Mathematical models for cathodic protection of an underground pipeline with coating holidays .2. Case studies of parallel anode cathodic protection systems

A boundary element mathematical model was used to assess the influence of cathodic protection (CPI design parameters on performance of a parallel-ribbon sacrificial anode CP system for coated pipelines. The model accounted for current and potential distributions associated with discrete holidays on coated pipelines that expose bare steel to the environment. Case studies, based on the CP system used to provide protection to the Trans-Alaska pipeline, were selected to show conditions under which a given CP system will and will not protect a pipe. In the cases studied, Rig ribbons provided adequate protection in 50 k Omega-cm soil, but almost no additional protection was achieved by retrofitting Mg anodes to a CP system using Zn ribbons if the Zn ribbons remained connected to the pipe. The model also was used to show the lack of sensitivity of aboveground on-potential surveys to localized corrosion on the buried pipe.