PR-003-063509-R01 Models for Predicting Delayed Fatigue Failure of Pipelines at Mechanical Damage

Because where potentially serious damage is found it is dug and dealt with, models that assess damage severity have no relevance in that context. But, as in-line inspection tools become better adept in identifying and characterizing potential damage, models will be needed to screen the ILI results, to determine whether a dig is warranted, and when that must happen. While criteria like B31G might be used to assess metal loss due to a gouge, the usual complexity of mechanical damage cannot be assessed simply in terms of length and depth, so validated models that can account for the complexity are required. Such models could then be evaluated parametrically to develop prioritization guidelines useful with modern inspection technologies.

PRCI Projects MD-4-3 and MD-4-4 were initiated to develop such models, with the results of full-scale testing generated in Project MD-4-1 serving as the basis to assess their validity. This report presents the work done as part of PRCI Project MD-4-4, which targeted an improved model for predicting time and/or cycle-dependent failure of dent and gouge damage. In addition to meeting that objective, this project also developed the basis to predict immediate failure consistent with the scope of Project MD-4-1, which as of this reporting had not emerged from the work of Project MD-4-3. Technology framed in the context of PR3-9305 was used to predict the immediate failure due to burst testing of gouged pipe done as part of MD-4-1, and found to accurately predict those collapse-controlled failures. Technology framed in that same context also was used to predict the delayed failure due to fatigue that developed in cyclic pressure testing of gouged pipes that also was done as part of MD-4-1, and found to reasonably predict those outcomes. However, there were some results that indicated the need to better quantify the crack driving force in terms of re-rounding, and the population of defects that can develop when the damaged pipe re-rounds in the wake of damage.