Category Archives: AC Modeling

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?


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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 on to 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 appendixes 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.

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