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. It is characterized by high inspection speed and instant recording of results. Unlike prevailing radiography, it provides immediate feedback to the welders.
Recent technical improvements in flaw sizing and recording have allowed the application of rejection/acceptance criteria for weld defects based on fracture mechanics principles. 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 pipeline (300.000 welds) were inspected with Rotoscan and its reliability proven.
The introduction of colour enhanced transit distance “C-scan mapping”, producing a coherent picture based on the signal’s transit distance, 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 Time Of Flight Diffraction (TOFD) function enables through-thickness sizing of defect. The present system is capable of achieving a high Probability Of Detection (POD) together with a low False Call Rate (FCR).
In the meantime, Rotoscan 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 system has demonstrated its capabilities also for use on lay barges as an alternative to high-speed radiography.