If you’ve ever taken a road trip, chances are you’ve seen the massive high-voltage power lines alongside and crossing over the road. In 2016, there were about 160,000 miles of high-voltage power lines in the United States, but likely more today as our population grows.
What you probably don’t see as often are pipelines. Utilities have recently begun running gas pipelines along the same paths as high-voltage lines along Right of Ways (ROWs) to save space. According to the National Pipeline Mapping System, every state in the contiguous United States has at least one pipeline. States like Texas, Oklahoma, and Louisiana have thousands of miles of long-distance gas, crude oil, and hazardous liquid lines buried underground.
Running two utilities parallel is a great idea because there aren’t many people interested in living near high-voltage power lines or underground pipelines. However, despite the space-saving impact of having them together, the pairing does present an issue.
High-voltage transmission lines are uninsulated and give off interference as electricity flows through them; that’s why your hair stands up when you get too close to them. The interference doesn’t cause much of an issue when the lines run together but may put people and assets at risk if they diverge.
Electricity always takes the path of least resistance. It’s also the problem we’re trying to solve with a form of cathodic protection called alternating current (AC) mitigation.
When pipelines operate near high-voltage power lines, they’re impacted by AC interference from the transmission lines. Transmission lines use air as their “insulation,” but air doesn’t do much to prevent interference from reaching the pipes. Essentially, the pipeline becomes a “secondary winding” of a transformer. It receives energy through electromagnetic induction or electrical current using a magnetic field.
In essence, the two structures become attached through AC coupling.
Three main types of coupling can occur between power lines and pipelines, including:
Conductive (Resistive) Coupling – This form of coupling occurs when there is direct contact between the two structures and typically happens during fault conditions. A lightning strike is one example of a fault condition.
Capacitive Coupling – Air is the dielectric between the pipeline and the power line when this coupling occurs. Sometimes electricity can arc from the high-voltage lines to the pipeline. Burying the pipeline prevents capacitive coupling from happening.
Inductive Coupling – When inductive coupling occurs, the pipeline is subjected to AC current due to an electromagnetic field.
Conditions are generally good for the pipeline and high-voltage lines when they run together along the same path.
It’s when the two utilities begin to diverge that problems pop up. When that happens, AC current wants to jump off the pipe, causing holidays (holes) in the pipeline. If enough holidays form, it can cause pipeline corrosion to set in faster.
That isn’t to say problems only occur when pipelines and power lines split. Transmission line towers are fantastic lightning rods, and when lightning bolts hit them, the electricity sometimes dissipates onto the pipeline. The massive influx of electricity creates pipeline holidays and encourages corrosion.
Cathodic protection is critical to protecting metal structures from corrosion, but how does it mitigate AC current?
Cathodic protection keeps direct current (DC) on the pipeline while reducing AC voltage as much as possible to protect the system. Grounding systems are attached to the pipeline to safely remove AC current while maintaining a constant DC current that slows corrosion.
DC current needs to be isolated for a cathodic protection system to work. Utilities rely on DC couplers to move AC current along while isolating DC current flow. When the couplers are applied to a pipeline or other metal structure, a low level of DC current is applied, preventing corrosion.
Computer modeling can also determine safe induced voltage levels, where potential problems like faults might arise, and soil resistivity along the pipe’s pathway.
There is no way to completely stop corrosion. Evidence suggests AC corrosion can still happen even when measurements are below the Association for Materials Protection and Performance’s (AMPP) 15VAC threshold, mainly due to surrounding soil conditions.
There are several ways to reduce pipeline corrosion caused by AC current, prolonging the life of the pipeline and reducing maintenance costs.
The impacts of AC currents get weaker the further the pipeline is from the power line. Though it’s nearly impossible to dig up thousands of miles of pipeline and move it somewhere else, it could make sense for new installs.
Moving the pipeline further from the high-voltage lines reduces interference between the two, and AC current will find other places to go. If the pipe cannot be moved further away from the transmission lines but still along the Right of Way, it may be possible to bury the pipe deeper underground.
DC decouplers may be your best tool to reduce AC current along the pipeline. When attached to the pipeline, decouplers reduce current along the line and dissipate energy into the surrounding soil.
Other options include galvanic magnesium anodes or surge protectors, which do the same thing as decouplers but use slightly different approaches.
Cathodic protection systems take several forms, but the most effective is a two-step process starting with applying a coating along the length of the pipe.
Once the coating is applied, an anode system is attached using DC current rectifiers and cathodic protection cable. The cathodic protection system searches for holidays along the pipe where the coating has been removed or disrupted, then completes the circuit in those spots to provide corrosion protection.
Although applying coatings is generally a good idea, it isn’t perfect. Film-backed coatings can sometimes block the protective current coming from the DC rectifier. This prevents the system from doing its job and leaves the pipe at risk of corrosion.
Without this critical protection, pipelines across the U.S. would be more dangerous.
Workers along the path would face higher risks of electrocution and shocks, and pipelines along the high-voltage lines would be at risk of arcs, surges, and other problems. This is on top of not having the basic protections that cathodic protection offers against corrosion.
Pipeline companies spend millions of dollars ensuring their assets are safe. Proper cathodic protection systems protect workers, homes, and businesses from danger and cost less to maintain over the years.
Cathodic protection technology may not be new, but the effort companies put into cathodic protection will continue to improve. And although we may not be able to stop corrosion entirely, we can keep these assets operating for many years.
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