Research on root appearance and fatigue life of steel catenary riser (SCR) using GMAW/GTAW-P double-sided root welding process

A steel catenary riser (SCR) is a flexible stand-up pipe that connects a subsea wellhead in the seabed (normally at a depth of more than 1500 m) to a floating platform. Waves, ocean currents, and winds apply substantial loads to SCRs that produce fatigue damage. Hence, the requirements for the appearance of SCR weld joints, particularly the weld root, are extremely strict. This paper proposes a new gas metal arc welding/pulse gas tungsten arc welding (GMAW/GTAW-P) double-sided root welding process for SCRs to obtain a nearly smooth root geometry. Test results show that the average values of root reinforcement, root transition angle, and transition radius achieved with this root welding process are only 169 mu m, 6.4 degrees, and 4055 mu m, respectively. These values are dramatically lower than those achievable with the best technologies available, such as the surface tension transfer (STT) root welding process and cold metal transfer (CMT) root welding process. The fatigue strength of the double-sided welding joints is improved by at least 45.8% because the root stress concentration is signifi-cantly reduced. However, incomplete penetration defects were found when the molten pool position was not appropriate. Solutions for this problem are discussed. The proposed process is believed to be a potential strategy for welding high fatigue life SCRs.

Automated summary

This paper proposes a new double-sided root welding process for SCRs, which can achieve a nearly smooth root geometry. Test results show that the average values of root reinforcement, root transition angle, and transition radius achieved with this process are dramatically lower than those achievable with the best technologies available. The fatigue strength of the double-sided welding joints is improved by at least 45.8% due to reduced root stress concentration. However, incomplete penetration defects may occur when the molten pool position is not appropriate. Solutions for this problem are discussed. The proposed process is believed to be a potential strategy for welding high fatigue life SCRs.