Combined Ultrasonic Tethered Tool for Inspection of Weld Cracks in Offshore Pipelines: Tool Qualification and First Inspection Results from Offshore Inspection
Typically inline inspection is carried out for geometry-, corrosion- and cracking defects. Regarding cracking, most of the time axial cracking (SCC, seam weld cracks....) is observed. Ultrasonic crack inspection tools are available for the reliable detection of such cracks using conventional ultrasonic shear wave technique suitable for detection but with limited depth sizing capabilities. Under special conditions (bending forces, poor welding) cracking can also appear in circumferential direction. For this type of cracks, the same type of tool can be used with a modified sensor carrier providing similar detection and sizing performance as for axial cracking. In offshore applications, however, accurate sizing in particular of crack depth is an important requirement as offshore verification and repair work is usually very cost-intensive. In order to provide a high POD together with good sizing performance a tethered 10" tool was developed for inspection of circumferential cracks. It contains a pulse - echo (PE) unit for reliable detection and a TOFD (time-offlight- diffraction) unit for accurate sizing of circumferential cracks. The PE technique is used for fast screening during the forward inspection. The TOFD measurement which is basically stationary is performed on the backward inspection by stopping at every location where a crack indication was detected by the PE inspection. Extensive blind testing of the new tool was performed. The comparison of the results from ILI inspection and destructive testing showed that the ILI tethered tool fully met the specification with regard to detection and sizing. After successful validation the tool was used for a tethered inspection of a 10" offshore pipeline (length 8 km). In the paper the inspection concept and the setup of the new tool are described. Results from the qualification tests as well as from the first inspection run performed with the new tool will be presented.
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