Case Study: AC Mitigation Design Criterion – It Matters

This case study highlights some of the challenges associated with choosing AC mitigation design criterion for a new pipeline construction project.  The specific project consisted of approximately 200 miles of pipeline with another 35 miles of lateral lines and included compressor stations, metering valves and a valve station.  The final approved right of way consisted of 41 identified transmission lines spanning 5 different utility owners.  MATCOR’s scope of work included designing both the CP system and modeling and designing an AC Mitigation system to address the extensive HVAC colocations.

For the AC Mitigation effort, MATCOR performed extensive field data collection along the right of way and MATCOR sent requests for utility operating data for AC Mitigation Design purposes.  After 6 months of requests from the various utilities, the results were inconsistent with some utilities providing only emergency and peak data, others provided seasonal average data and one utility would not provide any data.  Ultimately, modeling was performed based on actual data wherever possible, supplemented by assumptions based on experience from other AC Mitigation designs projects.

While this engineering project was being started, NACE was adopting its latest version of SP 21424-2018-SG “Alternating Current Corrosion on Cathodically Protected Pipelines: Risk Assessment, Mitigation, and Monitoring”.  This latest version presented an updated criteria standard based on DC Current Density.  Basically, if you can assure that the CP Current Density along the length of pipeline is controlled to below 1 A/m2 then you can tolerate a much higher AC Current density threshold of 100 A/m2 requiring less AC mitigation.  If you are unable to control CP Current to the 1 A/m2 level, then the acceptable AC current density drops to a much lower 30 A/m2 threshold often requiring more extensive mitigation.

For the initial AC modeling effort, MATCOR based the AC Current Density limit to the lower, more stringent, 30 A/m2 criteria that assumes that the CP Current Density could not be controlled to below 1 A/m2.  The modeling, given all the assumptions that we had to make, came back with:

  • Numerous locations where Step and Touch Potential concerns required mitigation
  • No concerns over fault current given the separation distances and fault currents presumed
  • A very significant mitigation requirement to bring the induced current densities below the 30 A/m2 criteria including approximately 81 miles of parallel mitigation and 8 x 600ft deep grounding wells.

These results warranted additional review given the extensive AC Mitigation requirements from the initial modeling.

After some sensitivity assessments, a second modeling effort would be taken based on some revisions to the HVAC operating data based on updated information and some revisions to the input assumptions.  This new modeling effort would also be run using 50 A/m2 as the AC Current Density limit.  Additionally, based on input from operations, it was determined that all deep grounding wells would be limited to 200 ft depth.  The new modeling effort resulted in a significant reduction in AC Mitigation required eliminating almost 50 miles of parallel mitigation.

The dual modeling efforts showed that there were numerous locations that were only slightly above the 30 A/m2 threshold but below that of 50 A/m2.  In those areas, the owner opted to install additional monitoring systems but forego the initial installation of AC Mitigation and instead focus on those higher risk AC Interference areas by installing AC Mitigation.

This case story highlights the role of AC mitigation design criterion selection and the complexity around the current criteria that correlates CP Current Density levels, which are not typically controlled, to the AC current design threshold to mitigate the induced corrosion AC Interference risk.  With judicious design decisions and a healthy amount of monitoring systems, there is significant value in your modeling criteria.


To get in touch with our team of cathodic protection and AC mitigation experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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AST Cathodic Protection System Tank Isolation Considerations

cathodic protection tank isolation considerationsEvan Savant, EnLink Midstream reached out to the MATCOR Technical Team asking about AST cathodic protection system tank isolation:

“Can you advise on the importance of isolation for a new AST connected to a Pipeline, and can you advise on the need to isolate the tank cathodic protection from the tank grounding?

MATCOR’s Director of Engineering, Kevin Groll PE, NACE CP4 responded:

I am unaware of any papers or technical documents on the subject, but I will summarize as follows:

  1. Why can a lack of isolation can hurt your cathodic protection?
    When trying to protect any type of structure from corrosion, cathodic current loss to nearby structures is always a concern. Losses can occur when the structure in question is directly bonded to other structures which may “steal” current. Offending metal structures that are close to the cathodic protection anode and structures with better resistance to earth (e.g., bare copper grounding, bare driven piles, etc.) will more likely take a significant amount of current.
  2. How do you obtain isolation without losing overvoltage protection?
    To prevent current loss, your target structure must be electrically isolated from the offending structures.  However, once you isolate a structure, you will lose grounding (if it was purposefully grounded) and you will lose protection against overvoltage events, AC faults, and lightning strikes.  Therefore, to obtain DC isolation but maintain AC continuity and overvoltage continuity, we use solid state decouplers (SSDs) and polarization cell replacements (PCRs). The primary difference between these devices is how much surge current they will carry.
  3. Tank cathodic protection design considerations.
    When we design an under-tank CP system with concentric rings, we assume that we will not have isolation from grounding and facility piping, and we also assume that most of the current will get to the tank bottom because of the proximity of the anodes.  This is not always the case, as we saw in a recent project, but for the most part concentric ring systems can be powered high enough to overcome the lack of isolation.

Horizontal directional drilling installed linear systems show approximately 1.5 to 2 times as much current is required as a concentric ring system due to current losses.  Again, we usually factor in enough current capacity to overcome these losses.

Deep anode systems and semi-deep anode systems suffer the worst losses. These systems will sometimes require isolation of the tanks to prevent critical current loss.  If a system is already in place, testing can be performed to determine how much loss there is to existing structures by measuring the current returned on ground rods and pipes. This is accomplished by using clamp-on current meters around wires/rods and Swain meters around pipes.

It is important to note that tank terminal isolation and grounding are factors in these complex tank terminal applications that must be considered in the proper design of Cathodic Protection.  MATCOR’s experienced team of engineers can evaluate your specific application and make the appropriate recommendations.


To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

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PHMSA Mega Rule Update

PHMSA Mega Rule
The PHMSA Mega Rule significantly impacts the US pipeline industry.

It has finally landed – the PHMSA MEGA rule has hit and will have a tremendous impact on the US pipeline industry, adding significant scope to the current pipeline integrity management requirements.  Some of the industry impacts that the PHMSA MEGA rule will have include:

  • An approximately 20% increase in the number of regulated pipelines in the United States
    The exact impact depends on some additional details not yet released; however, it is very clear that the addition of 20% more regulated pipelines will have a significant impact on an industry where highly qualified integrity professionals and related services are limited in supply and the industry is already struggling to meet demand.  These additional pipelines will require significant integrity resources.
  • Expedited reporting requirements
    The time restrictions for implementing the new rule have been accelerated, with initial reporting requirements in July, 2020, less than a year away. Time to comply with these regulations has been reduced 20% from the initial draft order timeline.
  • Increased cathodic protection requirements
    Many pipelines that previously were not regulated and have not had proper CP will now require a properly designed, maintained and tested cathodic protection system.

The PHMSA MEGA rule will be a challenge and an opportunity for MATCOR as midstream pipeline operators will be looking for partners to help them address these new regulations.  MATCOR provides a full range of cathodic protection and pipeline integrity services including:

  • Field Integrity Surveys
  • ECDA
  • CP Assessments
  • CP Design
  • CP Installations
  • Annual Testing Services
  • Test Station Installations
  • Remote Monitoring
  • CP Maintenance
  • AC Interference

It is going to continue to be exciting times in the midstream market.

Learn more about PHMSA regulations.


To get in touch with our team of cathodic protection experts for more information, to ask a question or get a quote, please click below. We will respond by phone or email within 24 hours. For immediate assistance, please call +1-215-348-2974.

Contact a Corrosion Expert