Chevron Pascagoula is switching out the old systems for Centum VP, Yokogawa's newest integrated production control system platform. Using a hot cutover method, engineers have so far successfully migrated two of the old control networks—one loop at a time, while keeping the units in operation.
"Each loop was surveyed out in the field prior to the cutover," Little says, explaining that DCS points and logic were tested and accepted by Chevron prior to beginning the hot cutover, and all transmitters and valve positioners were replaced in the transition. "All instruments that could be were mounted and connected beforehand."
Not Lost in Translation
Making the switch required putting together a team with considerable expertise. "We had to understand both the old system and the new and the differences between the two," Little said. "We had to make choices on how to best develop the new logic in the Yokogawa system to make sure we didn't lose any functionality."
A dedicated migration team was assembled for the entire project, Little said, including IT, control systems, operations, etc. "Everybody was affected. The team did loop-by-loop planning prior to cutover. If the loop was deemed a critical loop, then a procedure was developed to move that."
The move also required having extra operators on hand. When Chevron connected its subsystems (PLCs) to the new Yokogawa system, it was able to leave the connection to the old system to make sure everything was working properly. "During the hot cutover process, we had two complete sets of consoles, and we needed operators on each."
Little said that the migration itself has been a great opportunity to clean up the control system. "It's been there for 20 years. Things get left behind and built and never used. It's a good time to identify all the junk we had in our system and get rid of some of it."
The refinery expects to have much lower maintenance costs with the Yokogawa system.
"The new system is going to be much cheaper to maintain." He added that Yokogawa's hardware has thus far been "extremely reliable."
A new control room was built in 2008 to accommodate the new Yokogawa control system, and also to provide a shelter-in-place location. A new rack room contains the majority of the field control systems (FCSs) and 50 to 60 HP servers for the new control room. The site also uses remote FCSs, which are housed in old control room buildings.
Windows Pros and Cons
Although Little is concerned about running a process human interface stations (HIS) on a Windows operating system, with its greater opportunity for software failure and exposing the system to possible viruses, Windows offers benefits as well, he said. "With remote access from our office we can accomplish engineering functions and Windows patching offsite," he noted. "And most of the HIS parts are readily available, since the system uses a Windows platform. We used to have to go directly to the system vendor, which got costly at times."
Other benefits of the new system include easier configuration. "The Yokogawa configuration is much more apparent in the control drawing diagram," Little said. "The control drawings concept provides a good mechanism to group control calculations and functions together. The Calcu block is also very versatile, Little added. "It provides a good mix of features in one block, whereas in the old system, we had to use a combination of points to achieve the same results."
Switching to Cat 5 twisted-pair cables also has had its benefits: "They're much less problematic than the coaxial cables we had used," Little explained.
Despite the in-house expertise, the refinery team has had to do some learning along the way. "We never had fieldbus before," Little said. "There was a large learning curve for our refinery to learn fieldbus configuration and commissioning. It's not as easy as 4-20."