Influence of seamless pipes wall thickness variation for the effectiveness of Automated Ultrasonic Testing

Offshore pipelines construction is a technological challenge. Increasing water depth, harsh environments, engineering and commercial constraints are just some of the existing issues. Material selection therefore plays a key role and, for several applications, seamless pipes offer both the best technical and commercial compromise. Typically, NDT inspection of girth welds is carried out using the phased array based automated ultrasonic testing (AUT) zonal discrimination approach. AUT, compared to other techniques, guarantees a higher probability of detection of flaws and it also allows a really accurate assessment of the indications. However, since one of the peculiarities of this technique is that it uses focused beams, it is strictly linked to the geometry of the joint. According to the most credited international codes [1], performing AUT the allowable wall thickness (WT) variation from the nominal one is ±1.5 mm (or less for higher strain applications). Seamless pipes are likely to exceed this limit, particularly taking into account that the WT commonly varies point by point around the circumference. For this reason, several approaches have been developed during the years from AUT contractors such as using dedicated techniques (e.g. multi-shooting) or using multiple calibration blocks. However, all these solutions may deeply influence costs because of the additional qualification tests required and the potential impact on the cycle time. Starting from AUT validation data to build the model and using simulation software as tool, this study analyses the influence of WT variation on the reliability and accuracy of zonal discrimination approach. Real flaws geometry has been extrapolated from macro-sectioning. Actual focal laws settings have been considered. After validating the proposed model by comparing the simulation results with actual ones, several steps of base material WT variation are considered in order to understand the effect on the ultrasonic response of the flaws in object.