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Innovative Mitigation Strategies to Address Ac Corrosion At Higher Frequencies

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A corrosion investigation performed in 2018 on an NPS 6, approximately 4 km long, polyethylene coated pipeline determined that the accelerated corrosion anomalies detected during in-line inspections (ILI) were due to AC corrosion. The AC and DC current densities on the AC coupons adjacent to these anomalies were above the limits recommended in NACE SP21424. It was also determined that the line was cathodically over-protected, and that most of the AC voltage measured on the line was due to 120 Hz AC ripple from a foreign rectifier.

Product Number: 51323-19280-SG
Author: Wolfgang Fieltsch, J. Scott Farquharson
Publication Date: 2023
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$20.00
$20.00

A 2018 field investigation on an NPS 6, approximately 4 km long, liquid pipeline identified a rectifier as the likely source of the elevated DC and AC current densities, which resulted in AC corrosion anomalies detected during in-line inspections (ILI). Initial findings indicated that the 120 Hz rectifier ripple may have contributed to the accelerated AC corrosion at this location.


A literature review identified a gap in the industry with no existing standards that address AC corrosion at these higher frequencies, and very little research on the topic. This resulted in additional field and laboratory investigations to further quantify the risks of higher frequency harmonics on cathodically protected pipelines.


Due to the complexity of the system, innovative mitigation approaches were required to mitigate the AC corrosion risk and minimize DC interference risks, while at the same time ensuring adequate CP is maintained along the pipeline. A comprehensive mitigation and monitoring system was installed and commissioned. This paper focuses on the design and implementation of the mitigation system, along with a full assessment of the system commissioning and monitoring data. Lessons learned during the mitigation and commissioning process are intended to provide guidance to operators encountering similar conditions on their pipeline systems.

A 2018 field investigation on an NPS 6, approximately 4 km long, liquid pipeline identified a rectifier as the likely source of the elevated DC and AC current densities, which resulted in AC corrosion anomalies detected during in-line inspections (ILI). Initial findings indicated that the 120 Hz rectifier ripple may have contributed to the accelerated AC corrosion at this location.


A literature review identified a gap in the industry with no existing standards that address AC corrosion at these higher frequencies, and very little research on the topic. This resulted in additional field and laboratory investigations to further quantify the risks of higher frequency harmonics on cathodically protected pipelines.


Due to the complexity of the system, innovative mitigation approaches were required to mitigate the AC corrosion risk and minimize DC interference risks, while at the same time ensuring adequate CP is maintained along the pipeline. A comprehensive mitigation and monitoring system was installed and commissioned. This paper focuses on the design and implementation of the mitigation system, along with a full assessment of the system commissioning and monitoring data. Lessons learned during the mitigation and commissioning process are intended to provide guidance to operators encountering similar conditions on their pipeline systems.

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AC interference studies have become increasingly popular in an industry where shared right of ways have increased and there has been a better understanding of how AC interacts between pipelines and powerlines that are collocated with each other. While modeling software for AC interference studies have been developed since the 1990s, advancement in AC interference processes have occurred as more has been learned over the years. When performing an AC interference study there are three steps that need to be completed: field data collection, modeling, and mitigation design. Within this paper, we can compare a project from ten years ago to a project from today to understand the developments that have been made over the course of time to improve the way we develop our mitigation designs.
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