FULL-SIZE FOUR-POINT-BEND TESTING - A NEW APPROACH TO EVALUATE THE SSC RESISTANCE OF LARGE-DIAMETER PIPES FOR SOUR SERVICE

摘要

For investigation of the resistance of submerged arc welded (SAW) large-diameter pipes for application in sour service, a new full-size four-point-bend test method has been developed for performance of SSC tests on specimens of full pipe wall thickness. 45 specimen holders, designed for stressing specimens containing the original pipe curvature with a load up to 4 tons were machined. Specimens of 260 mm length, 25 mm width and up to 33 mm thickness were tested in solution A to NACE TM0177-96 at 0.1 MPa H_2S and exposure times of 96 and 720 hours, respectively. The specimens had been machined from the pipes in 12 o'clock position and contained the original inner and outer pipe surfaces including the longitudinal welds. The specimens were loaded to stress levels between 72 percent of the specified minimum yield strength and 110 percent of the actual yield strength of the grade X65 pipe material with the inner surface in tension. Load application was monitored by strain gauges which were fixed close to the notch between the weld metal and the base metal. The investigations revealed that the pipe material was not susceptible to SSC under the present test conditions up to a load of 100 percent of actual yield strength of the base metal. The results were confirmed by full scale H_2S testing of two girth-welded pipe sections under internal pressure equivalent to a hoop stress of 72 percent of the specified minimum yield strength of the same pipe material. The full-size four-point-bend test proved to be a useful tool to evaluate the susceptibility of large-diameter pipes to SSC, particularly in ease the weld area has to be tested. In contrast to four-point-bend testing of large-diameter pipe material on flat specimens, the new test method considers the real pipe geometry and a stress distribution close to service conditions.