A Powerful Tool for Assessing Locations of Defects Missed-Out by ILI

The paper describes essentials, features and possibilities of a statistical tool that permits assessing the possible location (log distance coordinate) of defects that were missed-out during inline inspection (ILI). The method is based on complete analysis of the results of the latest ILI, using the basic set of ILI tool performance metrics, introduced and substantiated by the first author. The method is based on analysis of a full set of possible outcomes of any ILI measurement, namely: true detection/non-detection; false detection/non-detection. The developed statistical tool uses combined data from the ILI tool specifications, ILI run results and verification digs. This method permits quantitative assessment of the number of falsely detected and undetected actual defects and also the distribution of their sizes for each segment of an inspected pipeline that was not verified by digs. Application of the method is described for use when deciding how many defects of different categories have been unobserved. A decision rule is described with which it is possible to be practically sure that no dangerous/reportable defects were omitted. A combined (actual and computer simulated) case of analysis of ILI and dig data accumulated for a one-kilometer long pipeline segment is presented. Using existing strength codes it is shown how to locate possible dangerous spots on pipeline segments, where additional digs or monitoring would be advised/recommended. The described method is recommended for use by pipeline companies in order to minimize the risk of leak/rupture and enhance the culture of pipeline maintenance in general.