Fracture Mechanics Approach for the Assessment of a Damaged Pipeline

Nowadays, the damage assessment of pipelines using fracture mechanics techniques is a state-of-the-art methodology which allows a judgement whether the pipeline is fit-for-purpose over the residual lifetime (perhaps with a reduced pressure load) or has to be repaired or replaced. However, in some circumstances, uncertainty exists about its correct application, as this may be a matter of interpretation. This paper describes the analysis of a damaged pipeline based on a fracture mechanics approach for which the straightforward application of the flaw assessment standard BS 7910 without additional consideration of individual conservatism criteria could lead to misinterpretation. The investigation was carried out for pipelines under internal overpressure. The methodology applied considered large plastic deformation introduced during a repair measure. Since the repair or a subsequent overload resulted in the damage of a pipeline and could also have caused the damage of further components, evidence had to be provided that the structure was still damage-tolerant, i.e. that cracks, the existence of which could not be excluded, would not grow to a critical dimension over the projected lifetime. The investigation comprised the following steps: In a first step, the maximum crack size which could have survived the repair measure had to be specified. In this context, the choice of conservative input data and model parameters was essential in order to avoid any underestimation of the postulated crack size and geometry. Various criteria are discussed briefly here, placing special emphasis on global and local plastic collapse criteria. Secondly, the growth of this initial crack due to the operation stress had to be quantified; this was done by applying the so-called NASGRO approach. Finally, the critical crack dimensions had to be assessed. Fracture assessments were performed using various procedures, such as BS 7910, R6 and FITNET. A parameter study was carried out to demonstrate the potential conservatism of these assessment methods.