In a remote north eastern area, we have a steel gas main with multiple abnormally low CP reads. Installed in 1983, the 8’’ pipe is protected with a sacrificial cp system and is coated with a two layer polyethylene coating using shrink sleeves at the girth welds. Functioning insulating joints are located at the city gate station exit and at all entrances of the gate stations
A close interval survey with a 10ft spacing was performed on the cp main with cp reads ranging from -1.05 to -1.15Vcse. However, multiple locations (roughly 100ft in length) were noted ranging from -0.5 to -0.6 Vcse. Similar reads were taken using a different test station and on a different day.
One local business had underground conduits protected with a rectifier. This rectifier was interrupted and a survey was done over on the pipeline. The foreign rectifier had negligible influence on the pipe. We then passed a pipe locator using a 120 hz filter to find any rectified voltage on the pipe. None were found.
A DCVG and ACVG survey were conducted over the line to find any coating holidays. Negligible signals were found indicating any major coating holiday. The soil resistivity using a 4 pin Wenner method was around 3500 Ω●cm at pipe depth (both parallel and perpendicular to the pipe).
Direct assessment digs were then done on three locations. The pipe coating was in a near perfect state. We then proceeded to install two 32lbs high potential magnesium anodes and a new test station at each site. The excavation was backfilled with new material.
The following month, the pipe potential stayed roughly the same and the anodes were reading around -0.8 Vcse and around 3 mA DC of current flowing towards the pipe.
We are looking at installing CP coupons and doing a soil analysis even though the backfill was new material only.
What do you believe could be the problem with this pipeline?
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