Interaction between internal and external defects on pipelines and its effect on failure pressure

A finite element model was developed to evaluate the interaction of internal and external defects with various orientations on pipelines and its effect on failure pressure in this work. With an approximately identical geometry, the external defect is associated with a greater maximum von Mises stress than the internal defect. The longitudinal limit spacing, below which the interaction occurs between defects, for a defect distribution type containing external defects only (S-L,ext(Lim)) is larger than the longitudinal limit spacing for a defect distribution type containing both external and internal defects (S-L,int(Lim)). There is a lower failure pressure for the defect type with all external defects. When both external and internal defects are present, the failure pressure of a pipeline depends on both the longitudinal spacing between internal and external defects (S-L) and the ratio of the internal defect depth to pipe wall thickness (r(bB)(,int)). For small r(bB)(,int) the failure pressure is almost independent of the presence of the internal defect. With the increase of the r(bB)(,int), the effect of the internal defect on failure pressure becomes apparent if S-L smaller than S-L,int(Lim). When the r(bB)(,int) is so large, the failure pressure decreases rapidly. Thus, for more accurate prediction of the failure pressure, it would be necessary to sufficiently assess the interaction between the internal and external defects on the pipelines according to the new S-L,int(Lim) rule.

Automated summary

A finite element model was developed to assess the interaction of internal and external defects on pipelines. It was found that external defects have higher maximum stress than internal defects, and pipelines with only external defects require a larger longitudinal spacing before interaction occurs.