Reliability of mechanised UT systems to inspect girth welds during pipeline construction

As an alternative to radiography, a field-proven mechanized ultrasonic inspection system is discussed. Called Rotoscan, this system has been developed for inspection of girth welds during construction of long-distance pipelines, both on- and offshore. The system is characterized by high inspection speed and instant recording of results. Unlike prevailing radio-graphy, it provides immediate feed-back to the welders. Recent technical improvements in flaw sizing and recording have allowed the applica-tion of rejection/acceptance criteria of weld defects based on fracture mechanics. The development and actual use of such modern acceptance criteria, particularly in Canada, supported the introduction of mechanised ultrasonic inspection. World wide applications proved that, contrary to expectations, ultrasonic inspection does not lead to higher weld repair rates than radiography does. Between early 1989 and now, over 5.000 km of pipe-line (300.000 welds) were inspected with Rotoscan and its reliability proven. Introduction of C-scan mapping enabled the system to cope with most existing ultrasonic procedures and acceptance criteria, because of its capability to detect and quantify volumetric defects. Moreover, the integrated simultaneous TOFD function enables defect height estimation. The present system is capable of achieving a high Probability Of Detection (POD) together with a low False Call Rate (FCR). In the meantime, the system has been qualified in various countries, for different customers and for a variety of weld processes, pipe diameters and wall thicknesses. Because of its features, the now mature the system has demonstrated its capabilities also for use on lay barges as an alternative to high-speed radiography.