Need: The ability to fatigue-rate dents in line pipe is important for three reasons. First, field failures give evidence that fatigue crack growth occurs in dents under certain circumstances in spite of the fact that plain dents are generally thought to have little effect on the integrity of natural gas pipelines based on the results of single-cycle burst tests. Second, many operators are discovering large numbers of dents on the bottom quadrants of their pipe associated with rocks and backfill loads. Guidance is needed for discerning dents for which excavation and inspection is economically wasteful and counterproductive to pipeline safety from those dents for which further action would be beneficial. Third, fatigue life may be a more rational basis for rating the severity of a dent than present criteria which rely solely on dent depth with a maximum depth of 6 percent of the pipe diameter as a generally accepted limit. In some cases deeper dents might be permitted to remain in service, while in other cases, shallower dents should be repaired.
Benefit: The goal of this project was to develop guidelines for pipeline operators to assess the severity of dents on the basis of their fatigue life in-service. The assessment uses pipeline operating pressures and simple geometric measurements of the dent.
Result: A theoretical model for a deformed cylinder under pressure was used to obtain the following results: (1) identification of a dent shape parameter that significantly influences the dent's rerounding and fatigue behavior; (2) derivation of a model for predicting dent rerounding due to pressure in the pipe; (3) derivation of a model for estimating the fatigue life of a plain dent that is free to reround; (4) confirmation that unrestrained plain dents of as much as 10% of the diameter may not pose a significant threat of fatigue within the expected service lifetime of most gas pipes; and (5) development of guidelines to determine which dents should be excavated for detailed inspection and which are better left undisturbed. The experiments also showed that dents undergo stress relaxation under steady pressure and incremental plastic rerounding under repeated pressure cycles, until subsequent cyclic stresses are elastic.
