Influence of Internal Pressure for Depth Measurement on a Dent Available to Purchase

The main cause of failure for most transmission pipelines is third party damages, which conduct to the creation of dents. Depending on their dimensions, these defects can be detrimental in term of pipeline integrity. Defect measurement is usually performed on field when the pipeline is under pressure. However, common assessment criteria are based on the defect characteristics measured in absence of internal pressure. An empirical relation — Ho = 1.43 Hr — links the dent’s depth in absence of pressure Ho to its depth under pressure Hr. It can be assumed that the dent’s depth decreases as internal pressure increases; however, the relation is used whatever the pressure value, during measurement, is. Therefore, this relation can be considered as too conservative, and not realistic, especially when the pressure during defect characterization is low. This study presents a new relation linking Ho and Hr based on results of finite elements calculations and experimental data. The calculated results are always conservative, and the difference between experimental and calculated data does not exceed one percent. Gaz de France Research and Development division has also been able to decrease the coefficient of the relation below 1.43 for most of cases, especially for low values of internal pressure. But some cases with a coefficient between Hr and Ho which overtook 1,43 were also found. The depth of a dent without pressure can then be determined by taking into account the internal pressure when measuring the size of the dent.