Abstract
Telluric currents cause variations in pipe-to-soil potentials, which can override a pipeline’s cathodic protection system. The cumulative effect of the “unprotected” time can be significant in areas where telluric activity is high, such as the Canadian north.
This paper presents a general approach to the assessment of telluric activity. Telluric currents observed in a pipeline are the result of three simultaneously operating factors: (1) variations of the Earth’s natural geomagnetic field, (2) conductivity of the underlying Earth and (3) pipeline electromagnetic properties and pipeline structure. The first two factors were combined to give an estimation of the telluric electric field that produces variations of the pipe-to-soil potentials in a pipeline. This geoelectric field then can be combined with a particular pipeline equivalent circuit model which gives the pipeline response to the telluric activity in an area.
We assess the occurrence of the geomagnetic and telluric activity for a typical year, assuming a general uniform conductivity structure across Canada. A more detailed case study has been done for the auroral zone than spans northern Canada. This includes not only a statistical evaluation of the geomagnetic activity but also specification of regions with different earth conductivities (layered earth structures), which when combined with the geomagnetic evaluation lead to the existence of different regions of telluric activity in the area of investigation. Using results from the assessment of telluric electric fields as an input into the pipeline model, the effects can be interpreted in terms of telluric variations in pipe-to-soil potential.
