Failure cause of bi-metal composite pipe of Xinjiang No.2 Keshen Gasfield

The bi-metal composite pipe used at Xinjiang No.2 Keshen Gasfield, exhibited cracking of welded joints, delayed crack, pressure washout, post-commissioning washout and other problems during service, causing significant economic losses. A thorough investigation into the various causes of the composite pipe failure was conducted, and laboratory experiments as well as theoretical analyses were carried out. It is considered that the direct cause of this instance of composite pipe welding failure was the dilution of the elements Ni and Cr in the weld seams, which led to excessive hardness of the cap in the filling layer, a deterioration of ductility, and a weakening of its capacity to prevent cracking. After winter construction, some of the pipe sections with elastic laying in river crossing experienced contraction, which led to areas of increased hardness on the welded joints experiencing greater bending stress, thus causing composite pipe failure. Based on the analysis results, the authors suggest a new welding technique to solve the problem of composite pipe failure, and put forward suggestions regarding composite pipe design and manufacture, welding techniques, and on-site construction.