ABSTRACT

Abstract

Close interval potential surveys (CIPS) are the backbone of pipeline integrity, because they determine the effectiveness of the cathodic protection system which provides protection against external corrosion. When dynamic stray or telluric currents influence CIPS survey results, accurate interpretation of the results may be difficult if not impossible. Fluctuation of the pipe-to-soil potential due to dynamic stray or telluric current makes it difficult for the corrosion engineer to validate the integrity of the cathodic protection system protecting a valuable asset. This paper will describe a method of recording the effect of stray and telluric currents on the pipe-to-soil potential and utilizing this information to correct the recorded CIPS data.

Dynamic Stray Currents

Dynamic Stray Current is Direct Current (DC) with varying amplitude and/or geographical paths, which is flowing in a path other than the intended path. DC stray currents principally originate from DC traction systems and are usually found in urban areas. With increasing urbanization and the reliance on electrified transportation systems, DC or low frequency AC stray currents can affect pipelines many kilometers from the source. The advent of regenerative braking as a power saving device has had significant impact on the ability of pipeline operators to safely mitigate DC stray currents. Regenerative braking converts the DC traction locomotive into a moving power source during braking operations. In the past, the source was at a fixed location and the load moved, but with regenerative braking both the source and the load move around the system making it difficult to design an effective stray current mitigation system. DC stray current can also be encountered in rural areas from the operation of commuter rail systems or mine railways. These traction systems generate stray currents, the magnitude and direction of which depends on the location of the load and the horsepower being expended in relation to the source. See Figure No. 1.

Low Frequency AC Interference

There are a number of transit systems operating that use low frequency AC power systems. This low frequency AC affects pipelines in a similar manner to DC current and causes fluctuations in the pipe-to-soil potential. Correction can be affected by utilizing the same principals as for DC and Telluric current.

HVDC Transmission

Another source of DC stray current is high voltage DC transmission systems (HVDC) that use the earth as a return circuit. Mono-polar HVDC systems use the earth as the return conductor and flow a relatively steady DC current through the earth, although some variation in the magnitude of the current flow through the earth may occur due to loading. Bipolar DC transmission systems use a metallic return circuit that is isolated from the earth; however any imbalance in the system will result in DC current flow through the earth circuit. Bipolar HVDC systems may revert to mono-polar operation during maintenance or system upset conditions, resulting in use of the earth as return conductors.

This content is only available via PDF.
You can access this article if you purchase or spend a download.