Abstract
Pipe-to-soil potential measurements are the primary means for monitoring the effectiveness of cathodic protection (CP) systems. All criteria for cathodic protection employ, in one form or another, a potential measurement of the pipe with respect to a reference electrode. However, very little is known concerning what portion of the pipe is sampled by the potential measurement. The primary emphasis of this research program was to examine the area of the pipe sampled during a pipe-to-soil potential measurement. The work described in this paper is concerned with bare pipe only. The approach involved field investigations and finite element analysis (FEA) computer modeling. The primary purpose of the field investigations was to verify the accuracy of the FEA modeling. The modeling was then used to examine a wider range of conditions than those found at the single field site. A field site was established consisting of 80 feet of bare pipe with 64 reference probes positioned to within 1 to 2 inches of the pipe's surface. The reference probes permitted measurement of the potentials at the pipe surface for comparison to potentials measured at ground level.
Excellent agreement was found between the FEA modeling predictions and the field measurements. The FEA modeling permitted quantitative relationships to be established between the area of pipe sampled and (1) depth of burial and (2) pipe diameter-to-depth ratio. It was concluded that gradual potential fluctuations at the pipe surface may be accurately followed by ground level pipe-to-soil potential measurements. However, local potential anomalies may be difficult to detect by ground level measurements. Also, potential fluctuations limited to the bottom of the pipe may go undetected.
