Recommended Procedures for Evaluation and Synthesis of Pipelines Subject to Multiple IMU Tool Surveys

This paper summarizes recommended procedures for evaluation and synthesis of geometry data from pipelines subjected to repeat in-line inspections (ILI) using strapdown Inertial Measurement Unit (IMU) tools. The paper provides an overview of IMU tool instrumentation (gyroscopes, accelerometers and odometers), a brief summary of IMU tool data processing and discusses IMU tool sensitivity and accuracy specifications. The paper also provides a review of the approaches that are used to identify ground movement signatures in IMU data profiles. Based on a single IMU survey, it can be difficult to determine with certainty if a given bending strain feature was induced during construction of the pipeline (e.g., by conforming the pipeline profile to an uneven trench profile) or was the result of post-construction movement of the pipeline (e.g., due to geohazards along the alignment such as landslides, karst, etc.). Because such a large percentage of vendor identified bending strain anomalies in IMU surveys are the result of construction, comparison of pipeline geometry changes between two (or more) inertial inspections is normally a more reliable method of identifying areas of actual post-construction movement of the pipeline. This is because a high degree of repeatability is observed in overlays of the pipeline geometry data signatures obtained from different IMU surveys at construction-induced bending features whereas the geometry signals associated with real pipeline movement frequently exhibit change patterns in out-of-straightness, pitch and heading and curvature/bending strain profiles that correspond to increasing pipe curvature and deformation. The primary focus of this paper is to summarize recommended data deliverables and plots of three-dimensional pipeline geometry profile data from different IMU tool surveys at vendor-identified bending strain anomalies including multi-panel overlay plot packages as well as survey-to-survey difference or change profiles that greatly assist in distinguishing if the feature is the result of construction or due to post-construction movement and to aid in the evaluation of pipeline deformation states.