the OPC Data Hub protects it from device reads." This has relieved the company from having to redesign their HMI and PLC configurations from the ground up, saving them ...