Using an inline inspection (ILI) tool to measure cathodic protection (CP) current provides real-time data that help operators identify areas where the CP system is not functioning properly and where active corrosion may be present
This ILI technique complements all existing direct assessment inspection and CP survey techniques and provides a new data set that was not available prior to development of the tool. Launched and received like a standard ILI tool, the CP ILI current measurement tool collects voltage data using direct electrical contact with the interior pipe wall to determine the direction and magnitude of CP and other current on the pipe. It records data for pipeline sections under pavement, below rivers, and under bodies of water where conventional CP test procedures are not effective.
The CP ILI current measurement inspection is able to locate areas where there is little or no CP current pickup on the pipeline between CP current sources, as well as areas where there are contacts or unknown bonds on the pipeline that are drawing away CP current intended for the pipeline. At any given spot along the pipeline, there is a direct correlation between CP current and the pipe-to-soil (P/S) potential. CP current on the pipe causes a change in the measured potential in the negative or more cathodic direction.
There is a very strong technical argument that supports the use of pipeline current and current density calculations to document adequate CP levels, and that utilizing P/S potentials alone is not the only means of determining that adequate CP is present. It is now possible to establish whether there is adequate corrosion protection using current as a stand-alone criterion. When a comprehensive CP evaluation is attained using CP ILI current measurement along with conventional CP potential surveys, close interval surveys, and the direct current voltage gradient technique, there is a higher level of confidence that corrosion control has been achieved on a pipeline and it is receiving current from the electrolyte at all points on the surface of the steel.
The technology, process, and methodology are discussed in CORROSION paper no. 3821, “Developing a Standardized Process for Cathodic Protection Current Measurement on In-Service Pipelines—Process and Procedures for a New Technology,” by D. Williams and D. Janda.\