Full-Scale Validation of a Flaw Assessment Methodology for Welded and Reeled Pipes

Pre-existing weld flaws in pipes subjected to high strains resulting from reeling installation may extend by combined ductile tearing and low-cycle fatigue (LCF). The increase in flaw sizes from this process will reduce the flaw tolerances of reeled pipes subjected to subsequent service high-cycle fatigue (HCF) exposure compared to the tolerances determined ignoring the reeling process. This paper describes a flaw assessment methodology that includes the synergy between ductile tearing and LCF flaw growth due to reeling. The methodology utilizes the Level 3C (J-based) failure assessment diagram (FAD) approach of BS 7910 to explicitly account for the effects of flaw and pipe geometries, as well as material stress-strain behavior, on the crack tip driving force, J. The Level 3C FADs used herein are derived from and validated against the results of a matrix of J-based finite element analyses (FEA) of pipes. They have been incorporated into the computer program FlawPRO™ developed for the offshore industry that enables a comprehensive engineering critical assessment (ECA) for flawed reeled pipes to be performed. The methodology incorporated into FlawPRO is validated against the results of full-scale tests on reeled pipes containing weld flaws subjected to a variety of reeling scenarios. The validation includes investigations into the effects on predicted flaw growth of using J-resistance curves measured on SENB and SENT specimens and of assuming strain versus load controlled reeling. The best agreement with the flaw extensions measured in the full-scale tests is obtained when an SENT J-R curve is used together with assuming reeling occurs under strain controlled conditions. It is concluded that the reeling methodology in FlawPRO is successfully validated by the full-scale test results.