Cathodic Protection at Transwestern Pipeline
Corrosion is a pipeline operator's enemy, a natural process that is constantly attempting to reduce the pipeline's metals back to their original state.
According to the United States Department of Transportation's Office of Pipeline Safety, corrosion was the cause of 25% of all natural-gas pipeline accidents in a recent reporting period. Still, thanks to modern methods of protection against corrosion, the actual number of pipeline failures due to corrosion is relatively small.
Cathodic protection, in conjunction with protective coatings, is the primary method of defense against corrosion on major cross-country pipelines. Cathodic protection requires the constant delivery of a steady electrical charge. In a cathodic protection system, the pipeline is negatively charged, acting as a cathode. A positively charged sacrificial anode completes the circuit. In this system, oxidation occurs in the anode while the cathode—that is, the pipeline—is protected from corrosion.
For Transwestern Pipeline Company, cathodic protection is essential for the maintenance of 2,600 mi. of pipeline that transport natural gas to markets in Texas, Arizona, New Mexico, California, and the Midwest. John Gormley, a former Transwestern corrosion specialist, describes the threat of corrosion as "always waiting to reduce the metal back to its origins."
Gormley's job was to ensure the constant protection of Transwestern's pipelines against corrosion. The remoteness of the pipeline's route through the southwestern US makes that job difficult because the necessary electrical supply is not readily available. In populated areas, AC power can be pulled off of overhead electrical lines and converted into the DC power needed for the cathodic protection system. Lacking AC power lines, however, Transwestern turned to distributed-power generators at each system location.
Transwestern moves 1.2 billion ft.3 of natural gas through its system every day, so natural gas is the obvious choice of fuel for electrical generation. Historically, thermoelectric generators from Global Thermoelectric Inc. have been the power source of choice for Transwestern. Natural gas drawn from the pipeline is burned, and the heat moves through a thermoelectric module containing an array of lead-tin-telluride semiconductor elements. This module converts the heat directly into electrical power with no moving parts.
The thermoelectric generators have provided good service over the years, but Transwestern's pipeline system is now 41 years old. As the pipeline ages, more energy is required to maintain adequate cathodic protection. "Cathodic protection requirements were increasing," reports Gormley. "Thermoelectrics could not give the output needed."
Transwestern was faced with two options: add additional thermoelectric generators to increase the power supply at each location or replace worn-out units with a different type of generator.
"The [thermoelectric generators] were wearing out; repairs would be two-thirds the cost of a new unit," says Gormley. "Sticking with thermoelectrics would have required the installation of additional units." Overhead AC power was considered but ruled out as too expensive; Transwestern has many remote locations without cost-effective access to overhead power lines.
While Transwestern was considering its options, they were offered the use of a Minotaur remote-power system for evaluation. Now manufactured by Marathon Engine Systems, the 2.5-kW Minotaur uses a small internal combustion engine driven by natural gas or propane to deliver 48–100 V of DC power (or 120 V of AC power). The Minotaur offered five times the power of a thermoelectric generator at roughly the same cost per unit. Cathodic protection on the Transwestern pipeline currently requires a 30-V, 35-A output, a fraction of the maximum output provided by the Minotaur. The Minotaur provides enough reserve output to allow for future increases in power requirements as the pipeline ages.
Transwestern's experience with its first Minotaur, however, wasn't without problems. The system provided by Marathon's predecessor included a lot of remote-control and remote monitoring devices that created problems. Gormley also notes that the unit's plastic fan blades broke almost immediately. After buying the Minotaur line, Marathon "bulked up the unit," which greatly pleased Gormley. Transwestern now has six Minotaur generator sets in place: three in western Arizona and three in central New Mexico.
Replacement of the thermoelectric generators with the Minotaur units, which have a footprint of only 41 in.2, has been very simple. "Concrete pads and all connections were already in place," says Gormley, so Transwestern simply could pull an outdated thermoelectric system and put a Minotaur in its place with minimal effort.
Operating costs are low for Transwestern. Natural-gas consumption is specified by Marathon as 40 ft.3/day, which is a negligible volume compared to the overall volume of gas handled throughout the pipeline system. Transwestern has never metered the actual consumption, which shows up as part of the overall system loss.
Transwestern's Minotaur units are equipped with remote monitoring capabilities using Bullhorn telemetry devices from American Innovations. Because it does not require the purchase or installation of any equipment other than the telemetry devices on each Minotaur, the wireless Bullhorn system provided a simple solution for Transwestern. The Bullhorn devices use cellular telephone technology to report equipment readings to an American Innovations operating center. The data are stored in a database that Transwestern operators can access over the Internet using a Web-based interface.
The remote monitoring devices transmit four channels of data, including voltage and amperage readings, at a specified time every day. The primary use of the remote monitoring equipment, however, is to notify the pipeline operator of any shutdowns in the cathodic protection equipment. If the daily data readings show that a unit has shut down, the American Innovations system automatically notifies a Transwestern technician by e-mail. Downtime is minimized because a maintenance team can be dispatched immediately rather than weeks later when crews visit the remote location on routine checks.
Remote monitoring using cellular telephone technology might not sound like a viable solution in the sparsely populated locations where Transwestern's pipelines are located. The Bullhorn operates at much higher power levels than those used for voice transmission, however, and high-gain directional antennas can be installed; American Innovations claims 98% coverage of the US.
The remote monitoring system is designed to provide Transwestern with prompt notification of system malfunctions, but those malfunctions have been rare with the Minotaur. In fact, Gormley notes that only one malfunction has occurred since Marathon took over the Minotaur product line two years ago. In that instance, a circuit card failed, and Marathon sent a replacement card, which Transwestern personnel easily installed with no operating downtime.
Transwestern has found that the Minotaur also requires very little routine maintenance. Marathon designed the Minotaur's internal combustion engine to provide up to 40,000 hours of continuous operation, allowing nearly five years of use on the pipeline before a major engine overhaul is required. In the remote locations traversed by the Transwestern pipelines, routine maintenance intervals are specified at 4,000 hours by the Office of Pipeline Safety, although Transwestern personnel visit each unit every other month to gather such operating data as voltage and amperage outputs. These trips are somewhat redundant, however, as the Minotaurs are equipped with remote monitoring capabilities. Gormley expects that the frequency of visits will decrease as Transwestern comes to rely more on the daily data reports sent by the remote monitoring system.
Often vandalism can become a maintenance issue. Transwestern's Minotaurs are secured behind chainlink fences, but chainlink doesn't stop a bullet when a piece of equipment is used for target practice. Gormley reports Transwestern is lucky that no one has taken any shots at the Minotaurs yet, so their resistance to this type of vandalism has not been tested. The unit enclosures, however, are constructed of 3/16-in. aluminum structural members with 1/8-in. aluminum skins.
Gormley expects that the cost effectiveness of the Minotaur units now in place along the Transwestern pipeline will be reevaluated in five years. That is when Marathon recommends an overhaul be completed, and the economics of overhaul versus replacement with a different unit will be considered.
After installing the Minotaurs at Transwestern, Gormley returned to his home state of Nebraska and is now a senior corrosion specialist for Northern Natural Gas. Most of the pipelines in Northern Natural Gas's system are in populated areas with easy access to existing overhead AC power lines, so there is less need for distributed-energy systems, such as the Minotaur. Gormley is a strong advocate for the Minotaurs, however, and he looks forward to promoting their use along the southern end of Northern Natural Gas's pipeline in western Texas, where overhead power lines are less available.
Author's Bio: Guest author Thomas M. Roth, P.E., is a geological engineer with Parson's Engineering Science in Atlanta, GA.