Pipeline Current mapping was first introduced in 1995 as a methodology of assessing pipeline coating condition from above ground using electromagnetic techniques. The technology was developed in conjunction with several gas distribution companies, including Southern California gas.
Later the methodology was adopted as a standard by NACE in "SP0502-2010, Pipeline External Corrosion Direct Assessment Methodology" and has been widely used within the pipeline corrosion industry. Per the AMPP website: SP502 "covers the NACE external corrosion direct assessment (ECDA) process—a process of assessing and reducing the impact of external corrosion on pipeline integrity. ECDA is a continuous improvement process providing the advantages of locating areas where defects can form in the future, not just areas where defects have already formed, thereby helping to prevent future external corrosion damage. This standard covers the four components of ECDA: Preassessment, Indirect Inspections, Direct Examinations, and Post Assessment."*
* Ref 1 - https://store.ampp.org/ansi-nace-sp0502-2010
This article will address the use of pipeline Corrosion mapping to find coating defects using A.C. current attenuation, and pinpointing those defects using D.C. Voltage Gradient with the use of an accessory "A-Frame".
A pipeline current mapper consists of three parts. A transmitter for applying the A.C. and D.C. current that is required for the surveys, an electromagnetic receiver that displays the signal strength, measured current flow and the depth to the center of the pipeline, and an accessory that plugs in to the receiver used for making D.C. voltage gradient measurements.
The systems typically apply multiple signals to the pipeline for the purpose of making the surveys. These are an A.C. current with a frequency of <1KHz used of locating the pipeline and for measuring the depth to the center of the pipeline (if the diameter of the pipe is known, this measurement can be used for a depth of cover measurement), and an A.C. current with a frequency of <10Hz used to mimic the cathodic protection current that is normally applied to the pipeline. The user can then survey the length of the pipe taking multiple measurements sets and observe areas where these is current loss to determine coating defects or connections to foreign or unwanted conductors, such as a shorted pipe.