This is a paper published in Corrosion Management Magazine ( September 2018, issue 145). It can be accessed from IoCC website.
The most important performance indicator for cathodic protection (CP) is the structure-to-electrolyte potential. In most of coated on-shore pipeline, the resistance between the reference electrode and the pipe-to-soil surface boundary is too significant to be ignored. The IR drop caused by CP current is a measurement error. It is stipulated in many CP standards that IR component to be evaluated and excluded while assess the pipe-to soil potential performance in pipeline integrity management.
For a well-coated pipeline, the most practical monitoring technique is to use synchronised interrupting technique by bring the current to zero and measure the potential momentary after the current source is off. By switching all sources of current to near zero, the measured potential is approaching to polarization potential by virtually eliminating the IR component in the CP electrical circuit. This technique has been widely accepted and adopted in pipeline industry for decades.
Most of modern pipelines are electrically isolated from above ground facilities, either through a flange isolation kit (FIK) or Monolithic Isolation Joint (MIJ) . This is to prevent excessive current drain which may result in CP being uneconomical or impractical. They prevent the CP current flowing to other facilities and equipment that otherwise electrically connected to pipeline. However, in practice, the effectiveness of these electrical joints can be compromised by internal debris deposition , which leads to the electrical short of pipeline to above ground facilities and their associated grounding system, from time to time. The earthing system, governed by local regulation and standards, is typically copper or copper-clad steel rode conductors. The implication of this electrical short to such earthing systems in CP measurement has not been fully explored or .fully understood by the industry.
|||Standards Australia, “AS 2832.1 Cathodic protection of metals,” 2015.|
|||ISO, “15589-1 Petroleum, Petrochemical and Natural Gas Industries- Cathodic Protection of Pipeline Systems- Part 1: On-Land Pipelines,” 2015.|
|||NACE, “TM0497, Measurement Techniques Related to Criteria for Cathodic Protection on Underground or Submerged Metallic Piping Systems,” 2012.|
|||BS EN, “BS EN 13509 Cathodic protection measurement techniques,” 2003.|
|||NACE, “SP0502 Pipeline External Corrosion Direct Assessment Methodology,” 2010.|
|||NACE, “SP0286 Standard Practice Electrical Isolation of Cathodically Protected Pipelines,” 2007.|
|||E. McAllister, Pipeline Rules of Thumb Handbook, Gulf Professional Publishing, 2009.|
|||S. M. Hesjevik, “09067 Isolation Joint Stray Currents Experimental Testing and Modeling,” in NACE Corrosion 2009, 2009.|
|||NACE, “SP0104 The Use of Coupons for cathodic protection monitoring applications,” 2014.|
|||R. Gummow, D. Boteler and L. Trichtchenko, “PRCI L51909: Telluric and Ocean Current Effects on Buried Pipelines and Their Cathodic Protection Systems,” 2002.|
|||CeoCoR, “Application of Coupons and Probes for Cathodic Protection Monitoring Purposes,” 2013.|
|||NACE, “SP0104 The Use of Coupon for Cathodic Protection Monitoring Applications,” 2014.|
Full paper can also be accessed from link below.