The Effect Of Weld On The "Elephant Foot" Buckling Behavior Of Large Oil Tanks

"Elephant foot " buckling is one of typical damage of oil tanks under earthquake load, which is the axial compressive instability in essence. Axial compression buckling is very sensitive to initial geometric imperfections. Following the uniform strength concept in desiging, large oil tanks are generally made by welding multi-layer cylindrical shells together, so welding residual stress and residual deformation are produced inevitably. The welding residual stress can be regarded as initial geometric imperfections in cylindrical shells, which will reduce the classic critical buckling load to some extent. Therefore, it is very significant to make research of the effect of weld on the buckling behavior of large oil tanks. The main work in thestudy is concluded as follows:1. The theoretical calculation of buckling safety is carried on 20×10~4 m~3 oil tank under 100% water-filled condition. The result shows that the tank will not occur "elephant foot" buckling during seismic intensety 6 to 9. In numerical simulation of the tank, quasi-static numerical simulation method and arc-length method are adopted to obtain the "elephant foot" buckling stress during different seismic intensity.2. To get the residual stress distribution feature, inherent strain method is adopted to simulate the process of cylinder welding. The consistent of simulation results and measured results verify the accuracy and feasibility of inherent strain method. The different effect of uniform distribution and trapezoid distribution of the inherent strain on the axial critical buckling load has been compared. The result shows that the uniform distribution is safer to simulate welding process.3. To reveal the effect of different factors of weld, such as welding residual stress and deformation, weld reinforcement, the number and the location of weld on the buckling stress, series of cylindrical shells with circumferential weld, longitudinal weld and combined weld have been simulated.4. Quasi-static calculation model with the considerateon of welding residual stress and deformation has been established in "elephant foot" buckling analysis. The main influencing factors of buckling stress are analyzed, such as weld types, welding residual stress and deformation as well as welding sequence. The results show that the buckling stresss is reduced because of the welding influence, and the combined welding style has the greatest effect with the same inherent strain. With the increasing of welding residual stress and deformation, the buckling stresss is also reduced. The welding sequence has little effect on the buckling load, but increases the initial location of buckling.