Numerical Simulation Analysis Of Underwater Welding Repair

During long-term operation, the wall of oil and gas pipelines will be damaged due to vandalism or environment, so they need to be fixed. In-service welding technology can be applied to repair these pipelines in active operation. The technology is not only efficient but also promising. However, it is very difficult and dangerous to weld underwater with high-pressure flowing natural gas inside and flowing water outside. This thesis is focused on the in-service repair welding for high-pressure natural gas pipelines underwater with numerical simulation technology providing a theoretical guidance for this kind of repair in the future.In this study, the process of in-service repair welding for a16Mn pipeline underwater was simulated in SYSWELD by building pipeline repair geometry with verified forced convection heat transfer coefficient and uniform load inside and outside the pipeline and various heat source model shapes at different water depths, analyzing the effect of gas velocity inside, water velocity and water pressure outside on temperature field, stress field, the Martensite’s proportion and hardness of in-service repair welding underwater.The simulation results showed that (1) the tensile stress exists in wall while the compressive stress on the ectotheca of the pipeline. The maximum tensile stress is greater than the maximum compressive stress.(2) water velocity outside has a significant impact on the size and distribution of temperature field, residual stress, the Martensite’s proportion and hardness. When water velocity increases from Omm/s to600mm/s, temperature field, the peak temperature, the maximum stress and hardness of inner wall all decrease, while both the Martensite’s proportion and hardness of the outer wall increase. The peak of temperature field decreases from2427.68℃to1982.33℃. The maximum residual tensile stress decreases from361.9MPa to231.5MPa, the maximum residual compressive stress decreases from361.9MPa to156.9MPa, the Martensite’s proportion becomes99.5%, the maximum hardness of the outer is up to708HV, as well as the maximum hardness of the inner declines from636.6HV to494.8HV.(3) both gas velocity and water depth have an insignificant effect on temperature field, residual stress and hardness. When gas velocity increases from lm/s to15mm/s, the peak temperature of the inner surface declines from733.8℃to704℃, but the stress and hardness change in a small range. As water depth increases, the same node peak temperature of the weld center along the wall thickness direction ascents, but the residual tensile stress reduces and trends of the compressive stress are not obvious.(4) results of residual stress occurred in-service multi-pass welding repair underwater in different conditions are similar to which of single-pass welding.