Strain Based Assessment of Wrinkles
Improvements in In-Line Inspection Technology has led to the increase in probability of detection and ability to characterise many pipeline anomalies including geometric features such as shallow dents and wrinkles. Dual measurement (deflection and proximity) technologies can characterise and measure geometric anomalies such as wrinkles more accurately than traditional calliper technology (deflection only). Wrinkles are not widely recognised as having a significant impact on pipeline integrity. However they can cause failure by two mechanisms: cracking at the wrinkle apex or shoulders or by fatigue failure. High resolution calliper technology consisting of two measurement components is capable of accurately measuring the surface profile of a wrinkle. The tool uses a combination of a mechanical calliper arm and an eddy current based proximity sensor which compensates for the data artefacts resulting from the sensor arms lifting off the pipe wall. Existing methods of assessing wrinkles generally consider failure due to fatigue only and require confirmation that the wrinkle is free from associated cracking prior to assessment. In practice, this restricts the assessment to wrinkles that already been investigated in-field and where the absence of cracking has been confirmed by non-destructive evaluation. In addition, the methods can be highly conservative, in particular for pipelines that are only lightly pressure cycled. Strain based assessments are widely used throughout the industry to estimate the likelihood of cracking on dents and appropriate strain limits have been defined. This paper discusses the use of a strain based assessment to determine the significance of wrinkles on the immediate integrity of a pipeline.