Cross country pipelines typically contain dents with the potential to affect integrity due to formation cracking or fatigue. Modern In line inspection (ILI) can provide dent shape profiles which can be used directly in modelling and assessing dent fatigue.
This paper describes a finite element analysis (FEA) modelling process which can be applied efficiently to all the dents reported in a pipeline, and uses ILI profile data as the starting point. The process accounts for:
- The internal pressure at the time of inspection
- The non-linear relationship between internal pressure and stress in the dent
- The presence of restraint
- The effect of material shakedown to elastic behaviour
- Interaction with welds and blunt metal loss
Together, these allow the fatigue damage caused by complex pressure loading histories to be determined.
This paper discusses approaches to effectively identify when dents are restrained, and presents a novel method using FEA to calculate the extent of dent restraint using ILI profile data.
Where a dent interacts with a weld, fatigue of the weld is of concern. However, the weld may not be located at the highest stress concentration. This paper discusses an extension of the FEA approach to determine the stress concentration (and hence fatigue loading) at the weld.
Benefits of the approach are also discussed, considering the accurate representation of dent shape from ILI data, the ability to automate assessments reliably, and avoidance of the need for closed form approximations.