ABSTRACT

In this case study we integrated 3D modeling including polarization into the design of Direct Current (DC) stray current interference mitigation associated with Light Rail Transit (LRT) systems and adjacent metallic pipelines. Simulation results identify interference hot spots by predicting interference current density and potential shifts along pipelines under study. This approach in analyzing stray current interference provides proactive monitoring solutions to address pipeline integrity concerns. The simulation tool also predicts pipeline polarized off potentials, which could be measured at recommended test post locations and compared against NACE potential shift criteria during future monitoring.

INTRODUCTION

During operation of DC rail transit systems, DC current will follow the path of least resistance when returning to the Traction Power Substations (TPSS) to complete the electrical circuit. If the track-to-earth resistance (resistance between the train rails and surrounding soil) is not sufficient, current can leak off the LRT track system into the surrounding soil. Metallic facilities such as pipelines in the soil offer lower resistance paths for the current while returning to the TPSS.

DC stray current will get onto the pipelines through microscopic coating deficiencies (coating holidays) along the pipeline and discharge at other coating holidays closer to TPSS. This discharge of current off the pipeline will result in significant corrosion at these locations.

At areas of DC stray current pickup, typically where LRT train passes over a pipeline or starts/stops at a passenger station, additional cathodic polarization on the structure could be observed. In cases of coated mild steel pipelines with existing cathodic protection systems, overprotection can become an issue and lead to coating disbondment. Coating disbondment, or blistering, occurs when there is electro-osmotic movement of water through the coating, which causes pressure buildup beneath the coating. The reduction reaction at the metal surface produces high pH which can destroy coating adhesion bonds.

At areas of DC stray current discharge, electronic current is converted to ionic current through an oxidation reaction. Corrosion results as the metal surface is consumed. More attention is typically given to mitigate areas of current discharge as this is where the most damage is likely to occur.

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