Simulation of cathodically protected pipeline with capacitive AC mitigation devices for interpretation of misleading instant Eoff readings

摘要

A cathodically protected buried pipeline is usually connected to earthing electrodes via several capacitive AC mitigation devices placed at various locations along the pipeline in order to decrease the AC voltage interference. During periodic switching of the cathodic protection circuit for the instant Eoff measurements, the capacitive AC mitigation devices tend to be discharged through the soil provoking an error in the instant Eoff readings, thus resulting to a possible false evaluation of the cathodic protection effectiveness. The aim of the present work is to investigate the role of the main factors, which influence this error. An electric circuit simulation of the pipeline with connected earthed capacitors is proposed. The circuit analysis has shown that the key determining factors of the aforementioned capacitive discharge time constant comprise the capacitance value of the capacitive AC mitigation device at every earthing site, the number of earthing sites where the capacitive AC mitigation devices are installed, the earthing electrodes resistance and the pipeline (or coating) resistance to remote earth. Measures to minimise this misleading error of Eoff readings through the control of the abovementioned factors and the modification of the time of the Eoff measurement are suggested. In this context, the Eoff should be measured at prolonged time after every switching, contrary to the usual practice. Furthermore, the capacitive AC mitigation devices should not be disconnected from the pipeline in order to prevent the adverse effects of AC interference on cathodic protection monitoring. The present work contributes to the understanding of the capacitive AC mitigation devices effect on pipeline cathodic protection monitoring, which has been rarely analysed in the past. Moreover, by recommending novel approaches, the paper can stand as a reference to the cathodic protection operators for a simple, reliable and economic monitoring of the cathodic protection effectiveness in presence of earthed capacitive AC mitigation devices.