This week’s Pipeliners Podcast episode features first-time guest Frank Onesto of Burns & McDonnell discussing the effects of alternating currents (AC) on pipelines and why this matters.
In this episode, you will learn about the common impacts of AC on pipelines, the benefits of modern pipeline coatings, and the three AC interference mechanisms that you should know about.
AC Interference Effects: Show Notes, Links, and Insider Terms
- Frank Onesto is a Pipeline Corrosion Engineer at Burns & McDonnell. Connect with Frank on LinkedIn.
- Burns & McDonnell is a family of companies bringing together an unmatched team of 7,600 engineers, construction professionals, architects, planners, technologists and scientists to help those who work in critical infrastructure sectors deliver on their imperative responsibilities.
- Alternating Currents (AC) is an electric current that reverses its direction at regularly recurring intervals.
- AC Interference Mechanisms include the following:
- Capacitive Coupling is the influence of two or more circuits upon one another, through a dielectric medium such as air, by means of the electric field acting between them.
- Inductive Coupling is the influence of two or more circuits on one another by means of changing magnetic flux linking them together.
- Resistive/Conductive Coupling is the influence of two or more circuits on one another by means of conductive paths (metallic, semi-conductive, or electrolytic) between the circuits.
- AC Mitigation is designed and installed to decrease the induced voltage on the pipeline.
- Metallurgy is the branch of science and technology concerned with the properties of metals and their production and purification.
- Ultrasonic (UT) Testing is a family of non-destructive testing techniques based on the propagation of ultrasonic waves in the object or material tested.
- Corrosion Engineering is an engineering specialty that applies scientific, technical, engineering skills, and knowledge of natural laws and physical resources to design and implement materials, structures, devices, systems, and procedures to manage corrosion.
- Epoxy Coating protects surfaces strengthens materials and protects them from corrosion and decay.
- Corrosion is the oxidation and electrochemical breakdown of the structure of a pipe used to convey any substance.
- Electrochemical Process is a chemical reaction caused by the applied electrical current.
- Cathodic Protection (CP) is a technique used to control the corrosion of a metal surface by making it the cathode of an electrochemical cell.
- High Tension Transmission Lines are used to transmit electricity at high voltages across long distances to reduce the amount of energy lost in long distance transmission.
- INGAA released a valuable white paper on the application to the pipeline industry, “Criteria for Pipelines Co-Existing with Electric Power Lines.”
- Collocation is when two structures are paralleling or near paralleling.
AC Interference Effects: Full Episode Transcript
Russel Treat: Welcome to the Pipeliners Podcast, episode 148, sponsored by Burns & McDonnell, delivering pipeline projects with an integrated construction and design mindset, connecting all the project elements, design, procurement, sequencing at the site. Burns & McDonnell uses its vast knowledge and the latest technology with an ownership commitment to safely deliver innovative, quality projects. Learn how Burns & McDonnell is on-site through it all at burnsmcd.com.
Announcer: The Pipeliners Podcast where professionals, Bubba geeks, and industry insiders share their knowledge and experience about technology, projects, and pipeline operations. Now your host, Russel Treat.
Russel: Thanks for listening to the Pipeliners Podcast. I appreciate you taking the time. To show that appreciation, we give away a customized YETI tumbler to one listener each episode. This week, our winner is Dyer Schlitzkus with LJA Engineering. Dyer, if I got your name wrong but got close, I should get credit because [laughs] it is a tongue twister, my friend. Anyways, congratulations. To learn how you can win this signature prize pack, stick around until the end of the episode.
This week, Frank Onesto with Burns & McDonnell is going to join us. I’m going to get educated about alternating current effects and mitigation with pipelines. Frank, welcome to the Pipeliners Podcast.
Frank Onesto: Afternoon, Russel. Thanks for having me.
Russel: Really a pleasure. I’ve got to tell the listeners right up front that I’ve asked you to come in and talk about alternating current effects on pipelines and mitigation. I’m just going to confess. I know very little about this. The quality of my questions in this episode may be suspect. That’s my disclaimer.
Frank: [laughs] That’s all right, Russel, because the industry as a whole still has a lot to learn about the exact mechanisms of AC interference. There’s been countless studies done throughout the 19th century and 20th century documenting the impacts that alternating current power lines can have on pipelines.
A lot of these studies conflict. There’s a lot of gaps in some of the information. Like I said, our industry as a whole still has a lot to learn.
Russel: Look, why don’t you give us a little bit about your background, your education, how you got into pipelining, and how you have developed your expertise around AC effect.
Frank: Sure. I studied mechanical engineering at Marquette University, and I was interested in manufacturing and metalworking. That led me to have a brief career in tool and dye engineering, specifically related to foundries and other machining processes. Through doing that type of work, I had some metallurgy experience, UT testing experience.
I was approached by a pipeline design firm who saw my metallurgy and UT testing experience and thought it would be a good fit in the corrosion world. That’s really how I got started in pipeline corrosion mitigation.
From day one, I was heavily involved in AC interference modeling and mitigation design. It’s a subject that I find absolutely fascinating particularly because there’s still a lot we have to learn.
Russel: I always think it’s interesting when you talk to engineers that it generally takes 5 to 10 years from the time you start working as an engineer before you land in that discipline that for some reason, you just find compelling. Generally, it’s some strange combination of factors. That seems to be thematic. At least, it is in my career.
I mean, I grew up a civil engineer. I did construction long enough to know that that wasn’t what I wanted to do.
Frank: I’ll admit. I didn’t know corrosion engineering was a thing when I was in school.
Russel: Neither did I.
Russel: Mechanical engineering is one of those things that nearly caused me to not be an engineer. [laughs] There you go. Frank, I feel you. You came into pipelining through the metal basically?
Frank: Yup, through the metalworking industry.
Russel: Why don’t we talk a little bit about…Just in general, you can educate me. How does alternating current affect a pipeline?
Frank: Sure. A lot of these impacts have emerged in recent years for a couple reasons. It’s very common nowadays to co-locate a pipeline in the same right away as a high voltage transmission line. It’s a very attractive option from a cost perspective, land acquisition.
Sometimes, the utility owns both the high voltage transmission and the pipeline. Land is becoming scarce. Routing these structures in the same corridor makes sense, but unfortunately, it has resulted in some of the AC interference concerns that we see nowadays.
Another reason why in recent years AC interference has become so prevalent is because of the improvement in our pipeline coating qualities.
The coatings that we use nowadays, like fusion bonded epoxy, are far superior to the older coatings that we used to use like coal tar. As a result, this high coating quality is not allowing alternating current to naturally dissipate off the pipeline because of the absence of small coating imperfections.
An old coal tar coating had a lot of micro imperfections. There were areas along the length of the pipe where steel was in contact with the Earth. When these pipelines were in close proximity to power lines, that current would get onto the pipeline. Just as quickly as it got on, it would naturally dissipate off of the pipeline.
Now with these modern fusion bonded epoxy coatings and other coatings, that current has nowhere to go. It’s being retained by the pipeline. It’s leading to accelerated corrosion and elevated voltage levels, which are a safety concern.
Russel: I want to unpack that a little bit. I did a podcast a while back on corrosion and the fact that that’s an electrochemical process. We talked about holidays and such. That was good because I knew that I needed to know about corrosion. I knew nothing about corrosion.
If I’m hearing you correctly — I’m going to try to restate what I think I heard you say — because the coatings are so effective at isolating the metal from the soil — the AC current that gets collected gets concentrated. Is that a fair way to say it?
Frank: That’s exactly right. The benefit that these modern coatings provide from a cathodic protection standpoint and a general corrosion mitigation standpoint, those benefits far outweigh the negative impact of alternating current.
It’s unfortunate that it has led to these alternating current concerns. Like I said, because the benefit outweighs those concerns, we have turned to mitigating the AC interference effect rather than not working towards an improved coating.
Russel: We’re not going to go backwards in terms of what we’re doing with coatings. We’re not going to have gas out with coal tar and brushes painting the pipeline before we drop it in the ditch.
It raises a question for me, though. How does that electric current get captured in the metal? What’s the mechanism that causes that to occur?
Frank: There’s actually three mechanisms of AC interference phenomena that we observe. I’ll start off with the first one, which is capacitive coupling. That generally occurs during pipeline construction when you have ungrounded segments of pipe resting on wooden skids before they’re lowered into the trench.
When these ungrounded segments of pipe are in close proximity to a high voltage line, they’re going to act as a capacitor. They’re ungrounded. We have an alternating current source in close proximity. The air between those pipelines and the conductors, that could be treated as a dielectric medium. Then your pipeline is the capacitor plate.
Those ungrounded segments of pipe are going to start collecting AC charge. It could lead to a rise in AC voltage which could become a shock hazard to construction workers.
That’s why it’s very common that you see a contractor usually ground the ends of these skidded pipes to reduce capacitive coupling. Once the pipeline is installed and backfilled, the capacitive effect is no longer a concern because the pipeline is grounded.
Russel: I want to ask a couple questions about that because a couple things come up. The first one is, for the pipeline to be grounded, there has to be someplace where there’s metal contact to Earth. If the pipeline is perfectly isolated, it’s not grounded. Is that correct?
Frank: Correct. No coating is perfect. When that pipe is backfilled, there are still places along that pipeline where there’s microscopic pinholes, metal in contact with the Earth.
Russel: At some point in the future, somebody’s going to come up with the perfect coating. Then this is going to become a bigger problem.
Frank: Then I’ll have a real problem.
Russel: The other question I want to ask, nowadays, we recharge our iPhones and such by setting them on these fancy little pads. We don’t actually have to plug something in. Are we effectively doing the same thing, this capacitive induction, to charge our iPhones?
Frank: Yeah, you could think of it as a similar concept. That’s going to lead me into the next interference mechanism, which is inductive coupling. When we talk about this, we could think about what you just said or an induction cooktop.
What’s going on here is that now the pipeline is backfilled and grounded, it’s still existing within that electromagnetic field.
Faraday’s Law of Induction says that when you insert a parallel metallic conductor into an electromagnetic field, you can induce a voltage onto that parallel conductor. That’s actually how transformers work.
You have two sides of a transformer, the primary and the secondary coils. One of those sides might be larger than the other. You induce a voltage across those coils to either step up or step down voltage. That’s how you can think of the system that we’re observing here. It’s a very poor transformer.
Russel: What’s the third mechanism?
Frank: That third mechanism of interference is called resistive coupling, otherwise known as conductive coupling. That occurs during a power line fault scenario. Just like pipeline operators have overpressure events on our systems, high voltage transmission line operators have fault events on their high voltage lines.
That could be a result of a lightning strike, high winds, maybe in the worst-case scenario, a tower has fallen and made contact with the Earth.
Either way, there is unintended current flowing to ground from those transmission line conductors flowing through the tower grounds into the Earth. Because it’s in close proximity to the pipeline, it’s going to use the soil as a direct path to couple with that pipeline.
We have these large currents injected into the Earth. That current is looking for a path of least resistance. It sees this nice pipeline. Steel’s very low resistance. It’s going to jump on there.
Russel: That’d be similar to a lightning strike.
Frank: Exactly. With inductive coupling or resistive coupling, where that current jumps off of the pipeline and returns to its source, that’s where we observe accelerated corrosion. We measure that discharge of current in the units of amps per meter square. That’s current density.
Russel: Couple other questions. These may be nonsensical. I don’t know. The voltage that the high tension lines are carrying, does it matter? Does higher voltage cause more of this problem, or is it more about current?
Frank: It is entirely dependent on current. You could have a 345 kilovolt power line operating at 500 amps. It could operate at 1,000 amps, 1,500 amps, whatever it may be.
That’s why the actual voltage rating of the power line doesn’t provide enough insight to determine if it’s going to cause a risk to our pipelines or not. We really have to take a look at the steady state currents that are flowing through those conductors, emergency currents, and those fault currents.
Russel: Does it matter if that current is cycling up and down?
Frank: Yes, it does. That has multiple impacts, particularly to some of the corrosion mechanisms that are occurring at the steel to soil interface.
Russel: If the current is cycling up and down, then the induced voltage on the pipeline is going to cycle up and down. That’s going to cause difficulties for the cathodic protection. They’re more designed to operate at a constant voltage.
Russel: Good. You took that as a question, not as a statement. [laughter] Because it was not a statement.
Frank: We operate power lines at 60 hertz. In Europe, they use 50 hertz. That constant cycling does have an impact on the overall magnitude of interference and then also how it impacts some of the cathodic protection levels and the performance of our CP systems.
Russel: Interesting. This is all awesome. I want to ask a couple of more just what is the problem we’re trying to deal with questions.
If I have a pipeline, it’s running in a high tension transmission line. For people that don’t know what high tension means, that’s just basically the tall electrical towers carrying the big, fat cables that are carrying a lot of voltage.
When those pipelines are moving down that right-of-way, there’s one impact. What happens when it moves out of the right-of-way?
Frank: Great question. We define that paralleling length as collocation. When these two structures are paralleling or near parallel, we call that a collocation. We generally consider everything within 2,500 feet to be collocated with these transmission lines. That plays into the magnitude of interference.
Let’s say these two routes are parallel. All of a sudden, the pipeline diverges away from that power line right of way. At that point of divergence, we generally observe a spike in interference. That has to do with the pipeline orientation changing in relation to the magnetic field. That disruption of the magnetic field causes a spike in voltage.
Russel: Electrical engineering’s one of the other reasons I nearly didn’t make it out of engineering school. [laughter] I did very well at structural engineering just for the record, and I did very well in math. Just so that the listeners know that I wasn’t a complete imbecile in college. I might have been a complete imbecile in college but not for those reasons.
Frank: Like I said, I studied mechanical engineering. The two things I studied the least are chemistry and electricity. It’s ironic that I’m having this conversation with you today. [laughs]
Russel: Exactly. I think that’s also thematic with engineers as well. The other question I want to ask is, to what degree does proximity from the pipeline to the power source matter?
Frank: It matters a lot. There’s studies that show that pipelines can experience significant interference when that separation distance is in excess of 1,000 feet. Even if you’re a good distance away from the power line, that power line still has the ability to induce current onto the pipeline.
That’s why I said earlier we consider everything within 2,500 feet to be collocated with the power line. Obviously, the farther you are away, the lower the risk. But generally, we consider everything within 100 feet to be at a high risk for AC interference.
Of course, that’s dependent on dozens of other variables. We can’t just look at one or two and then get an idea of how these transmission lines are going to impact our pipeline. We’ve got to look at all the variables. They paint a whole picture of the exact interference.
Russel: That’s right. They all interact. Managing one has implications for the other, from a risk standpoint. Does it matter if you’ve got other pipelines in the right of way? Does that have any impact?
Frank: It has some impact. I wouldn’t say it is the most influential of variables. If you have an electromagnetic field and you insert one parallel metallic conductor in there, that metallic conductor is going to absorb some of that field.
Then the more and more conductors you insert into the field, they’re each going to absorb a little bit of the field. It does have an impact, but I would say that it’s more difficult to observe how that would impact the overall magnitude of interference.
Russel: What about another pipeline that’s crossing over your pipeline? You’re in the right of way with the high tension lines. You’ve got a line that’s crossing over you. What impact would that have? That’s going to modify the field, for sure.
Frank: Sure, yeah. At a crossing location, let’s say that you have one pipeline that has a severe AC interference concern. You have a second pipeline crossing that. There is the chance for interference between those two structures, especially if the structures are bonded.
In the cathodic protection world, we bond a lot of structures to maybe protect multiple pipelines with one system or to mitigate DC interference between cathodic protection systems. If these two lines cross, especially if they were bonded, they have the ability to influence each other. That second pipeline could potentially pick up some of that harmful AC influence.
Now, it’s actually interesting. If we’re talking about pipeline crossing transmission line, if those two structures cross at 90 degrees, induction actually is not possible. It ties back to paralleling length. You need that paralleling length to induce a voltage or current onto that pipeline.
When these two structures are crossing at 90 degrees, there is no paralleling length. Induction is no longer possible. When we’re talking about crossing angles, we always recommend that pipeline operators, or if it’s a new transmission line being constructed, we always recommend that these structures try to cross as close to 90 degrees as possible.
Russel: Interesting. Makes sense but now I’m starting to think about does it matter if I’ve got an eight-inch line and I’m crossing a 36-inch line.
Frank: Again, that variable would be farther down on my list of variables that impact overall magnitude of interference. Pipeline diameter definitely does play into the overall interference. Smaller diameter pipelines generally experience a little bit more interference as opposed to large diameter lines. The diameter does play into it.
Russel: Interesting. Great explanation of the problem and its complexity. What do you do to mitigate these impacts? Fundamentally, what we’re trying to do is mitigate metal loss due to how we’re conducting that electricity, right? What do you do to mitigate all this?
Frank: Yeah. There’s several techniques that we can utilize to reduce AC interference. They are all different types of grounding. We want to ground the pipeline in some sort of way at specific locations. That is going to safely remove the alternating current from the pipeline.
If this alternating current is discharging into the soil through the coating, it’s going to result in accelerated corrosion. That’s similar to your discussions on CP. When current is leaving metal, it brings metal with it, causes corrosion. We want to provide a safe path for that current to travel to ground.
I’ll start off with probably the most commonly utilized method for AC mitigation. That is the installation of a parallel grounding cable or sometimes it’s called a gradient control wire. Basically, it’s a copper or zinc ribbon cable that we would bury at pipeline depth or close to it. It could be as close as one foot, or maybe it’s separated by upwards of 10 feet.
What it’s doing is it’s going to become that sacrificial cable. Rather than current discharging from the pipeline coating into the soil, the current is going to see that low resistance copper or zinc. It’s going to look for the path of least resistance.
It’s going to prefer to flow onto the copper or zinc. Then it’ll discharge into the soil from there. That copper zinc will corrode in preference to the pipeline. That is probably the most common and effective method.
In terms of fault interference, I mentioned earlier when power line faults occur and there’s large current dumped into the Earth, these same cables can provide shielding from those harmful currents.
You have a current traveling towards your pipeline instead of directly flowing onto your pipeline. It’ll see this low resistance cable that you’ve strategically situated between your pipeline and the interfering source. That cable will help to absorb that current. There’s a few other techniques as well.
Russel: What is the most elegant?
Frank: The most elegant of all these techniques would be this parallel grounding cable. It has the ability to mitigate steady state AC interference and also provide that fault shielding.
Other techniques we use, they’re very effective, some in both the steady state and fault scenario, but some are less effective at mitigating fault because of the inability to position these cables in such a way where they’re in between your pipeline and the interfering source.
Russel: I guess you also have to maintain those cables, right?
Frank: Yeah. They do have a life.
Russel: Because they’re sacrificial, once they’re gone, then you have nothing there to protect you.
Frank: Right. They do have a finite life. Someday, they will corrode. Similar to cathodic protection systems and anodes, it’s approximately 20 plus years of life you would get out of these systems. There’s ways we can measure the performance of the system and determine if maybe it’s time to replace a cable or add more grounding. Yeah, you’re absolutely right.
Russel: How does this affect your cathodic protection? How does this AC affect your cathodic protection systems? I would assume that what you’re doing for cathodic protection within the magnetic field is different than what you’re doing outside of it…
Russel: …and that at the areas of transition, they would require special handling, I guess would be the way to say that.
Frank: Right. That’s a topic that’s still up for debate. We have a lot to learn about the exact interactions between cathodic protection and AC interference, but we do have a pretty good idea of what is going on.
A lot of studies have shown that the presence of AC interference could effectively depolarize your pipeline cathodic protection levels. You’ll bring them down. You would be receiving less protection. The natural thing to do would be to turn them up.
However, other studies show that excessive cathodic protection could lead to accelerated AC corrosion. It has to do with the chemical reactions that are occurring at the steel and soil interface. We have changes in pH going on and changes in soil resistivity. All of these variables effectively either raise or lower magnitude of interference.
The general consensus is that you want to find a sweet spot with your cathodic protection levels. You don’t want to be under protected. You don’t want to be over protected.
Russel: I think this is one of the reasons why people with lots of years of field experience in this domain, even more so than some others, is really important. There’s a fair amount of this that goes to local knowledge too because particularly soils and drainage affect this a lot.
Frank: Soils are one of the biggest variables when looking at AC interference, specifically soil resistivity and the environment that the pipeline is in. If you have a very low resistance soil exterior to the pipeline coating, it’s going to be very easy for that current to discharge into the soil. Your current densities are going to go up. Your corrosion rates are going to increase.
On the other hand, if you had a very high resistance soil exterior to the coating, it’s not as easy of a path for that current to travel into, so more of that current is going to be retained by the pipeline. It’s going to make your voltage go up, which is a safety concern.
That soil resistivity, it’s so important to note. It really is a double-edged sword with how it impacts both current density and AC voltage.
Russel: It’s like a lot of things. It’s all easy till you know enough about it.
Frank: It’s very important if you’re trying to measure the impacts of AC interference on your pipeline. Something that would always go along with that would be the collection of soil resistivity measurements. That is one of those variables that’s just very difficult to assume.
An incorrect assumption there could severely under or overestimate what your pipeline is actually experiencing. You could have soil resistivity varying drastically over a pipeline route. You’ve got creek crossings and river crossings where there’s low resistance or rocky areas with high resistance. It’s very important to know the environment your pipeline is in.
Russel: Frank, what would you say that every pipeliner should know about AC effects and mitigation? How would you boil this whole conversation down?
Frank: It’s a phenomena that is not cut and dry. It’s very difficult to look at a pipeline route and just come up with a solution for mitigating AC interference, or sometimes, you may not even be able to determine if your pipeline is going to experience interference or not.
It’s important if your pipeline is in close proximity, not to make an assumption and skip the analysis that could really determine if your personnel is going to be exposed to shock hazards or if your pipeline is going to be exposed to accelerated corrosion.
Russel: Listen. This has been great. I feel more intelligent than I did when we started this conversation, so I thank you for that.
Frank: [laughs] Absolutely. Thanks for having me.
Russel: I hope you enjoyed this week’s episode of the Pipeliners Podcast and our conversation with Frank Onesto. Just a reminder before you go, you should register to win our customized Pipeliners Podcast YETI tumbler. Simply visit pipelinerspodcast.com/win and enter yourself in the drawing.
Russel: If you have ideas, questions, or topics you’d be interested in, please let me know on the Contact Us page at pipelinerspodcast.com or reach out to me on LinkedIn. Thanks for listening. I’ll talk to you next week.
Transcription by CastingWords