01 December 2000
Salted away for a rainy day: A half-billion barrels of crude
By Lisa Nicholson, Deborah Hojem, and Bruce Philippi
Smart-instrument technology leads upgrade at the U.S. Department of Energy's facilities on the Gulf.
The Strategic Petroleum Reserve (SPR) is the U.S.'s emergency supply of crude oil stored in huge, underground, solution-mined salt caverns along the coastline of the Gulf of Mexico. President Ford created the SPR after the oil embargo of 1973–1974, which had a significant impact on the U.S. economy and created unprecedented rises in gas prices and long lines at the pumps.
|DOE||U.S. Department of Energy|
|LEP||Life Extension Program|
|RTU||remote terminal unit|
|SPR||Strategic Petroleum Reserve|
The Gulf of Mexico was a logical choice for oil storage sites because it contains more than 500 salt domes along its coast. Many U.S. refineries, distribution points for tankers, barges, and pipelines are also located in the area.
Today, the SPR holds more than 500 million barrels of crude oil, the largest emergency oil stockpile in the world. The facilities and crude oil together represent more than a $20 billion national investment.
Worn out and obsolete
In 1994, the SPR created the Life Extension Program (LEP) to replace and refurbish the equipment and systems to extend the life of the complex through the first quarter of the next century.
The LEP used a reliability-centered approach. This approach included the upgrade and standardization of mission-critical equipment and processes to extend the useful life of the SPR to 2025. We did this by the following means:
- Replacing major portions of the piping and pumping systems to accommodate process design changes.
- Installing new state-of-the-art instrumentation and control system technology.
- Designing standardization into the control logic.
System design features included well-defined levels of operational modes, definition of operator interfaces, consistent equipment sequencing, 99.9% system availability, data and alarm/event history capability, and smart communications capacity.
The high system availability requirement necessitated system redundancies and automatic transfer from primary to secondary devices. Everything from the operator console to the data highway had to be redundant. Even the valve actuator communication network had to be redundant in order to meet this requirement.
Get smart the only way
The advantage of implementing smart communications was obvious: One pair of wires for all device inputs and outputs means faster and lower-cost construction. The increased device data would also help predictive maintenance and problem diagnostics.
The major decision was which communication protocol to use. In 1995, as today, there was no clear industry standard. One option was HART protocol. Its key selling point was the fact that manufacturers' products had to effectively pass tests performed with a data test suite.
This requirement ensured easy integration of multiple manufacturers' equipment. However, a survey indicated that few, if any, manufacturers' product lines addressed electric and electrohydraulic actuators for the large 24-, 36-, and 40-inch valves.
Thus, we chose a second smart technology for actuators. HART would handle only the process instruments, including pressure, flow, and temperature elements from various vendors.
The second technology we chose was Modbus—specifically, Modbus RTU using RS-485. RS-485 had been in use for several years, and multiple actuator manufacturers claimed to support the technology. At least they did until we asked for redundant communication channels. That stipulation significantly narrowed the vendor pool.
Lastly, we chose Fisher-Rosemount's Provox to handle the distributed control.
Hassle the maintenance
A key design decision for the actuator installation related to the wiring scheme. Modbus protocol using RS-485 allows several setups, including loop configuration, star configuration, and a combination of the two.
Each has advantages and disadvantages. A loop network installation entails significantly less wiring installation and fewerdevices to maintain in the field. After all, we could use the actuators' onboard repeaters to drive the signal from one actuator to the next, foregoing the need to buy, install, and maintain separate repeaters.
However, troubleshooting communication problems is more difficult, as some actuator failures can result in the failure of all downstream actuators. A star network installation, on the other hand, is just the opposite. There are more devices to install and maintain, but troubleshooting is easier.
Much to the maintenance department's chagrin, we chose a loop configuration.
The procurement process resulted in the following becoming the standard HART smart instruments on the SPR: Krohne ultrasonic and magnetic flowmeters, Bailey-Fischer & Porter pressure transmitters, and Rosemount temperature transmitters.
Redundancy raises the bar
Securing the Modbus RTU actuators was more complicated, largely due to the redundant communications requirement. Also, the majority of actuators procured under LEP did not come separately. They were part of a valve package.
We ended up with three standard actuators: EIM electric actuators for isolation valves, Limitorque Domgas electrohydraulic actuators for fast/accurate response control valves, and a combination of EIM and Limitorque electric actuators for the remaining control valves. Later we added Rotork control and isolation valves.
With design and vendor selection complete, the next big challenge was system start-up.
One of the first tests was integrating the operation of the three vendors' versions of redundant Modbus RTU products in a single loop. After configuring the actuators with 10 on one loop—five on either side of a center communications driver—only the first actuator in the series communicated with the control system.
What we initially thought was an installation error was actually a difference in design philosophy among manufacturers. Apparently there is room for interpretation of the concept of redundancy. The three vendors did not define polarity the same way.
By swapping the first pair of wires, two of the remaining nine actuators came online. The same change had to be made further down the series to make them all function. However, communications did not last, cascading to total communications failure in a short period of time.
Combo doesn't wash
After much consternation and gnashing of teeth, and with the help of representatives from all three vendors, we finally discovered that the manufacturers' designs for handling redundant communications were not compatible.
Two products handled each channel independently, meaning their products could talk on both channels simultaneously. One design didn't include simultaneous communication on the redundant channels. The combination of these designs on the Fisher highway didn't work.
Fortunately, we had months to resolve this problem. It took all of them. We are still working on resolution of the final problems from integrating a fourth vendor. Start-up of the HART devices was easier. After determining the correct configuration for the devices, the only major problem was the implementation of an optional HART data block by one vendor that was not compatible with the Fisher control systems diagnostics.
Beer barrel blues confuse
Thankfully, this problem did not affect overall system integrity status, but it did take a significant amount of time to pinpoint the problem. One vendor made a programming change, and all of our HART devices have been up and communicating reliably ever since.
We encountered several notable design considerations and constraints during the smart instrumentation start-up. The vendors have different requirements and guidelines for grounding and signal cable shielding; cable types (each device can require a different cable type); cable lengths; polarity; device communications control parameters; and end device register configurations.
With no standardization, the start-up team had a tremendous task to determine and resolve device inconsistencies. It took quite some time to troubleshoot our erroneous readings on the Krohne flowmeters. We learned the hard way that beer barrels and oil barrels are not equal. It seems that Krohne, with its roots in Germany, has two barrel settings, with the first being for the brew. IT
Editor's note: The opinions stated in this article are those of the authors and not those of the DOE or DynMcDermott Petroleum Operations.
Behind the byline
Lisa Nicholson is senior electronics engineer at the U.S. Department of Energy's Strategic Petroleum Reserve. She has B.S. and M.S. degrees in electrical engineering from Tennessee Technological University. Deborah Hojem is manager, control systems, at DynMcDermott Petroleum Operations Co., which is a contractor for the SPR. She graduated from the Tulane University School of Engineering. Bruce Philippi is principal control systems engineer at DynMcDermott. He graduated from the University of Alabama at Birmingham and the University of New Orleans with B.S. degrees in electrical engineering and computer science.
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