Oil-Pipe Cracking and Fitness-for-Service Assessment

A new D457 x 7.1 X65ERW oil pipeline leaked twice during the pressure test. The two failed oil pipes belonged to the same batch of products from the same manufacturer. Failure analysis showed that the cracks in the 1# and 2# cracking pipes propagated from the outer surface of the pipe wall to the inner surface along the weld fusion line. The inclusion phase distributed in chains in the weld zone is speculated to be the root cause of the cracking in the pressure test. The fitness-for-service of this batch of steel pipes was analyzed using the failure-assessment chart technique. The batch of steel pipes could be used safely under the design pressure of 10 MPa, and the re-evaluation period of three years was recommended for the pipeline based on the fatigue results of the steel-pipe weld zone.