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 akey 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 ofdetection of flaws and it also allows a really accurate assessment of the indications. However, since one of thepeculiarities 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 , 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 variespoint by point around the circumference. For this reason, several approaches have been developed during theyears from AUT contractors such as using dedicated techniques (e.g. multi-shooting) or using multiplecalibration blocks. However, all these solutions may deeply influence costs because of the additionalqualification 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 analysesthe influence of WT variation on the reliability and accuracy of zonal discrimination approach. Real flawsgeometry has been extrapolated from macro-sectioning. Actual focal laws settings have been considered. Aftervalidating the proposed model by comparing the simulation results with actual ones, several steps of basematerial WT variation are considered in order to understand the effect on the ultrasonic response of the flaws inobject.
|Numero di pagine||0|
|Stato di pubblicazione||Published - 2015|