How to Assess AC on a Pipeline with a Mix of Coal Tar Enamel (CTE) and Fusion Bonded Epoxy - A Case History

摘要

Assessing and mitigating AC corrosion on pipelines with multiple coatings presents some unique challenges especially when there are significant coating resistance changes between them. Gas pipelines are constantly being upgraded due to class locations/relocations which result in changes in the pipe grade and coating. The older CTE coating never experienced AC corrosion issues in this same environment; however, now with mix of FBE coated sections changes, the dynamics are now different for AC corrosion to occur. This paper will present the challenges of assessing a poor-quality coating with 50 years of service versus an excellent coating with less than 5 years of service in a low resistivity environment. This is a case history of a 275.05 mm (10.75 inch) Outside Diameter (OD) natural gas pipeline that experienced AC corrosion over several years with subsequent leaks. There were previous attempts to mitigate using a distributed anode system; however, the anode placement was not adequate to prevent additional damage. After multiple repairs and replacement, a decision was made to model and analyze and this area of concern.