The construction of new pipelines within Europe has inevitably meant the increasing use of common corridors for pipelines and overhead power transmission lines. This co-location often results in the occurrence of induced ac voltages and currents on a pipeline and, in some relatively uncommon instances, in pipeline failure. The traditional approach of installing the pipeline and then mitigating any induced ac problems after construction is no longer considered valid, due to the availability of proven, proprietary modeling software. This software, which is able to model the pipeline and power transmission line interaction conditions, and optimize mitigation design, is proving to be of significant benefit in decreasing both the safety hazard and construction costs associated with new pipelines. The software can also be used for existing pipelines. Results of modeling from a recent onshore pipeline project are presented and discussed herein.
During construction and subsequent operation of a pipeline, areas of close proximity between the pipeline and an alternating current (ac) transmission line can give rise to several problems during both steady state and fault conditions. Induced ac voltages, which are with respect to ground potential, arise as a result of electromagnetic capacitive and inductive effects. The magnitude of the induced voltage depends on the ac transmission line voltage/current, pylon dimensions, the separation distance of the pipeline from the ac transmission line, the distance over which the pipeline runs in parallel with and crosses the ac transmission line, soil resistivity, coating conductivity of the pipeline and whether the ac transmission line is experiencing a fault or large transient surge associated with switch on.
In cases where the overhead line has an earthed neutral, unbalanced or fault conditions can result in substantial currents flowing in the soil. For an unbalanced single-phase supply, e.g. an electrified railway, or earth faults on the ac transmission line, hazardous voltages may be induced even over short parallel lengths [Ref. 1].
Voltages may also be induced conductively where large currents flow in the ground as a result of a fault at a pylon, a lightning discharge or where a severed ac phase wire comes into contact with the ground.
The magnitude of measured induced ac voltages at cathodic protection (CP) test posts along a buried pipeline will usually be low, e.g. up to a few hundred mV ac (as these measurements will probably be taken at a time of steady-state usage of the ac transmission line), but can be up to several volts. Corresponding induced ac currents will also be flowing in the pipeline steel. As an example, the ac volt drop across an existing two-wire pipeline current measuring span was measured over a twenty four hour period and was found to fluctuate during periods of highest and lowest power demand by a factor of about three. The presence of any measurable induced ac voltages and ac currents should initially be taken as significant with further investigation being warranted. The problems requiring attention can include:
Ensuring the continued integrity of the pipeline external coating (coated linepipe and field joint coating) and pipeline wall
Minimizing the danger to personnel and livestock (aboveground appurtenances and buried structures)
Preventing damage to electrical isolation joints/flanges (typically installed at each end of a buried pipeline)
Monitoring the levels of induced ac voltages to allow some control of any external corrosion caused by ac influences
Proprietary and proven software is available and was used recently to m