White Box Engineering

A Validated Assessment Methodology for Dent Fatigue

Lockey A.

Pipeline Pigging and Integrity Management Conference; Houston; February 2016.

Dents in onshore pipelines are common: most thin-walled onshore pipelines around the world are found to contain them. Many of these dents have depths less than 2% of the pipe diameter, and have only recently begun to be reported reliably by high resolution in-line geometry inspections. A dent causes localised stress concentration, and generally has a lower fatigue life than plain pipe.

There are many methods available to assess the fatigue life of dents. Closed-form methods based only on dent depth can be applied simply, using readily available ILI data. These have been validated against full scale test data, but can be overly conservative. Methods based on finite element analysis can take full account of dent geometry measured by high resolution in-line inspection, and are therefore potentially more accurate. However, a wide variety in the detail of finite element method implementations makes them complex to apply, and impossible to validate universally. The accuracy of some finite element methods may therefore be doubtful.

This paper gives details of a simple finite element based method for dent fatigue life assessment. The method generates the finite element geometry directly from high resolution in-line inspection results. It requires minimal material data, which is often not available to operators.

The paper then provides detailed validation of the method against 31 full scale tests carried out for PRCI. The improvement is accuracy compared with closed-form methods is demonstrated.