In this paper, a case study on AC interference measurement and mitigation was introduced. A buried pipeline was running in parallel with 500 kV high-voltage alternating current (HVAC) transmission line for about 10 km. The distance between them was about 25~600 m. Field test showed that the maximum AC voltage was 44.3 V and AC current density was 1248 A/m2 at cathodic protection (CP) polarized potential of -0.90 VCSE ~ -0.95 VCSE. Overall, these results indicated that the buried pipeline suffered serious AC interference. Therefore, corrosion coupons were buried and electrically connected with the pipeline to investigate AC corrosion, which was analyzed according to weight loss test results and corrosion morphology analyses. Based on the collected information, numerical simulation model was established and adjusted to meet field test results. And then, mitigation schemes were designed and optimized with the help of the numerical simulation. The final mitigation scheme was got to reach the mitigation target.
In the recent thirty years, more and more AC related failures of cathodically protected pipelines were reported in succession, and thus AC corrosion attracted extensive attention in the international corrosion field. 1-3 In 1991, a high-pressure gas pipeline was parallel to high voltage transmission line (HVTL) for 4000 m. The maximum AC voltage on the pipeline reached 28 V. 4 Its corrosion rate at the pipeline coating defects reached 1.4 mm/y even though the CP potential was -1.45 VCSE. In 2002, a natural gas pipeline from America was parallel to HVTLs. Corrosion perforation occurred even though the CP potential of the pipeline was -1.3 VCSE. 5 Subsequently, corrosion leakage of a buried metal pipeline occurred under AC current density of 144 A/m2 and CP polarized potential of -1.23 VCSE. 6 Similarly, crude oil leaking caused by AC corrosion at northern Alaska resulted in great economic losses. 7 In 2018, Andreas Junker has found that AC corrosion rate of a buried pipeline was ~0.2 mm/y as CP potential of -1.3 VCSE. 8 Taken together, these cases suggested that in the case of AC interference, buried pipelines still encountered corrosion even under good CP level. These AC corrosion cases have been reported in Canada, America and Europe. In China, more and more pipelines were buried in parallel to HVAC transmission lines with rapid development of electricity, petroleum and transportation industry. Therefore, buried pipelines suffered from serious AC interference in these conditions. 9-10 However, few AC corrosion was measured in field currently. In this work, a buried pipeline running parallel with HVAC transmission line was investigated.
