Reducing Cost of ILI Inspections by Assessing and Improving Accuracy of Past Inertial Surveys

Abstract Although IMU has been part of ILI surveys for 35 years, there is no standard method of reporting inertial mapping accuracy to operators. ILI vendors often report the accuracy of IMU runs as a standard tool specification rather than run-specific values, and when such specific values are reported, they can be easily misinterpreted. A method of assessing IMU survey accuracy explained in this paper could be adopted by the ILI industry to ensure reporting clarity and consistency between different vendors. The error estimation is also the basis for producing a master pipeline listing using single or multiple IMU data sets. The presented method delivers more accurate pipeline centerline shape, improved odometer distance, and corrected GPS locations, particularly when data from multiple surveys is available. Incorporating master pipeline listings into an ILI vendor’s program will reduce operator costs. It produces more accurate dig locating and allows for accurate distance (chainage) calculations in the event of an odometer failure. It also eliminates the requirement for repeat high-accuracy GPS surveys of AGMs, valves and other pipeline features. It can even eliminate the need for collecting future IMU data, unless pipeline movement and bending strain monitoring are required, or there is a major re-route of the pipeline. Using the same distance for all future surveys would simplify data alignment, cross-reference, and movement analysis between runs. In the process of calculating master listing, more accurate data for pipeline movement analysis can also be delivered for the entire run. Evaluation and improvement of past inertial surveys accuracy is necessary for deciding whether additional GPS or IMU surveys are needed to produce master listing with the required accuracy.