Simulation of electromagnetic interference on underground pipelines taking into account AC corrosion

Induced AC voltage is clearly identified as a potential hazard, from both safety and corrosion standpoints, for all buried pipelines coming into proximity with overhead electric transmission systems. The likelihood of electromagnetic interference (EMI) increases with rising operating currents in the overhead lines, with increasing quality of the coating on the pipeline, and with the length of a line parallel to and close to power lines. It is possible to simulate the conditions that are expected to exist on these pipelines, and calculate the anticipated levels of induced AC voltage and current as well as its intensity and density flowing into the soil through the coating defect. Determining a pipeline's response to EMI is not an easy task since it depends on three factors, namely the location of the structure with respect to the magnetic field generated by the AC source, the magnitude of the interfering field, and the electrochemical response of the structure to the interference. Many computer programs can be used to calculate a pipeline's AC voltage by taking into account the worst case scenario under normal operating conditions of the interfering systems. The programs however do not take into account the electrochemical phenomena on the pipeline surface at pipeline coating defect locations. It should be noted, that the mechanism of AC corrosion is not very well understood, particularly as it applies to corrosion in soils. A simulation model of a metal pipeline under inductive influence, in which AC corrosion is taken into account has been presented in the paper. An electrical equivalent diagram is presented and the corrosion current, the pipeline potential and the corrosion rate are calculated. The use of the Matlab-Simulink platform for multidomain simulation and model-based-design of dynamic systems permits the complex analysis of the interference on pipelines, whereas the electrochemical phenomena on the interface metal - soil are taken into account. EMI in the circuit is presented by Simulink in the form of a block diagram - a graphical representation of the process, which is composed of an input, the system, and an output. The block connected with the AC corrosion, in which the electrochemical phenomena are represented by the non-linear Butler-Volmer equations, has been implemented in the simulation package. The system is described by state equations, which can be iteratively solved by tools provided by Matlab.