Electricity and natural-gas delivery company CenterPoint Energy – Mississippi River Transmission LLC (CNP-MRT) owns and operates 8,200 miles of transmission pipeline that carried 1.6 trillion cubic feet of gas throughout a nine-state, mid-continent region during 2012. Transmission-business success depends on meeting complex, capital intensive and data-critical infrastructure challenges, and moving large volumes of natural gas – reliably, over long distances – to 22 gas-fired power plants.
Competing for utility customers against other transmission companies and alternate forms of energy requires CNP-MRT to leverage technology that minimizes operating, maintenance and downtime costs. This allows them to achieve price and service levels that win and retain utility customers.
Transmission profitability is complicated further because the throughput-dependent segment also must accommodate variables, such as demand-reducing mild weather, fluctuating gas prices and regulatory-compliance overhead.
Efficiently, reliably and intelligently controlling, automating and monitoring the performance of reciprocating engine compressors represents one of the most significant challenges in moving the highest quantity of natural gas at the lowest cost. The school bus-sized engines maintain a pressurized flow (up to 1,500 psi) to reduce gas volume up to 600 times and propel it through a pipeline.
Control and automation opportunities to reduce infrastructure maintenance costs, minimize downtime and leverage real-time diagnostics are significant. But so is the potential capital investment and resource allocation attached to deploying upgraded control technology across almost 70 transmission-line compressor units.
Each compression station – located at 40- to 100-mile intervals along a pipeline route – utilizes two to 10 compressors in the 2,000-horsepower range. Aging systems typical in the energy-delivery space are built to last. Even equipment that went online in the 1960s can have another 20 years of life. At the same time, the decades-old machines are not as efficient as current technology. As a result, control systems tend to underperform current standards.
Upgrading control systems across the sizeable Midwest compressor fleet represents an enormous capital undertaking – particularly under a traditional outsourcing model that contracts control design and implementation to multiple engineering firms and system integrators. Over time that approach created a mix of control products and solutions at CNP-MRT – often based on black-box proprietary logic. Because this scenario tends to keep management and station operators at arm's length, over reliance on external vendors limits in-house ability – and significantly increases the cost – to configure, integrate, expand and upgrade control.
The CNP-MRT control and automation group saw the company's Horseshoe Lake compressor station as a test case for designing and deploying an internally executed station upgrade. The four-compressor facility in Grant City, Ill. is on the CenterPoint Mississippi River Transmission (MRT) pipeline that transports the bulk of natural gas moving state to state in the Missouri, Arkansas and Illinois region.
The company inherited Horseshoe Lake from another entity that specified and built the facility before turning it over to CNP-MRT to operate. The seven-year-old station was relatively new, but at the time of construction the original, economically minded systems integrator opted for proprietary architecture and PLCs already nearing the end of their life span.
“Relative to the potential of today's open architecture, the result was an antiquated control system programmed in a very antiquated way,” said Steve Starkey, the CNP-MRT instrumentation and electronics specialist overseeing the company's Northern pipelines. “In addition, CNP-MRT uses Rockwell Automation exclusively and doesn't support the platform that was in use at that facility. So we couldn't work on the control system ourselves without investing a significant amount of time and money in training.”
Simply adding another step in an engine startup sequence, for example, required a control vendor to reprogram it at an additional cost for each improvement. Troubleshooting a problem or obstacle, particularly in the middle of the night or on a weekend, opens compressor operations to downtime risk.
“It's often said that there are not many ways to troubleshoot a black-box problem,” said Starkey. “CNP-MRT doesn't believe that. We wanted something that we can manage directly, so we can be in control of the troubleshooting. In addition, a proprietary system typically is not modular. You can't replace things individually when you need to integrate and expand.”
Finally, CNP-MRT needed significant improvement in the information solution, from data access, collection and sharing, to real-time remote monitoring and alarming. Major expense variables for large compressors include engine maintenance to optimize operating efficiency and performance, as well as the cost to repair failures that should be avoidable.
An engine that goes down or fails means an energy company does not make money. With contracts based on high standards for delivery reliability – including fines for not moving gas as promised – a lack of data-driven visibility prevents being proactive on maintenance, reacting quickly with adjustments, and efficiently expanding or scaling up.
CNP-MRT used the Horseshoe Lake station to create and prove a potential model to cost-efficiently develop, manage and implement a major control upgrade relying heavily on in-house resources. Partnering with oil and gas specialists from the Global Solutions team at Rockwell Automation, CNP-MRT set out to design a solution that was specific to Horseshoe Lake, but that could serve as the standard, or template, for upgrading the rest of its Midwest region's compressor fleet.
CNP-MRT selected a PlantPAx™ distributed control system from Rockwell Automation that integrates control and information, using an open architecture to combine the capabilities of a DCS system with pervasive access to engine operation data. Vastly improved asset visibility and production information gives station operators the data necessary to respond faster on maintenance, operation and flow-control issues.
“The trend in compressor operations is collecting, integrating and using higher amounts of data,” Starkey said. “We're trying to optimize every operational aspect, in real time, that contributes to improved uptime, efficiency and profitability. That includes monitoring for unit performance, emissions and even the safety of engines that pump gas and run on gas.”
“Anything that we need to do in the process world, Rockwell Automation has done it,” Starkey said.
The Horseshoe Lake upgrade enables station operators to more easily manage speed and load control. The control system also determines which compressor units should start based on engine hours and the number of engines the system controls. “In the past, when you turned on engines individually,” Starkey noted, “you started No. 1, then No. 2, and so on. As a result, those units always had more hours. Now, the PlantPAx system knows to start the engine with the lowest amount of hours, and takes the highest-hour engines offline first.”
The visualization capabilities within the PlantPAx system provide a window on compression by incorporating performance metrics and the situational display of production information at engine, station and pipeline system levels. Accurate reporting of real-time engine events, along with analysis tools and management dashboards, deliver contextual, localized, role-based information for better decision making.
“Anything that we need to do in the process world, Rockwell Automation has done it,” Starkey said. “Having access to that domain expertise helps us, as an operations group, to turn out an effective control solution. It's very full-function, which makes application development and improvement easier. If you can dream it, you can do it.”
CNP-MRT summarises the upgrade returns in four key areas.
Cost of ownership – Design through installation and delivery of the new system at Horseshoe Lake came in well below previous systems. Based on experience with previous systems, Starkey estimated the capital investment at approximately 20 percent of the total for control projects equal in scale.
Simplified HMI configuration – Not having to learn every HMI application from scratch cuts training time considerably. With a baseline to work from, even if a station is different, the in-house control group can make modifications to fi t nearly every type of engine, which reduces integration time significantly.
Improved operability – The Horseshoe Lake experience indicates that operators, who are responsible for their respective stations, can be confident in most anything they will be asked to do. “Previously, there were many gray areas; too many instances where operators didn't have control or the necessary insight,” Starkey said. “The approach is different now. Operators themselves helped build really good HMI and PAC programs. It's their control system.”
Reduced downtime – The PlantPAx system has reduced station downtime and the capital outlay for repairs. Previously, there were no alarms other than shutdown. Today, with alarms programmed to catch engine issues before failure, operators initiate repairs ahead of engine failure. By watching the data remotely, staff can make an engine the least available until they resolve an issue.
Horseshoe Lake represents the first time that CNP-MRT has undertaken an entire compressor control station using in-house resources. There was understandable skepticism on the part of top management. “We had to demonstrate that the new system was substantially easier for our team to manage than previous systems,” Starkey said. “It had to be more scalable, more efficient, and more reliable than anything purchased previously.”
With Rockwell Automation supporting the project through goal setting, engineering advice, guidance and energy-specific best practices, the PlantPAx solution is the consensus control and automation model for the entire compressor fleet. From the faceplates to the look and feel of how the control system operates, the design and implementation is the CNP-MRT standard for upgrading other compressor stations in the future.
“The PlantPAx system has outperformed on every metric, from reliability to scalability and integration of most anything we can think of,” Starkey said.