In Canada and in the northern portions of the United States, soil resistivities increase rapidly by up to several orders of magnitude once water in the soil freezes in the winter. Frost-penetration can vary from centimeters up to a meter or more in depth.
Although it is standard industry practice in the electrical industry to consider seasonal variations in soil resistivity when modeling grounding facilities for substations and generating stations, this is often not considered in AC interference studies related to pipelines.
The effect on AC induction is expected to be minimal, as this is predominantly dependent on the deeper soil layers. However, frozen soil conditions can have a significant impact on the resistive coupling under a phase to ground powerline fault, and the effectiveness of the AC mitigation system.
This paper involved a case study of a project involving AC interference on pipelines located in Canada. summer, and winter soil conditions were modeled to determine the effect on the subject pipeline under steady state and fault conditions. Allowable safe touch potentials were also calculated for these two scenarios, and used to assess the safety risks. A mitigation system was designed to ensure that the safety and integrity risks are mitigated to acceptable limits, for both seasonal conditions.
It is well known that soil resistivity is a key factor in the assessment of AC interference on pipeline infrastructure. Therefore, it is common practice to perform extensive site surveys to measure soil resistivities along areas of powerline/pipeline collocation, at crossings and at stations. The soil resistivity stratigraphy, is a critical parameter in the calculation of AC induction and resistive coupling between powerlines and pipelines, the calculation of AC current density in the assessment of AC corrosion risk, and the design of AC mitigation (grounding) systems.
Although the role of soil resistivity in assessing AC interference risk is well established in the National Association of Corrosion Engineers (NACE) (1) SP01771 and Canadian Standards Association (CSA) (2) C22.3 No. 62 standards, there is no consideration given to the effect of seasonal variation in soil conditions. Therefore, this effect is often neglected in the pipeline industry.