MECHANIZED UT ON GIRTH WELDS DURING PIPELINE CONSTRUCTION - A MATURE ALTERNATIVE TO RADIOGRAPHY

As an alternative to radiography, a field-proven mechanised ultrasonic inspection system, is discussed. Called 'Rotoscan', this system has b een developed for inspection of girth welds during construction of lon g-distance pipelines, both on- and offshore. The system is characteris ed by high inspection speed and instant recording of results. Unlike r adiography, it provides immediate feedback to the welders. Recent tech nical improvements in flaw sizing and recording have allowed the appli cation of rejection/acceptance criteria of weld defects based on fract ure mechanics. The development and actual use of such modern acceptanc e criteria support the introduction of mechanised UT. Worldwide applic ations proved that, contrary to expectations, ultrasonic inspection do es not lead to higher weld repair rates than radiography. Between earl y 1989 and now, over 5000 km of pipeline (400,000 welds) were inspecti on by our organisation. Recent introduction of C-scan mapping enable t he system to cope with most existing UT procedures and acceptance crit eria, because of its capability to detect and quantify volumetric defe cts. Moreover, the integrated simultaneous TOFD function enables defec t height estimation. The present system is capable of achieving a high Probability Of Detection (POD) together with a low False Call Rate (F CR). In the meantime, Rotoscan has been qualified in various countries , for different customers and for a variety of weld processes, pipe di ameters and wall thicknesses. Because of its features, the now mature system has demonstrated its capabilities also for use on lay barges as an alternative to high-speed radiography.