Relationship Between Defect Shape Resolution Accuracy and Calculated Remaining Strength of Corroded Pipelines

Reliable predictions of the remaining strength of corroded pipelines are dependent on the assumptions made during the defect assessments, and the availability of an accurate description of the defect shape that is detected by in-line inspection (ILI) tools. If only the maximum defect dimensions are recorded and subsequently used in a defect assessment then a low remaining strength is likely to be predicted for the corroded pipeline. However, if the defect profile had been more accurately mapped then improved predictions of the remaining strength can be determined from the subsequent defect assessment. This improved assessment could be enough to justify safe operation of the corroded pipeline rather than to undertake expensive repairs. For these reasons, PRCI has sponsored Advantica to investigate aspects of both ILI data accuracy and assessment methodology to determine the benefits gained in the remaining strength predictions. This has been undertaken using a combination of in-house specialist computer software and finite element modelling. Work has been undertaken to determine the benefits of an improvement in the measurement accuracy of the three characteristic flaw dimensions (depth, length and width) when determining the remaining strength of a pipeline, and to determine the effect that changes to the defect interaction criterion, used in the assessment, has on remaining strength predictions. Case studies using example ILI data from two pipeline operators are used to determine how measurement accuracy affects remaining strength predictions. Using deterministic assessment methods, such as RSTRENG, it is concluded that sizing accuracy for the defect depth and length are most critical in predicting remaining strength predictions. The defect width is used only to determine interaction of closely spaced defect clusters. In addition, it is concluded that the interaction criterion generally used in industry is overly conservative and could be relaxed.