During service, pipelines are likely to have mechanical damage, such as dents that can contribute to local stress concentration and alter the proper functioning of the structure. A dent may cause significant structural damage, especially if the pipeline is subject to water hammer waves. In this article, the investigation of pipes subjected to water hammer waves due to the event of a sudden change in valve status or pump failure is carried out. A numerical damage accumulation model that takes into account the load interaction effect is developed and validated in order to assess the harmfulness of the dent defect. This model uses the pressure peaks induced by the water hammer phenomenon as input. Based on a fatigue life estimation model, taking into account the mean stress effect, the damage is calculated for each loading level. A non-linear interpolation analysis is performed on the results obtained from a finite element model in order to determine the stress concentration factor for any dent depth value, which is used to estimate the fatigue life of the damaged structure. A model is used to take the effect of dent re-rounding into consideration. Then the developed damage accumulation model is used for a parametric analysis. The parameters considered are the dent type (spherical, rectangular longitudinal and rectangular transverse), the dent configuration (constrained and unconstrained), the dent depth, and the fluid inside the pipe. This study will allow the most damaging indentation defect to be concluded, as well as the factors that could make the defect more harmful.