Need: Historically the effects of telluric currents on pipelines have been considered a curiosity and an inconvenience when conducting cathodic protection surveys for compliance with the pipeline codes and regulations. Recently however, as more pipelines have been constructed at higher latitudes and in higher resistivity soils, and as higher quality coatings have been used, the resulting telluric potential and current variations, being more severe, have prompted concerns about the following issues; 1) whether or not the pipe is corroding during periods of telluric current discharges, and 2) will the coating be stressed and possibly disbonded during periods of pick-up, and 3) how can the effects of telluric current activity be mitigated, and 4) what techniques are available to measure accurate pipe-to-soil potentials during periods of telluric activity.
Result: This report describes two methods for modeling telluric currents in pipelines wherein the distributed source transmission line (DSTL) model has been used successfully to predict the magnitude of telluric currents and the associated potential variations throughout pipeline networks. The model addresses typical pipeline situations including pipe bends, pipe junctions, branch lines, insulating flanges, grounding points, changes in pipe dimensions, and changes of coating conductance.
Benefit: Correcting for the telluric activity while conducting close interval potential surveys is somewhat more complex. Most of the techniques that have been developed require multiple recorders to collect data from stationary electrodes, prior to or during the survey, so that a correction factor can be derived and applied to the potential measured by the moving electrode. This operation requires accurate time stamping, usually by reference to the global positioning system. No single technique corrects for all the possible voltage drops in the measurement circuit.
