Abstract
An initial AC interference study and mitigation design was performed prior to construction, on the subject NPS 36, 107 km long pipeline (Pipeline A) installed in 2016. The subject pipeline is collocated with an NPS 30 pipeline (Pipeline B) constructed in 1999, for the entire route, and several other pipelines in some areas, all owned by the same operator and with shared cathodic protection systems. Five areas of powerline collocation were identified and modeled in the AC Interference study.
During the commissioning survey, elevated AC voltages were recorded in areas with no identified powerlines. Furthermore, it was discovered that some pipeline bonds, existing mitigation systems and other pipelines were not modeled as per the final “as-built” installation or incorporated into the AC mitigation system. Subsequent site investigations were performed to confirm bonding, assess AC interference corrosion risks, and identify additional AC interference sources. The pipelines were remodeled to include the existing “as-built” configuration and these additional interference sources. Re-modeled results were assessed based on field collected data, remotely monitored data, and In-line Inspection (ILI) data on both pipelines to develop an integrated AC mitigation and monitoring system. Several lessons were learned, that can benefit future projects. These lessons include the importance of remodeling the as-built pipeline configuration, complexities related to AC interference in multi-pipeline corridors, and the difficulty in assessing the AC corrosion risk via modeling alone. Assessment and incorporation of site and ILI data into the final AC mitigation and monitoring system design leads to a more holistic result.
