Elizabeth Nicholson, B. Eng, Poland 2010
Abstract
Stray currents can impact the ability to protect a pipeline or other buried metallic structure from corrosion. They can be generated from a variety of manmade and natural sources. Common sources of stray currents are cathodic protection on other lines, DC transit systems and telluric activity. Detecting these stray currents is done by deploying stationary data loggers to monitor the pipe to soil potential over an extended time period. Dynamic stray currents can affect the pipe to soil readings used in CIPS surveys to analyze the effectiveness of the cathodic protection system. By using GPS synchronization between the current interrupter, CIPS survey instrument and stationary logger, the time stamped data from the logger can be used to correct the CIPS data for the influence of the stray currents. Once the source and magnitude of the stray currents are found, mitigation measures can be put in place if necessary to protect the pipeline.
Introduction
Stray currents are caused by sources of current flowing through unintended paths. These can have significant effects on buried pipelines and other metallic structures. Water combined with dissolved minerals, such as in soil, becomes conductive and allows the conduction of current. This is important to the pipeline operator as these stray currents can initiate and/or accelerate the corrosion of a pipeline. It is important to the pipeline operator to determine whether stray currents are affecting the pipeline and then define the probable source of the stray current. The stray currents will affect the effectiveness of any cathodic protection system on the pipeline. Test station readings and close interval survey data can be distorted by stray currents. Once the extent of the stray currents has been identified, mitigation strategies may be necessary to protect the pipeline.
Stray current is particularly destructive at the location the current leaves the pipeline. This creates a localized anodic area on the pipeline which can overcome the cathodic protection applied. Often these areas are in close proximity to the generator of the stray currents or at a holiday in the coating or at a crossing of lines. There are two types of stray currents, static and dynamic. Static stray currents maintain a steady current flow and are often from other cathodic protection systems on pipelines or other structures. Dynamic stray currents fluctuate over time and can come from natural or manmade sources.
