Abstract
There are several factors that can introduce an error to the off-potential such that it no longer represents an accurate measure of the polarized potential at the steel surface. One such phenomena created between two areas of pipe due to a potential gradient on the pipe. Although long-line currents have been discussed for many years, very little is known as to their magnitude and their effect on measuring an off-potential.
The objective of the research discussed in this paper was to establish and verify the amount of error introduced by long-line currents when measuring an off-potential. The scope of the project included field testing on operating pipelines and computer modeling utilizing circuit analog models. It was shown that under certain conditions, long-line currents can produce an IR-drop error in the pipe-to-soil off-potential measured by interruption techniques, such that the off-potential is no longer an accurate measure of the polarized potential of the pipe. The driving force for long-line currents is any potential gradient on the surface of a pipe. This potential gradient can be due to local coating degradation, varying environmental conditions, or to the use of remote anodes to protect a long length of pipe.
