FATIGUE LIFE ASSESSMENT FOR PIPELINE DENTS UNDER HIGHWAY CROSSINGS

A routine inline inspection pigging operation revealed a number of dent anomalies in an onshore pipeline transporting refined products. A detailed fitness for service assessment was subsequently performed by a leading engineering consultant company using the best industry practices, concluding that the dent defects with varying level of ovality in the order of 5% presented no immediate threat to the normal service under projected pressure cycles. The plain dent defect features were classified into a non-urgent category well down the scheduled repair list, requiring no priority intervention or replacement programme except further monitoring inspections and operating pressure control before a repair was planned. However, the dent defects in the proximity of highway crossings predicted to have longer remnant fatigue life failed unexpectedly within a much shorter time span, leading to costly emergency replacement and disruptive system downtime. This disparity has raised a question about the likely failure cause, uncertainty and reliability of the defect assessment. To this end, one anomaly was re-examined and its study is presented in this paper on how a fitness for service assessment for such dent anomalies can be improved using FEA modelling and consideration of some of the lessons learnt from similar failures and assessment experiences. It demonstrates that certain dynamic loading and additional fatigue promoting factors commonly ignored are of importance for fatigue life assessment.