The owner of an onshore X52 pipeline (diameter 12 inch) wanted to assess the safety risk of the pipeline over time. The risks were identified as fatigue failure initiating from either crack-like defects (e.g. weld defects) or zones with metal loss (e.g. due to corrosion) and propagating as a result of internal pressure variations in the pipeline.
A fatigue testing program was therefore set up. Different defects were simulated in full-scale pipe samples:
• Crack-like defects were simulated by sharp machined notches
• Metal loss was simulated using local reduction of wall thickness
The damaged pipe samples were subjected to controlled dynamic internal pressure variation. The maximum pressure was set equal to the highest possible pressure in the network and the amplitude was chosen to match maximum service pressure variations. The tests were carried out until a leak appeared due to fatigue crack propagation.
In all test samples, failure occurred at a number of cycles larger than the expected number of cycles in service. For the considered metal loss reduction and crack depth, the pipes can be considered as safe over its fifty year service life.
Simulation "metal loss like" defect
Simulation "crack like" defect
Internal pressure fatigue test set-up
Detail of fatigue test set-up