Pipeline Bending Strain Assessment using ILI Tools: A Case Study
Mapping the position of a pipeline using pipeline location coordinates (x, y, z) collected from an in-line inspection (ILI) survey has become routine practice for many pipeline operators over recent years. When an IMU (Inertial Mapping Unit) tool is included as part of an ILI survey it provides a synchronized stream of x, y, z mapping information which aligned with the ILI data provides the means to accurately and easily locate pipeline anomalies, features and fittings. Indeed, regulations in some regions require that the precise location of pipeline assets are documented as part of managing the integrity of a pipeline and certainly this is considered to be good practice. The x, y, z mapping information collected by an IMU tool has an additional use which is less widely understood. The mapping data can be used in the determination of sections of a pipeline indicating potential deviation from the pipelines’ original position. Specialized assessment of the IMU data can be performed to calculate curvature and to derive the consequential bending strain levels throughout the pipeline. The use of data from repeat runs can identify where even small changes to pipeline shape changes are occurring. However, the use of the IMU mapping data is less known for this purpose and has not yet been widely adopted across the industry. There is no requirement in pipeline regulations to conduct this type of study and furthermore little guidance in industry codes on what level of movement or bending strain should be considered actionable. This paper presents case studies describing the rupture failure of a 10 inch natural gas pipeline caused by a spontaneous and localised landslide event. The metallurgical failure investigations carried out on the pipe material taken from the failure site did not find any evidence of sub-standard pipe material or of any pre-existing pipeline defects, this was further confirmed from ILI and IMU data collected from the failed pipe several months prior to the event. Hence, it can be concluded that the rupture was caused solely by the catastrophic external loading on the pipeline resulting from the sudden landslide. The paper goes on to discuss the specialized assessment of IMU mapping data to derive the magnitude of sub-critical bending strains present on the pipeline prior to the landslide event and whether such assessment could be used to indicate the pipeline segments in unstable ground conditions and potentially at risk from environmental outside force events. Taking experience from these case studies and from other known pipeline events, the level of bending strain that may be considered actionable is also discussed.
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