The Effect Of Welding Residual Stress On The Fatigue Life Of LNG Liquid Cargo Tank Bulkhead

LNG (Liquefied Natural Gas) is one kind of high quality and clean energy with a large reservation, and the development and application of LNG are becoming more and more important in industries. So the building of LNG carrier attracts more and more attention. One of the key technologies of membrane LNG tank is the welding of Invar steel, and the reliability of the welding beam has a great effect on the safety of the structure.The paper simulated the temperature and residual stress field of TIG welding of Invar steel with MSC.Marc FEM software. The result of the temperature field showed that the heat source model is similar to double ellipsoid heat flux. The temperature of the heat resource center is 1619℃. The residual stress field mainly distributed in the welding seam area and nearby. The maximum value of equivalent residual stress is 258Mpa, a little lower than the material yield limit 281Mpa, and the longitudinal residual stress is up to 335Mpa.Subroutine of gauss rotation body heat source is used to simulate laser flux, and used Visual Fortran6.6 programmed the secondary development of flux. The simulation results showed that equivalent residual stresses caused by Laser/TIG welding methods are 295Mpa and 258Mpa respectively. However, longitudinal residual stresses caused by Laser/TIG welding methods are 295Mpa and 334Mpa respectively. Longitudinal residual stresses caused laser welding is 39Mpa less than TIG welding. At the same time, the welding residual deformation caused by laser welding is 71.1% less than TIG welding.The liquid sloshing load on the bulkhead of LNG carrier calculated with the application of CCS related rules, and studied the fatigue life of the bulkhead under the condition of the coupling effect of the welding residual stress and sloshing load. The result indicates the fatigue life of Invar steel bulkhead structure decreased 77.6% with the processing of TIG welding. At the same time, the fatigue life increased 72.3% with the using of laser welding compared with TIG welding.