Trusts and Beliefs in UT of Girth Welds

摘要

In the German pipeline industry ultrasonic testing (UT) has been and still is the method favoured over radiographic testing (RT) for the inspection of girth welds. However, in the internationally widely applied Standard API 1104 (Welding of Pipelines and Related Facilities) till the nineteenth edition, September 1999, UT was of secondary importance when compared to RT. The NDT requirements in API 1104 are contrary to the NDT requirements in API 5L (Line Pipes), which are applied in pipe mills for the longitudinal and spiral welds, specifically for the continuous and uninterrupted inspection of welds by ultrasonic means (Automatic respectively mechanized UT). Common line pipe specifications like API 5L and EN 10208 (Steel Pipes for pipe lines for combustible fluids) include clear definitions about the reference standards used and define clearly the acceptance criteria for the applied ultrasonic inspection. As long as the NDT of girth welds is based on European Standards, the testing parameters including reference criteria and acceptance criteria for the ultrasonic inspection are also clearly defined. Depending on the quality requirements of the pipeline girth welds, a quality level, a testing class and an acceptance level are to be specified and on that basis reliable and reproducible UT results can be maintained. If UT is to be applied in accordance with API 1104 a qualification of the testing procedure is required prior to application. Basis for the acceptance of UT is the capability of the UT technique to circumferentially and axially locate imperfections, size for length, determine the distance between pipe surface and the imperfection and to compare the results with RT on defects and acceptable imperfections. In addition, the UT procedure must accurately determine the acceptability of welds based on the imperfection type, length and distribution, as cracks, linear surface, linear buried, transverse, volumetric cluster, volumetric individual, volumetric root and aggregate lengths. Many papers and inspection procedures reflect much belief in the UT capabilities to determine imperfection types and to locate and to size imperfections. From comparison reports on purposely made weld imperfections and from SGS in-house research work it can be shown to what extent UT results are trustable when inspecting girth welds in pipes made from thermo mechanically rolled plates.