Determining the acceptability of bottom side dents with metal loss

摘要

The assessment presented in this paper demonstrates how finite element analysis can be coupled with reliable ILI data to perform a fitness-for-service assessment for a dent with corrosion. This methodology is appropriate and beneficial for dents that may have small amounts of corrosion that can be accurately identified and sized. In pipelines where the pressure cycling is light to moderate, fatigue is unlikely to occur, and a detailed assessment can demonstrate the acceptability of the dent. The case study reinforced several known concepts and demonstrated a few key concerns in the assessment of dents. a. When performing assessments of restrained dents, operators should use all available means to confirm the nature of the metal loss. The input of an experienced data evaluators is considered to be paramount. b. The nature of the restraint should be evaluated by considering the shape of the dent or performing initial elastic finite element analysis. c. The case study demonstrated that treating a dent as unrestrained will produce a larger SCF and a shorter fatigue life, consistent with previous research. However this approach may result in excessively large SCFs for many restrained dents. d. The assessment should consider multiple indenter scenarios and/or variations in the relative motion of the indenter and pipe as this may have an impact on the magnitude of the SCF. e. The assessment should also consider the effect of any historical pressure tests. f. Finally, the analysis should address both the burst pressure and fatigue life of the feature ensuring that plasticity is appropriately accounted for in the calculation of the SCF and remaining life of the dent. It should be noted that the fatigue lives in this paper are largely contingent on the mild pressure cycling data. This data was assumed to be representative of past and future pressure cycling on the line. When performing a remaining life assessment for pipeline anomalies it is important that operators periodically monitor pressure history data to ensure that the fatigue life of the feature is not reduced by a change in operations. It is also important to note that the assessment assumed that the corrosion had been mitigated such that the depth of metal loss had not increased since the time of the inspection and would not increase in the future. This assumption can be confirmed through repeat ILI inspections.