ABSTRACT
Telluric currents produce variations in pipe-to-soil potential (PSP) that take the pipeline outside the voltage range for cathodic protection and interfere with potential surveys. The size of the PSP variation depends on a variety of factors related both to the natural environment and the characteristics of the pipeline itself. This paper presents a methodology for assessing how different pipeline features influence the vulnerability of the pipeline to telluric effects. Derivation of the distributed-source transmission line (DSTL) equations for induction in a pipeline show that the critical parameters are the series impedance of the pipeline steel and the parallel conductance to ground through the pipeline coating. Large PSP variations occur where there is a disruption in the flow of telluric currents along the pipeline, such as happens at the end of the pipeline, at flanges, at bends, and changes in pipeline characteristics. A series of plots are presented showing the dependence of PSP variations on coating conductance, changes in series impedance and bends in the pipeline. Also examined are how the PSP variations are modified when overlapping effects occur. Finally, the paper considers the effect of various mitigation strategies and presents model results of the reduction in PSP variations that can be achieved.
INTRODUCTION
Telluric currents in pipelines create variations in pipe-to-soil potential (PSP) that make it difficult to monitor cathodic potentials and, if large enough, raise concerns about the amount of time a section of a pipeline may not be cathodically protected. For engineers, encountering telluic currents for the first time they may seem a capricious phenomenon, coming and going in an apparently random way and with size varying from time to time and place to place. An understanding of the phenomenon is not helped when the only measurements often made on a pipeline are spot values or short-interval recordings as part of CP surveys. However, systematic longer-term multi-site recordings of PSP have helped to identify that there are patterns to when telluric PSP variations are seen on a pipeline [1,2]. This has been coupled to modeling work that has provided the theoretical framework for understanding the PSP produced by telluric currents [3]. Thus a lot is now known about telluric currents and this understanding can be used to mitigate their effects. The causes of telluric currents and environmental factors influenced their severity are described in a companion paper [4]. This paper examines how the pipeline structure contributes to the size and location of telluric PSP variations on a pipeline.
Modeling
Telluric current effects on pipelines can be modeled using distributed source transmission line (DSTL) theory in the same way as is done for AC induction [5]. The electrical properties of the pipeline are described by the series impedance of the steel and the parallel conductance through the coating to ground. These are used to define the propagation constant, ¿, and the characteristic impedance, Zo: (available in full paper)
The voltage and current along the pipeline are then given by (available in full paper)
