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 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(1) SP214241. 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.
A literature review indicated a gap in the industry with no existing standards that address AC corrosion and mitigation 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. The results provided insight into additional potential risks to pipelines from higher frequency interference, a better understanding of the influence of soil chemistry in the corrosion process, and direction in the development of a mitigation strategy.