Qualification of a Combined Ultrasonic Inspection Tool for Detection and Sizing of Circumferential Weld Cracks in Offshore Pipelines

Abstract

The target of inline crack inspection is normally related to the detection of axial cracks (weld cracks, SCC etc.) as axial cracks are usually expected under typical loading conditions in pressurized pipe. Ultrasonic crack inspection tools for this type of cracks have been available for over 20 years and have become a standard application in the ILI business. However, under certain conditions circumferential cracking may occur and the inspection technique needs to be modified accordingly. Especially under offshore conditions with limited pipeline accessibility not only the early detection of crack-like defects is required but also a precise depth sizing is important in order to minimize the risk of crack-related pipeline failure. In order to ensure a high probability of detection together with state-of-the-art depth sizing, a 10″ inline inspection tool was developed for the detection of circumferentially orientated weld cracks. The tool combines the advantages of the pulse-echo technique on the detection side with the excellent capabilities of the TOFD (time-of-flight-diffraction) technique for accurate sizing. Both techniques are implemented into a tethered tool where the pulse-echo unit serves for fast screening while the relatively slow TOFD-unit is used for sizing of any detected crack-like features. In order to qualify the new tool for a special offshore application (inspection of circumferential cracking at welded anode pads) extensive testing was performed using a 10″ test pipeline prepared by Statoil. The test line contained 64 artificial weld defects for reference purposes as well as an unknown number of fatigue cracks in the weld area which were generated by fatigue testing of the corresponding pipes. The tests were full blind tests with no advance knowledge on locations and sizes of the cracks. The sizes of the fatigue cracks (length, depth) were determined by destructive examinations carried out after inline testing. The subsequent comparison of the destructive results with the non-destructive results showed that the specification of the tool with regard to detection and sizing was fully met. In the paper, the inspection concept and the setup of the new tool are described, and the results of the qualification tests are presented.