| Ultrasonic testing technology is a kind of commonly used non-destructive testing method, and is widely used in the welding defect detection and evaluation of the welds on oil and gas pipeline. The ultrasonic testing technology researched in this paper is mainly used in pipeline repair lap fillet welds and butt longitudinal welds, which can detect the common welding defects in repair welds effectively, such as crack, porosity, slag inclusion, incomplete fusion, lack of penetration and so on.This paper analyzes the common welding defects appeared in repair lap fillet weld and butt longitudinal weld on pipeline, as well as their causes and distribution in the welds. On the basis of the analysis results, two different types of reference blocks are designed and manufactured with different artificial defects included respectively.According to the special type of lap fillet weld and ultrasonic beam's coverage range in the weld, an ultrasonic testing plan of multi-channel and multi-probe based on ultrasonic pulse echo is formulated. Then a series of detection tests are made on the reference block of lap fillet weld using two probes with different angles, K1 and K1.5, with the frequency is 2.5MHz and the length is 9mm respectively. The testing results show that the locations of defects inside the weld can be detected easily and the error is less than 1mm compared with the actual value. For the defects in the area of fusion lines, this paper analyzes the influence of weld surface shape on the ultrasonic testing signals, and discusses the relationship between the weld surface shape and the ultrasonic beam's propagation path, two new concepts are proposed which are effective detection acoustic beam and acoustic beam's effective detection range, as well as the computational formulas. The testing results of ultrasonic phase array verify the concepts and show that, all flat-bottomed holes in the area of fusion lines are detected successfully and the position errors are less than 0.5mm compared with the actual value. Furthermore, the blind detection areas of the reference block are identified, which will provide references for the on-site testing.According to the special type of lap fillet weld and ultrasonic beam's coverage range in the weld, an ultrasonic testing plan of ultrasonic TOFD is formulated. Then some detection tests are made on the reference block of butt longitudinal weld using a pair of TOFD probes and wedges with the frequency is 5MHz, the diameter is 6mm and the angle is 70°. The results show that all artificial defects except the near-surface defect in the weld can be detected effectively and the errors are less than 1mm compared with the actual value. For the near surface defect which is hard to identify by conventional TOFD method, a new analysis method based on the signals of shear waves diffracted from the defects' tips, called Shear-wave TOFD method. And its applicable conditions and computational formulas of sizing the defect are deduced, through which the TOFD images are reanalyzed, the results show that the errors of the near-surface drilled hole's depth and height are less than 1mm compared with the actual value, this is taken as proof that Shear-wave TOFD method is effective for the recognition of near surface defects in the longitudinal weld, and it can provide effective supplement for conventional TOFD method. |
PDF Full Text Request