Need: In the recent past years there has been a considerable amount of effort devoted to developing methods and instruments to correct measured pipe-to-soil potentials for IR drops that may occur from currents (from the cathodic protection system or stray sources) in the soil to obtain the polarized potential. However, many of the methods or instruments available are either time-consuming, cumbersome to use in the field, applicable to only certain types of cathodic protection systems and under particular circumstances, subject to influences from stray current sources or not fully developed as of yet. Thus, there is a need to develop a practical method of determining the polarized pipe potential free of IR drop errors.
Benefit: This report examines and compares seven available methods for measuring polarized potential and describes the advantages and disadvantages of each. The following methods for measuring polarized potential were evaluated: (1) pipe-to-soil measurements with cathodic protection; (2) pipe-to-soil measurements with interrupted cathodic protection (also called instant-off); (3) measurements based on rectifier AC waveform analysis; (4) stepped current reduction; (5) a four-terminal network; (6) extrapolation to zero resistance (IR drop); and (7) a combination of DC/AC transverse measurements.
Result: Of these methods, the instant-off approach produced the most reasonable compromise between accuracy and simplicity of operation. Also as part of this study, a new measurement method was developed that uses a combination of AC and DC pipe-to-soil potential measurements. The advantages of this method are that the cathodic protection current does not have to be interrupted, and the polarized potential is determined with higher resolution along the pipeline.
