Geomagnetic Effects on Pipelines

Introduction

Pipelines are used widely to transport gas, oil and water from their sources to processing plants and consumers. Damage to a pipeline is very costly; not only the replacement cost of the pipeline itself be must considered, but also the potential damage to the environment and the threat to people's lives.

To carry large amounts of liquid or gas underground or under the sea (from off-shore oil fields) or even on the surface, pipelines are built from steel to be able to withstand the pressure. Therefore, damage to pipelines can come not only from physical cracking, but from corrosion of the pipeline steel. To prevent corrosion, the pipeline steel is covered with an isolating coating and connected to special devices, called cathodic protection rectifiers.

Corrosion Protection Through tiny holes in the pipeline coating, not detected by pipeline surveys because the pipeline are usually buried or placed under the water, the pipeline steel may come into contact with the soil, water or moist air and be subject to corrosion. This electrochemical reaction can be inhibited by maintaining the pipeline steel negative (cathode) with respect to the surrounding soil (anode). It can be done by connecting the negative output of a DC power supply to the pipeline and the positive output to the anode devices placed in the soil so that electric currents flow from the anode to the pipeline. In this arrangement the pipeline is the cathode of the circuit; that is why this method is called "cathodic protection". The protection system keeps the pipeline potential with respect to the soil in a safe region from -0.85V to -1.35V.

How Geomagnetic Variations Affect Pipelines Time-varying magnetic fields induce time-varying electric currents in conductors. Variations of the Earth's magnetic field induce electric currents in long conducting pipelines and surrounding soil. These time-varying currents, named "telluric currents" in the pipeline industry, create voltage swings in the pipeline-cathodic protection rectifier system and make it difficult to maintain pipe-to-soil potential in the safe region. During magnetic storms, these variations can be large enough to keep a pipeline in the unprotected region for some time, which can reduce the lifetime of the pipeline. As an example, the geomagnetic storm on the 6-7 April 2000 is shown on the figure. The top panel shows geomagnetic field variations at Ottawa magnetic observatory; the bottom panel shows the pipe-to-soil potential difference on a pipeline in Canada, recorded at the same time. During the magnetic storm the pipe-to-soil potential difference went outside the safe region. That can increase the possibility of corrosion.

Effect of Pipeline Characteristics The whole pipeline system is quite complicated. It consists of a gathering system of pipelines from the oil or gas fields; a transmission pipeline (often two parallel pipelines); a distribution network of pipelines close to the city. To follow a certain route, pipelines must bend. For the electrical separation of one part of the pipeline from the other or from the processing plant, engineers use insulating flanges. They stop electrical currents but allow gas or oil to flow. All the above mentioned pipeline features cause the pipeline system to respond to the geomagnetic variations in a more complicated way. The figure shows an example of records which have been taken at different test posts of the same pipeline. To help engineers monitor the pipe-to-soil potential and make some predictions of the expected values during a geomagnetic storm, some research and mathematical modeling of the pipeline response to the geomagnetic variations has been carried out in the Ottawa Geomagnetic Laboratory. More about this research can be found in various papers and conference proceedings by the members of our research team: