Category Archives: AC Interference

AC Mitigation Video | Overview of Goals, Strategies and Materials

We’ve talked about AC Interference and we’ve talked about AC Modeling. The topic of our newest training video is AC Mitigation. The video is about 9 minutes long and we’ve included timeline indicators below so you can easily find your topic of interest in the video.

The goal for AC mitigation is to reduce your fault condition stress values to protect against stress coating damage and arcing potentials (arcing is less common because you need to be very close to the pipeline for arcs to appear). This includes:

  • Reducing current density below your threshold value. Typically in the US we use 20 amps per meter squared for a one CM2
  • Maintaining AC step and touch potential below 15 volts so that people working in and around pipeline areas are not subject to shock due to a fault condition

AC Modeling Aids in Predicting Conditions [0:55]

We use AC modeling to provide predictions and look at the mitigated and unmitigated conditions. Some cases warrant building a model, in other cases we can use “ad hoc” methods (such as experience) to come up with an effective AC mitigation plan.

Learn more in our AC Modeling Video.

AC Mitigation Schemes [1:14]

For our example pipeline application, AC modeling results show some locations to be concerned about where the 20 amps per square meter threshold is exceeded. These locations are indicated below, where the red line is above the yellow 20 amps per square meter reference line. What do we do to mitigate this risk?

AC Modeling Results

In this case, we’re going to put in a gradient control line in the areas of concern next to the pipe. This is a grounding system that attaches to the pipeline so that AC being picked up by the pipeline has a place to go. The coating system is “too good” with only a few small holidays, which means all of the current being picked up tries to rush out of those few small holidays. This is how you end up with AC induced pipeline corrosion.

By putting in a grounding system at strategic locations along the pipeline, we can reduce the AC voltage being picked by discharging it and giving it a place to go.

There are several ways to design an AC mitigation system but they are all basically grounding systems. Our solution in this case is shown as the blue line representing a grounding mitigation line.

AC Mitigation Grounding

Typical AC Mitigation Strategies [2:23]

  • Install a gradient control mat at locations where people can touch the pipeline
  • Maintain safe pipeline to power line separation distances to avoid arcing problems during fault conditions
  • If separation distances are too close, include a shield that picks up current as it is dumped to the earth and deflects away to protect the pipeline
  • Provide grounding of the pipe to the earth to dissipate current being picked up during steady-state conditions

What is a gradient control mat? [3:00]

A gradient control mat is a simple device that is connected to the pipeline to protect workers from step and touch potentials.

It is connected to the pipeline appurtenance where a person can touch the pipeline, and extends out enough so that somebody standing on it will not have that step and touch potential. Since it is connected to the pipeline, the entire gradient control mat has the same potential as the pipe.

As soon as I step on to that gradient control mat, I don’t have a voltage difference between me and everything else. Even if I touch the pipe, the ground below me is at the same voltage as the pipe, so no current flows through my hand, to my body and into the ground.

It is a fairly significant effort to install a gradient control mat but they protect people close to that appurtenance. Once they are above that gradient control mat, touching or being near the pipeline is not going to cause a problem. Current doesn’t flow unless there is a voltage difference. You can actually be in an environment where there is high voltage all around you, and as long as you are at the same potential (or equipotential), there is not going to be any current flow, and current is what can injure or kill you.

AC Mitigation Case Study [4:33]

In the case study shown, a pipeline runs parallel for 8 km to a transmission line, with the towers next to the pipeline. In this case the towers are too close so we use a zinc ribbon shield wire to protect from fault conditions. The zinc ribbon picks up the current and dissipates it before it can cause damage to the pipeline.

AC Mitigation Reduces Coating Stress Voltages

The chart below shows the effects without any mitigation, where you can see the voltage spike where it goes above 12000 volts of coating stress voltage.

Effects of Zinc Ribbon | AC Mitigation

You can see once various forms of mitigation are added, stress voltage drops below the limits. And depending on the type of coating, there’s a certain voltage limit that coating can withstand.

Pipeline Grounding Methods [5:46]

  • Spiral mat at pipeline valve stem location
  • Anodes in the earth that are connected to the pipeline; these become grounding rods for the pipeline
  • Horizontal ground conductors, connected at various lengths to the pipeline (gradient control line mitigation)
  • Deep anode ground beds


Deep Anode Case Study [6:25]

Deep anode ground beds are a little more expensive, however they a good solution in high resistance areas where you can’t discharge current into the ground effectively near the surface.

We did a project out west in the desert of the United States, where a new pipeline parallel to a transmission line was picking up AC voltage. In the very dry desert environment there was nowhere for this current to discharge. Grounding rods next to the pipeline do not work well in this case because the environment is so dry. We drilled holes 1000 feet into the earth and installed grounding cells. These were run up to the surface and connected to the pipeline to dissipate the AC voltage being picked up.


[7:11] There are a variety of ways to ground a pipeline; AC mitigation is basically how we ground the pipeline effectively.

AC Mitigation Materials [7:16]

The most common materials used for pipeline grounding include:

  • Zinc ribbon laid parallel to the pipeline
  • Bare copper, which is used predominantly in the corrosion industry
  • Engineered copper grounding systems; The MATCOR MITIGATOR® is an example of this type of system
  • Conducrete® systems where conductive concrete is used to enhance the earth’s surface area

AC Mitigation and Grounding Concerns [7:58]

  • Ease of installation
  • Performance
  • Life, how long is it going to last
  • Cost

Optimum AC mitigation [8:12]

  • The AC mitigation system is only as good as the modeling, so it is critical to ensure that modeling is accurate
  • Gradient control lines parallel to the pipeline are the most common grounding system used currently, although there are also quite a few locations using deep anode systems
  • For fault conditions, short lines at tower footings tend to be the most effective AC mitigation strategy

Have questions after viewing our AC mitigation video, or need a quote for AC mitigation materials or services? Contact us at the link below.

CONTACT A CORROSION EXPERT

AC Modeling Introduction Video

In this video training session we talk about AC modeling. The summary below includes video timeline indicators so you can easily find your topic of interest in the video.

In the previous AC Interference video we talked about the effects of AC interference from transmission lines on parallel pipelines. We discussed three different modes of impact:

AC Interference Recap—3 Issues (0:24)

  1. Step and touch potential-must be below 50 volts AC
  2. Conductive coupling where a fault condition dumps current into the earth, causing potential damage to the pipeline
  3. AC induced voltage from the transmission lines on the pipeline

AC Modeling and Design

Best Guess Approach (0:57)

In some cases you can do a best guess AC mitigation approach, where it might not be worth the effort to put data and information into a model to determine the impact of AC interference on the pipeline.


Example: You have a simple application where you have one mile of pipeline collocated with high voltage transmission lines. You have measured that you’re picking up AC and decide that you’re going to put grounding in from point A to point B and be done with it. It is over-designed, but the cost of AC modeling would exceed the cost of this simple solution. This approach is based on experience in the field.


Complex Pipeline Arrangements Require AC Modeling (1:42)

When you get into more complex pipeline arrangements—multiple pipelines in the same corridor, multiple AC transmission lines coming in and out, multiple towers and circuits—you cannot just throw grounding in the earth and hope it is going to work. You may ground it in one location, and it may push the current somewhere else. In these cases you need to consider AC modeling.

AC Interference Modeling (2:08)

AC modeling is data intensive. And just like any model where we use a computer to predict what’s going to happen, the quality of the data impacts the quality of the results.

At MATCOR, we use a program called Right-of-Way Pro, a software package developed by Safe Engineering Services and Technology out of Canada. It is the leading AC modeling software available today. There are other packages that are less expensive and less accurate, but Right-of-Way Pro is the gold standard for AC modeling software.


AC modeling is a service that MATCOR provides. We have trained professionals with years of experience modeling AC systems in the pipeline industry, and experience with this complex software.


AC Modeling Goals (3:16)

The goals when you’re performing AC modeling are simple:

First, we calculate the fault condition stress values.

What is the worst fault condition that can occur? What is the worst that can happen at each tower and how does that affect the pipeline given the relationship of the pipeline to that tower? How far away is it? How deep is it? What is the resistance in that location? We model every tower along that collocation.

Next, we calculate induced voltage.

This is the impact of having an electrical field with the pipeline running through it and picking up voltage. We model this for every collocation. This can get rather complex when you have multiple pipelines and multiple AC towers in the same corridor.

Then, we predict the AC current density along the length of the pipeline.

In the AC Interference video we talked about AC induced corrosion being a function of how much current is being discharged off small holidays. There is a certain threshold we do not want to exceed or we will be concerned about corrosion occurring.

The model of the pipeline will show where it is picking up current. At every point along the pipeline, the model indicates, given a holiday of a certain size, whether we have a problem with AC corrosion. This will change depending on where we are along the pipeline collocation and what the soil resistivity is around the pipeline in that location. Lower soil resistivity tends to mean higher current discharge, and these tend to be the areas of concern.

Finally, we evaluate mitigation measures.

Where should we put mitigation, how much mitigation, and how effective will it be?

As we are calculating induced voltage along the length of the pipeline, we are looking for areas where we exceed 15 volts because this is a safety concern. We are also looking for areas where a 1 cm² holiday would have more than a maximum threshold of current density. 20 amps/M² is a typical threshold in the US, since that is where corrosion can occur.

AC Modeling Data Requirements (5:50)

We need a lot of data to build the model out, including data on the HVAC transmission line, the pipeline location and characteristics, and the soil resistivity. In addition, we need information about changes in the collocation relationship. These changes are called excitation points; if there is suddenly a change in the pipeline or the high voltage power line, it is often a hot spot in the system and where you will likely have problems.

HVAC Line Data (6:39)

For the HVAC line we want to know the tower geometry, which can change. How high are the towers? How long are the spans? What is the separation of the different phase conductors? AC is always a 3-phase system, with an A, B and a C line. We need to know if there are phase shifts and where they are located for the model. We also need to know the phase conductor arrangement, the conductor height and distances, if shield wire* exists, and current loading information. What is the average current flowing through that line? What is the maximum, or peak current expected? Is there anticipation that the rating will increase in the future? Finally, what are the fault and ground fault currents?


*Shield wire helps in fault conditions; if there is a fault, instead of dumping current to the earth it will travel along the shield wire.


(8:08) Collecting HVAC line data can be a challenge. It is often a combination of going out into the field and physically measuring, and contacting the power company to request information like maximum fault conditions, length of maximum fault, and how quickly will breakers trip to clear a fault. Power companies don’t always like to provide this information and will often ask operators, or consultants like MATCOR, to pay a fee. When this information is not available we sometimes make assumptions, however the more assumption we make the less valid the model becomes.

Pipeline Characteristics (9:12)

We want to know pipeline characteristics, which are generally easier to get. Either they are easier to measure or the pipeline company has good data already.

We need to know the pipeline diameter or diameters, as sometime this can change. A 16-inch pipe may become a 20-inch pipe at some location. This would be an excitation point because a change has occurred. What is the wall thickness and material? What is the coating type, thickness & quality? What is the coating conductance?


(9:52) If your pipeline has an older coating, you probably do not have a big AC problem. If you have a brand-new, high quality coating means you probably have a bigger AC problem.


Depth of cover survey—how does the pipeline change its depth relative to the earth? We need to know the accurate GPS centerline of the pipeline in addition to the location of valves, casings, bonds and foreign pipeline crossings. Finally we need to know soil resistivity at different depths and multiple locations along the pipeline. The AC modeling software has the ability to look at multiple layer effects of the soil.

All of this data must be collected and put into the model. Often the pipeline company can provide the data or we can go out and measure it.

AC Modeling Software Key Features (11:35)

  • Up to five layers of soil resistivity modeling
  • A large conductor database—including most transmission line conductors, all copper conductors and mitigation devices such as zinc ribbon and the MATCOR MITIGATOR®
  • The ability to model large networks with multiple pipelines and multiple power lines along a corridor in the same model

AC Modeling Costs (12:11)

For a relatively simple collocation, you may not need to perform AC Modeling. A very simple AC modeling job might be $20,000. The cost can be well over $100,000 for a project requiring a lot of data collection and modeling effort.

In some cases it may be simpler to put in $10,000 worth of grounding and “overdesign” the AC mitigation system since the cost of modeling would be greater. In other cases, AC modeling is an absolute necessity.

AC Modeling Software Capabilities (12:42)

AC Modeling Example
(13:17) This is an example of AC modeling with multiple high voltage transmission lines and a pipeline going from one location out to a terminal. We have to model each of those transmission lines as well as the pipeline characteristics
  • Transformers, insulators, substations and other devices
  • Different phasing, pipe diameters and coating type configurations in one model
  • Solid state decoupler sizing
  • 3-D viewing and plotting
  • AC corrosion output
  • Fault modeling

AC Modeling Software Output (13:42)

The output from the software shows you along the length of the pipeline, those areas where you have concerns about steady state touch potentials, from a safety standpoint where it might be above 15 volts.

AC Modeling Software - Touch Potentials
AC Modeling Software – Touch Potentials

It will also provide your 1 cm holiday leakage current density, indicating areas where you are at risk for AC induced corrosion.

AC Modeling Software - Current Density
AC Modeling Software – Current Density

In addition the software provides areas where fault currents can exceed the maximum allowable for your coating stress.

When is AC Modeling Not Required? (14:16)

  • Simple applications
  • When overdesigning AC mitigation solves the problem

In our next segment we will talk about AC mitigation. What do you do once you know where the problems are?


Have questions after viewing our AC modeling video, or need a quote for AC modeling? Contact us at the link below.

CONTACT A CORROSION EXPERT

AC Mitigation: 4 Approaches

AC mitigation is the process of designing and applying pipeline grounding systems to:

  • Prevent voltage spikes during fault conditions
  • Reduce AC current density to protect against AC induced corrosion
  • Maintain AC step and touch potentials below 15 Vac to protect personnel from shock hazards
AC mitigation prevents voltage spikes and corrosion and protect workers
AC mitigation prevents voltage spikes, protects pipelines from corrosion and protects workers in areas where the pipeline parallels high voltage transmission lines.

Pipelines that parallel overhead high voltage AC transmission power systems are subject to AC interference. AC interference has several potential adverse impacts on the safety of personnel and pipeline integrity. Assuming that these conditions exist, there are several measures that can be taken to mitigate the AC interference present on a pipeline. These AC mitigation strategies are detailed in various international standards including NACE SP0177-2014 Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems.

There are four basic approaches to mitigating AC Interference. These mitigation strategies are:

1. Fault Shielding

One of the primary concerns with high voltage AC transmission systems parallel to buried pipelines is the risk that a fault condition at a transmission tower could result in the rapid discharge of fault current near the pipeline. This could lead to direct current arcing in soil – rare but very damaging. More common is the rapid ground potential rise that subjects the pipeline coating to large voltage gradients that result in coating damage. Fault shielding is a suitably designed grounding system that is installed between the tower footing and the pipeline that acts to shield the pipeline and shunt harmful currents away from the pipeline by providing a low resistance path to earth. This typically takes the form of a parallel shielding wire, either copper or zinc, connected to the pipeline.

2. Gradient Control Mats

When high levels of AC voltage are present on a pipeline, either during a fault condition or as the result of an inductive coupling during normal steady-state operations, personnel in close proximity to and/or touching any above ground or exposed appurtenance are at risk for electrical shock step or touch hazards. Installing gradient control mat, which is a system of buried bare conductors, typically galvanized steel, copper or zinc, connected to the structure, provides localized touch and step voltage protection by creating an equipotential area around the appurtenance.

3. Lumped Grounding Systems

Lumped pipeline grounding systems consist of shallow or deep localized grounding conductors that are connected to the structure at strategic locations to reduce the AC voltage level along the pipeline. This provides protection to the structure during steady-state or fault conditions from nearby electric transmission.

4. Gradient Control Wire

Gradient control wire grounding systems function the same as the lumped grounding system. With this type of system, long continuous grounding conductor(s) are installed horizontally and parallel to the pipeline. They are strategically located and sized to reduce the AC induced voltage along the pipeline during steady-state or fault conditions from nearby electric transmission.

For mitigating high levels of AC induced voltage along a pipeline, gradient control wires are the most common form of AC mitigation. Hybrid systems that combine lumped grounding systems with gradient control wires are also common. Regardless of the type of pipeline grounding system used, all of these AC mitigation approaches involve installing a grounding device to the affected structure to allow AC induced current and fault current to be quickly discharged off of the pipeline.

AC Modeling

Prior to installing an AC mitigation system, it is common to use a complex AC modeling software to evaluate the impact of fault currents and estimate the steady state induced currents that can be expected along the pipeline. This information is used to determine the quantity and location of mitigation required based on numerous factors, including the resistivity of the soil, the physical characteristics of the pipeline, the operating parameters of the HVAC transmission system and the spatial distances between them.

Engineered AC Mitigation Systems

Based on a thorough assessment of the pipeline and high voltage AC transmission system interaction, including modeling results when available, an AC mitigation system is designed by experienced engineers familiar with the mitigation strategies detailed above. This engineered AC mitigation system would detail the quantity and location of grounding installations required for a specific application. MATCOR’s MITIGATOR is an example of this type of AC mitigation system.

Other features of an engineered AC Mitigation system include:

Special Backfill

It is quite common to install the grounding conductor in a special backfill material. The purpose of the backfill can vary depending on the conductor material chosen and the type of backfill used. The benefits of various types of AC mitigation backfill include:

  • Enhanced surface area – conductive backfills such as carbon or conductive concrete are used to effectively increase the surface area of the grounding conductor reducing the overall resistance to earth.
  • Corrosion/Passivation Protection – some backfills are designed to protect the grounding conductor from corrosion or passivation of the conductor that could adversely affect the life or impede the performance of the grounding conductor.
  • Hydroscopicity – some hygroscopic backfills readily attract and retain water from the environment, helping to maintain a low uniform resistance around the grounding conductor.

Solid State Decouplers

These devices are almost always used in conjunction with AC mitigation systems and are usually installed wherever the grounding system is connected to the pipeline. These devices are designed to allow AC current to flow off the pipeline during steady-state or fault conditions while blocking all DC current. This effectively isolates the pipeline’s cathodic protection (CP) system from the AC mitigation system, preventing the mitigation system’s grounding conductors from taking CP current from the pipeline.

MATCOR provides complete AC Mitigation solutions including design, supply of materials, turnkey installations and comprehensive testing services.


Have questions or need a quote for an AC mitigation system or services? Contact us at the link below.

CONTACT A CORROSION EXPERT

AC Interference – Basic Theory | Video Training Course

What is the impact of AC interference on pipelines?

This 16-minute AC interference video training course reviews the 3 basic effects of AC interference on pipelines, including:

    1. Fault conditions
    2. AC induced corrosion
  1. Safety and the 15 volts AC threshold

The summary below includes video timeline indicators so you can easily find your topic of interest in the video.

What is AC Interference?

(0:25*) AC interference is an interaction that occurs between high voltage power lines and pipelines in a common utility corridor.

*References the time in the AC Interference Video where this topic is reviewed.

1. Fault Condition Interaction Modes

In the video, our AC mitigation expert Ted Huck explains fault currents and two modes of interaction with pipelines, conductive coupling and stress voltage.

Conductive Coupling

(1:09) Conductive Coupling is a relatively rare occurrence when there is a fault condition along the power transmission line and a large amount of electricity is dumped to the earth. The collocated pipeline is subject to this discharge of electricity through arcing, defined as the flow of current through the soil. Although rare, conductive coupling can burn a hole through the pipeline and cause a catastrophic failure.

Determining the Safe Distance from Tower to Pipeline for Arcing

This chart from our ac interference video shows how you can determine the safe distance from an electrical transmission tower to a pipeline and the potential for arcing to occur to aid in mitigating AC interference.
(2:08 in the AC Interference video) Arcing depends on soil resistivity and voltage.

Arc Length

This chart shows how far arcing can occur through the soil based on system voltage.
(2:49) We can predict how far arcing can occur through the soil by measuring soil resistivity and system voltage.

(2:16) In this segment, our AC mitigation expert describes a real customer case scenario where arcing caused catastrophic failure of a gas pipeline.

(4:01) Another issue that can occur with conductive coupling is a voltage rise radiating out from the location where the electricity is dumped to the earth. Newer pipeline coatings cannot handle excessive voltage stress.

Stress Voltage

This chart shows maximum AC interference stress voltage for various types of pipeline coatings.
(4:41) Exceeding maximum voltage stress can damage the pipeline coating.

Electromagnetic Induction

(5:44) Electromagnetic Induction is a steady state occurrence where current flowing through the line creates an induced current flowing in the opposite direction along the parallel pipeline. If the pipeline is close enough to the power transmission line, and runs parallel to it for some length, it will be in the electromagnetic field that exists around the AC transmission system. Being in that electromagnetic field, it will inductively pick up current throughout the longitudinal electrical field.

Longitudinal Electrical Field (LEF)

This illustration shows how you can measure the longitudinal electrical field (LEF)
(6:56)  The LEF field can be measured by running a copper cable along the pipeline and measuring the current.

2. AC Induced Corrosion

(7:26) AC induced corrosion occurs when alternating current is picked up by the pipeline that cannot effectively dissipate back to the earth. Well coated pipelines have very few places for the current to exit the pipeline and are at risk for significant, rapid AC corrosion. Older coating systems have many defects, or natural grounding points enabling AC on the pipeline to naturally dissipate, so AC corrosion is a relatively new concern. With newer coatings, AC current continues to build until it finds a small coating holiday (typically 1-3 cm2) to exit the pipeline, risking catastrophic failure.

This image shows a closeup of AC interference induced corrosion, which appears as round craters in the pipeline coating.
(9:00)  AC induced corrosion on a pipeline appears as round craters in the pipeline coating.

How likely is AC corrosion to occur?

This chart shows how likely it is for AC induced corrosion to occur for various current densities.
(9:43)  We can determine the likelihood of AC induced corrosion based on the current density.

Current Density Formula

This formula shows how to calculate current density for a given holiday size; our example is based on a holiday surface area of 1cm².
(10:05)  This formula shows how to calculate current density for a given holiday size; our example is based on a holiday surface area of 1cm².

In our example, 4.4 volts AC is all it takes to cause AC corrosion. With older pipeline coatings that threshold is in the 15 volts AC range.

100 A/m² Threshold – When will AC corrosion occur?

This chart shows holiday size and AC voltage required to exceed the 100 A/m² AC corrosion threshold at varying soil resistivity.
(11:37)  This chart shows holiday size and AC voltage required to exceed the 100 A/m² AC corrosion threshold at varying soil resistivity.

Refer to NACE Report 35110, AC Corrosion State-of-the-Art: Corrosion Rate, Mechanism, and Mitigation Requirements for additional information about the 100 A/m2 threshold.

In Europe, refer to standard BS EN 15280:2013, evaluation of AC corrosion likelihood of buried pipelines applicable to cathodically protected pipelines.


The Relationship Between AC Induced Corrosion and Cathodic Protection

(12:00)

  • No cathodic protection – high likelihood of AC corrosion
  • Excessive CP current, or over polarization may increase AC corrosion
This chart shows how cathodic protection affects the likelihood of AC corrosion.
(12:35)  Properly applied cathodic protection reduces induced AC corrosion risk.

3. Safety

(12:50) Pipelines have above ground appurtenances such as valve stems and test stations that are subject to the AC currents picked up by the pipeline. These can pose a serious safety risk to workers, including shock or death. These risks are referred to as step and touch potential.

Touch Potential is defined as current flowing from touching an electrified device, through the body and down to the earth.

This image illustrates step and touch potential.
(13:38)  Step and touch potential can cause serious safety risks, including injury or death. A maximum of 15 volts is the industry standard threshold for safety.

Step Potential can occur even without the worker touching the electrified device. In this case, current can flow up one foot, through the body and back down to the earth through the other foot, potentially causing serious injury or worse.


Refer to NACE SP0177-2014 (formerly RP0177), Mitigation of Alternating Current and Lightning Effects on Metallic Structures and Corrosion Control Systems,  Paragraph 5.2.1.1 for additional information on the 15 volt safety criteria.


Summary

This AC interference video reviews the 3 basic effects of AC interference on pipelines, including the maximum 15 volt safety threshold—how much voltage can accumulate on the pipeline before it becomes a safety hazard to a person touching the pipeline? If there is more than 15 volts AC, we must do something to drop that voltage. Then there are rare but potentially catastrophic fault conditions, or the dumping of current to the earth. Finally there is AC induced corrosion, a result of the interaction of the electromagnetic field generated by current flowing through the lines and how it reacts with the pipeline. Pipeline operators must be prepared to mitigate these risks.


Have questions after viewing our AC interference video, or need a quote to mitigate the risks of AC interference? Contact us at the link below.

CONTACT A CORROSION EXPERT


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AC Modeling – The Basics

AC Modeling Overview

Pipeline Right-of-Way
AC Modeling enables pipeline operators to evaluate and plan for mitigating AC corrosion.

There continues to be much greater awareness by pipeline owners and regulators of the adverse interactions (AC Interference) that can occur between buried pipelines and above ground high voltage AC transmission systems that share some parallelism in a common right of way. When AC Interference conditions exist, it is important that the potential impact is evaluated and when necessary mitigated. For many applications, the most cost-effective approach to assess and mitigate the impact of AC Interference is to use a complicated computer AC modeling program.

The term AC Modeling really covers multiple modeling evaluations, as an AC corridor can often be quite complex. They may include multiple HVAC transmission systems and multiple pipelines in a common corridor or multiple shared right of ways along a long length of pipeline. Each may require its own AC modeling. In addition, the modeling looks at several different risks assessing how the pipeline is affected by steady state AC induced current, the impact of fault current along the pipeline and an evaluation of the impact of a fault current on above ground appurtenances to assure safe operation in accordance with IEEE std. 80 step and touch potential criteria.

Thus, it is very important for any successful AC modeling effort that the modeling software be of an extremely high quality and capable of properly handling the complex interactions of these various networks. The engineer or technician developing the model must also have sufficient experience and expertise to properly configure and operate the model, and evaluate the results.

AC modeling involves four key phases:

  1. Data Collection
  2. Creating the Model
  3. Establishing criteria
  4. Evaluating mitigation strategies

Data Collection

The data collection is critical to a successful modeling effort (the old adage garbage in = garbage out is quite applicable for these projects). The data requirements can be broadly broken out into three categories:

  1. The characteristics of the AC Transmission Line(s)

    • Physical geometry data on the tower including GPS location, height, # of AC circuits, tower configuration, height of each conductor, lowest point of each conductor, separation distance between conductors, shielding wire type and location, location of any phase transpositions, etc…
    • Electrical data on the Transmission Line(s), including peak and average AC Load (in each direction), fault current max and duration.
  2. The characteristics of the Pipeline(s)

    • GPS location, depth of cover, coating type, coating resistance, pipeline diameter, pipeline wall thickness, location of all above ground appurtenances, location of all CP test stations and bonds to foreign structures.
  3. The characteristics of the Environment

    • Detailed soil data at multiple depths along the length of the pipeline, location of any crossings, presence/location of any foreign CP Stations or other interference conditions.

Collecting all the appropriate data often requires some field studies and working with both the pipeline owner(s) and the transmission line operator to get the required data. In some cases, the modeler cannot get all the required information and must make an educated guess – the accuracy of which can affect the quality of the results.

Creating the Model

AC Modeling
AC modeling software enables input of pipeline, transmission and environmental characteristics

Once all the data is collected, the modeler creates the model space, detailing all the pipelines and HVAC systems and providing the requisite parameters associated with each of these elements. There are several commercially available AC modeling software packages that each have their own format for inputting the pipeline, transmission and environmental characteristics. Once the model has been built, it can take hours, days and in some cases weeks, of processing time to run simulations and for the model to provide the results of the simulation.

Evaluating the Model Results Against Established Criteria

The results of the initial model run need to be evaluated against the criteria that is established by the pipeline owner. In the absence of specific guidance from the owner, MATCOR’s default criteria are:

  • No more than 20 A/m2 AC current density for mitigating AC corrosion during steady state induced AC current
  • 3000 volts maximum coating stress during fault conditions for newer FBE type coated pipelines in accordance with NACE standard SP0177-2014
  • 15 VAC for step and touch potentials at above ground appurtenances

For any given application, one or more of these criteria may be exceeded along the model’s area of analysis.

Adding AC Mitigation and Reevaluating the Modeling Results

Once the initial unmitigated results have been evaluated against the criteria that has been established, the modeler then adds, based on their experience with these systems, a mitigation scheme to the model with grounding at selected locations. This is often an iterative project where the model is run and the results evaluated and then if necessary, additional mitigation can be added or excess mitigation can be removed and the model rerun again in search of an “optimized” modeling solution that addresses all of the threats and results in meeting the requisite criteria.

Final Report

Once the AC modeling effort has developed a solution, the modeler develops a final report. Typical components of a final report include an introduction detailing the scope of the study, graphical illustrations of the pipeline(s) and transmission line(s) overlaid onto a satellite image, description of the modeling software used, detailed graphs/charts showing the results of the modeling, detailed drawings and bill of materials for the AC mitigation solution being recommended and appendices with the underlaying data.

Summary

AC Interference issues can be quite complex and modeling often offers the only valid way to assess and mitigate the risks from AC faults and steady state induced currents. When considering AC modeling it is important to look at the model being used and the modeler performing the evaluation.

Learn about our AC modeling and mitigation solutions:

Questions about AC interference, modeling or mitigation? Please contact us at the link below. Our experts are happy to help.

Contact a Corrosion Expert

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