Examination of Laminations in the Base Material on a Section of High Pressure Gas Supply Pipeline

Abstract

During the installation works of high pressure gas supply pipeline (working pressure 54 bar, pipe diameter Ø508 mm, wall thickness 6,4/7,9/9,53 mm, material X60M according to API5L, PSL2) at the performance of the in-field welding, appeared indications that in the base material of the pipe exist laminations. They are one of the most severe defects that cannot be tolerated in this kind of structures. That fact imposed the need for additional investigations in order to confirm and evaluate the severity of these defects before performing the 81 bar hydro test. Considering the fact that the entire section of the pipeline (cca 36 km) was already installed and buried under the ground, UT inspection of the section had to be done with special equipment that was moving inside the pipeline filled with water. Only by this manner it can be achieved to have inspection of the entire internal surface of the pipeline, in its full length of cca 36 km. With this inspection all the defects in the pipe wall were discovered at their exact location. On particular critical locations, to find out the exact dimensions and characteristics of the imperfections, additional examinations were performed with NDT methods. These findings in the critical locations of the entire 36 km pipeline obliged to be performed reparations. This paper covers the categorization of the defects on the critical locations (discovered by the UT examination and additional NDT testing with classical methods), the specifications and causes of their appearance. The proposed reparations on the entire gas supply section, due to these defects, are presented too.