The new NACE Standard SP21424-20181 provides a set of simple criteria for assessing the risk of AC corrosion on existing collocations between powerlines and cathodically protected pipelines. However, the task of developing design criteria for new collocations is left to the pipeline operators.
This paper covers a number of challenges related both to developing design criteria for new collocations and applying the new Standard to existing collocations.
Topics like selection of the AC current density limit for mitigating AC corrosion on a new pipeline, recording AC and DC currents on 1 cm2 coupons and dealing with high AC and DC average current densities at low AC voltages are discussed in detail in this paper.
The new NACE Standard SP21424-2018 provides a set of simple criteria for assessing the risk of AC corrosion on existing collocations between powerlines and cathodically protected pipelines. Specifically, the AC current density should not exceed a time-weighted average of 30 A/m2 if the DC current density exceeds 1 A/m2 or 100 A/m2 if the DC current density is less than 1 A/m2.
This paper covers a number of challenges related to converting risk assessment criteria to design limits, receiving accurate recording data from installed AC coupons and finally dealing with unexpected high AC and DC average current densities at low AC voltages.
When a new AC interference study is conducted for a proposed pipeline running in a common right-of-way with an existing powerline, the accepted technical approach is to collect powerline, pipeline and soil resistivity data, to use dedicated software to predict the pipeline voltages and then to calculate predicted AC current densities based on predicted voltages and measured soil resistivities at selected locations. Finally, mitigation shall be designed to ensure that the average AC and DC current densities or alternatively the average corrosion rate to be recorded during and after commissioning will satisfy the new NACE criteria.
