Pipeline Plain Dent Fatigue Assessment: Shedding Light on the API 579 Level 2 Fatigue Assessment Methodology

Abstract

For continued safe operation of pipelines, thousands of integrity digs are conducted every year to repair ILI detected defects. Integrity-driven pipeline excavations can be quite costly, present significant scheduling challenges with landowner consultation and seasonal access limitations, and an unmitigated defect may have required a pressure reduction or service outage, resulting in a loss of revenue from the asset. Dents are known to be one of the drivers for many integrity excavations, especially for liquid pipelines. A pipeline with a minimal mechanical deformation is not expected to fail immediately, however, severe pressure cycles combined with the geometric distortion can cause fatigue crack initiation and growth that can lead to failure. To account for the possibility of fatigue failure, recent changes to pipeline codes, such as CSA Z662, are requiring pipeline operators to repair any dent susceptible to fatigue failure unless an engineering assessment proves it is fit for service. A commonly used dent fatigue assessment methodology is outlined in API RP 579, also known as the EPRG-2000 model. The assessment methodology uses an S-N curve from DIN 2413 part 1 with a safety factor of 10, which has been derived from undamaged pressurized pipe sections experiencing pressure cycles with stress ratios of zero, and separate stress enhancement factors for dents and gouges which take into account the shape of dents and gouges. To account for the effect of mean stress, Gerber mean stress correction, which has been developed for pressure cycles with stress ratios of −1 (i.e., for fatigue bar specimens), is also applied on pressure cycles. According to the literature, API 579 Level 2 fatigue assessment methodology results in very conservative estimates of fatigue lives compared to experimental data. This paper will discuss the potential factors resulting in conservative assessments and propose refinements in the methodology. This will include the safety factor used for pipes with known operating pressure fluctuations and the mean stress correction model suitable for a pipeline with pressure cycles that have R ratios greater than zero. The acceptable number of cycles obtained using the proposed refinements were compared to experimental data and EPRG-1995 model’s predictions — the comparison revealed that the proposed methodology results in a more realistic safety margin for dented pipelines. The proposed methodology can be used as a part of engineering assessments in mechanical damage integrity management programs to improve the pipeline operator’s understanding of a dent’s remaining life and enable a more appropriate repair timeline.