Defect Sizing in Pipeline Welds: What Can We Really Achieve?

Pipelines are now using Fitness-For-Service (FFS) for accept/reject of weld defects. FFS requires accurate measurement of defect height for Fracture Mechanics assessments. The standard pipeline weld inspection technique of radiography is incapable of such measurements. However, the newer technique of ultrasonics can measure defect height, in principle. Initially ultrasonic amplitude methods were used for height measurement, but these proved unreliable. Now diffraction methods, especially Time-Of-Flight-Diffraction (TOFD), are being used in conjunction. This paper reviews previous work — mainly large nuclear studies like PISC II — and published pipeline sizing studies. The best nuclear sizing was within a few millimetres, using diffraction. In contrast to nuclear, pipeline AUT uses zone discrimination, focused transducers, much thinner material and simpler analysis techniques. Current accuracies are typically ± 1 mm (terminology undefined), which correlates with the beam spot size and typical weld pass. Requests for accuracies of ± 0.3 mm are probably unachievable, though future R&D should significantly improve pipeline sizing.