Adaptive ultrasonic imaging of electric resistance welded pipeline seams

Ultrasonic imaging using the Total Focusing Method (TFM) is a technique which is well suited for the in-service inspection of axial seam welds of pipelines fabricated by electric resistance welding (ERW). For the assessment of flaws in the ERW seam, both the identification of flaw type and accurate sizing play an important role. Initially, the inspection approach used ultrasonic arrays on plastic wedges placed on both sides of the axial weld. However, depending on the quality of the fabrication process, a good mechanical fit between the pipe and wedges could not be obtained for all pipe seams inspected, which affected the quality of the images. As a result, a second approach was developed replacing the plastic wedges with an immersion setup. In this second approach, adaptive processing of the recorded full matrix capture (FMC) data is required to determine the outer pipe surface (OD) and adapt the focal laws accordingly. Using these adapted focal laws, it is possible to determine the profile of the inner pipe surface (ID). The information on the position of the boundaries can then be used to image the weld not only directly through the OD, but also to generate images based on reflections from the ID and OD. Combining these images into one cross-sectional view can then be used for flaw assessment in the ERW bond and surrounding heat affected zone. Results show that accurate knowledge or calibration of the geometrical setup and material parameters is of vital importance for obtaining focused and aligned images. Both inspection approaches are compared, and key parameters for obtaining the required capabilities in terms of characterization and sizing accuracy are discussed.