Modelling of Dent and Gouges, and the Effect on the Failure Probability of Pipelines

One of the main causes of the failure, and resulting casualties, in onshore and offshore transmission pipelines is external interference (also known as ‘mechanical damage’, or ‘third party damage’). External interference can result in a dent, a gouge, or a combination of the two. The combination of dents and gouges is a severe pipeline defect. It creates an unstable stress state at the defect root where work hardening and micro-cracking may be present. This can lead to a variety of immediate and delayed failures in pipelines. The available assessment models for a dent containing a gouge contain significant variability which leads to both conservative predictions of failure pressure and wide scatter when compared to test data. Additionally, these models are used extensively in pipeline risk analysis, and the differing models can cause wide variation in pipeline failure frequency predictions. This paper is a critical evaluation of existing dent/gouge models, using the full scale test database from the ‘Pipeline Defect Assessment Manual’ (PDAM). It compares two published methodologies for assessing dents containing gouges to determine the most robust model. The paper then uses probabilistic methods to determine the effect of these models on the probability of pipeline failure in a range of pipeline geometries. The paper ends with a gap analysis on dent and gouge models, and identifies requirements for targeting further research in this area, and also makes recommendations for ‘PDAM’.