ECAs, PROBABILITIES AND AXIAL MISALIGNMENT

摘要

An engineering critical assessment (ECA) is commonly conducted during the design of an offshore pipeline in order to determine the tolerable size of flaws in the girth welds. API 579-1/ASMEFFS-1 2016 andBS 7910:2013+A1:2015 Incorporating Corrigenda Nos. 1 and 2 give guidance on conducting fitness-for-service assessments of cracks and crack-like flaws. The essential data required for an assessment (nature, position and orientation of flaw; structural and weld geometry; stresses; yield and tensile strength; fracture toughness; etc.) is subject to uncertainty. That uncertainty is addressed through the use of bounding values. The use of extreme bounding values might be overly-conservative. A sensitivity analysis is one way of investigating the sensitivity of the results of an assessment to the input data. A structural reliability-based assessment (a probabilistic assessment) is an alternative. A probabilistic assessment is significantly more complicated than a deterministic assessment. API 579-1/ASME FFS-1 and BS 7910:2013 note that a sensitivity analysis, partial safety factors or a probabilistic analysis can be used to evaluate uncertainties in the input parameters. Annex K of BS 7910:2013 gives partial safety factors for different combinations of target reliability and variability of input data. ISO 16708:2006 gives guidance on the use of structural reliability-based limit-state methods in the design and operation of pipelines. The structural reliability-based assessment of circumferentially-orientated, surface crack-like flaw in a girth weld in a pipeline is used to illustrate the significance of the distributions of the difference between the wall thickness and the ovality (out-of-roundness) of two pipes when calculating the bounding value of the stress concentration factor due to axial misalignment. The (assumed) distributions of diameter, wall thickness, out-of-roundness, yield strength, etc. are based on Annex B of ISO 16708:2006. The (nominal) probability of failure is calculated. It is then used to inform the choice of an appropriate bounding value (i.e. a characteristic value) for axial misalignment.