The interpretation of the indirect inspection data is a critical factor in conducting a successful ECDA process. The data must be validated, discrepancies must be resolved, and indications must be identified, classified and prioritized, in order to select the sites for direct examinations.
The pipeline operator must carefully analyze the interrelation between indications identified by various indirect inspection tools, based on their relevance to corrosion activity, and threat to pipeline integrity.
For example, a high AC induced voltage in a low resistivity soil in conjunction with a small holiday (minor DCVG indication) presents a significant threat of AC induced corrosion, while the same voltage and resistivity conditions in conjunction with a large holiday reduces this threat.
A sharp drop in pipe potential, recorded during the Close Interval Potential Survey, could indicate a holiday and therefore validate a DCVG indication, or identify a severe “DC Interference” indication, representing a major integrity threat to the pipeline.
This paper analyzes the various types of data provided by the indirect inspection tools and especially their interrelation. Additional types of indications related to AC and DC interference are also discussed. “Threat related” classification and prioritization criteria, as successfully applied on more than 50 gas pipelines in Ontario, are presented and rationalized. Possible error scenarios resulting from neglecting the interaction between various types of indications are developed from existing sets of data.