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AC induced corrosion on onshore pipelines- Part 1

From- UKOPA

By Roger Ellis Shell UK, Stanlow, Pipeline Manager.

This article describes the history of the investigations and how the phenomena was ultimately attributed to the effects of induced AC. Discussion and background research findings is given on probable causes of AC induced corrosion, how it can be predicted and how the effects can be mitigated against.

AC induced corrosion is a significant threat to integrity of buried pipelines, due to its very high localized corrosion rate. It can and has resulted in metal loss of more than 1 mm per year.

Shell UK constructed a new 412 km long 10 ins diameter high pressure ethylene pipeline in 1992. In 1996 a 100km section of the pipeline was intelligently pigged which resulted in the identification of significant metal loss features. Initial investigations following an intelligent pig investigation assigned the cause of significant pipeline metal loss to microbial action. Improvements to the levels of cathodic protection were made to ensure adequate protection. A further intelligent pig investigation in 1999 confirmed the reoccurrence of similar defects.

AC induced corrosion is a potentially serious phenomena and could lead to failure of a buried pipeline.

AC corrosion can however be predicted and the following are considered to be the main ingredients.

  • A source of induced AC
  • A coating of high dielectric strength
  • A soil of low resistivity or good earth.
  • Small coating defects.
  • A high current density.

Monitoring an data logging of induced AC and current densities running to earth should form an integral part of pipeline integrity management.

Mitigation in areas of high susceptibility can be achieved by installation of a preferential earthing system.