Pipeline Coating Holiday and CP Interference
A 12-inch buried pipeline with a fusion-bonded epoxy coating has been in service for 15 years. A close-interval survey shows a 200 mV drop in pipe-to-soil potential over a 30-meter section, while the rest of the line maintains a steady -0.95 V vs. CSE. The CP system is a impressed current system with a rectifier output of 15 A and 50 V. Soil resistivity in the area is 5,000 ohm-cm. A coating holiday is suspected, but the pipeline also crosses a high-voltage AC transmission line corridor at a 45-degree angle. The operator must decide whether to increase CP output, investigate AC interference, or perform a direct examination.
1. What is the most likely cause of the potential drop, and what actions should be taken to confirm it?
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The potential drop over a short section suggests a coating holiday or coating disbondment causing increased current demand. However, the proximity to the AC transmission line raises the possibility of AC interference, which can affect potential readings. To confirm, perform a DCVG (direct current voltage gradient) survey to locate coating defects, and simultaneously measure AC pipe-to-soil voltage and soil resistivity. If AC voltage is high (>15 V), AC mitigation may be needed. Also, check rectifier output and current distribution. Increasing CP output without understanding the cause may worsen AC interference or cause overprotection. Therefore, first conduct DCVG and AC interference assessment.
2. If a coating holiday is confirmed, what CP criteria should be applied to ensure adequate protection, and what are the risks of overprotection?
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According to NACE SP0169, a minimum pipe-to-soil potential of -0.85 V vs. CSE (with instant off) is required for steel in aerobic soils. However, for coated pipelines, a more negative potential of -0.95 V to -1.05 V may be used to ensure protection at holidays. Overprotection (more negative than -1.2 V) can cause coating disbondment and hydrogen embrittlement. The risk is increased with high-strength steels. Therefore, maintain potentials within the protective range and monitor with coupon tests. If AC interference is present, use AC mitigation and ensure CP criteria are met considering AC effects.