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 par allel-ribbon sacrificial anode CP system for coated pipelines. The mod el accounted for current and potential distributions associated with d iscrete holidays on coated pipelines that expose bare steel to the env ironment. Case studies, based on the CP system used to provide protect ion to the Trans-Alaska pipeline, were selected to show conditions und er which a given CP system will and will not protect a pipe. In the ca ses studied, Rig ribbons provided adequate protection in 50 k Omega-cm soil, but almost no additional protection was achieved by retrofittin g 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 se nsitivity of aboveground on-potential surveys to localized corrosion o n the buried pipe.