Researches On The Residual Stresses Of Typical Welds Of Uprights And Lower Buoys In Semi-submersible Offshore Platforms

With the rapid development of the world economy,the demand for oil and natural gas is gradually increasing.The oil and gas resources on land and offshore are far from meeting all demands.Because of its unique advantages,semi-submersible offshore platforms have become the best choice for exploration and exploitation of deep-sea oil and gas resources.The structure of semi-submersible offshore platforms is mostly welded by high-strength steel and ultra-high-strength steel.This paper is based on the theories of thermal-elastic-plastic correlation and fatigue strength calculation methods,and the structure of flat-plate butt joints and different types of groove.The T-shaped structure was simulated by welding,and the effects of welding procedures such as different welding sequences on the welding residual stress were studied.Finally,the fatigue strength of the T-shaped structures with four different groove types was analyzed.The main research work and related conclusions are as follows:(1)The widely-used flat butt welding on the semi-submersible offshore platform pillars and the lower floating body was studied.The post-weld residual stresses of different welding sequences,different sizes of plates and different constraints were taken into consideration,respectively,and the transverse residual stress was emphasized.The analysis shows that the change of the welding sequence of the segmented welding does affect the peak value of the residual stress of the welding seam,and the welding sequence of the segmented welding in the middle of the first welding and the middle of the welding later can obviously increase the residual of the entire plate.In the low stress region,even the original tensile stress can be converted into compressive stress;the increase in the width of the plate can reduce the transverse residual stress of the plate,and the greater the width,the more significant the reduction;reducing the constraint control will reduce the residual stress after welding,for the transverse direction For the residual stress,the reduction effect of X-direction constraint is most obvious.(2)The T-shaped structures of three different groove types(fillet weld,groove fillet weld,and full penetration)were numerically simulated for the welding temperature field and stress field,and the distribution of the initial welding temperature field and stress field was obtained.According to the law,the results show that the T-shaped structure adopts different types of groove welding,the cooling rate after welding is different,the cooling rate of the fillet weld structure is the fastest,the groove fillet weld is the second,and the structure of the full penetration is the slowest And the cooling rate of the structure in the initial stage is much greater than that in the later stage.In the comparison of post-weld residual stress peaks,the residual stress after full penetration structure welding is the largest,the groove fillet weld is the second,and the fillet weld is the smallest.(3)The welding residual stress of the T structure with elbow plate was simulated,and the influence of welding sequence change on residual stress after welding was explored.The results show that the change of welding sequence not only can affect the peak value of the structure,but also can affect the stress concentration of the structure.The change of welding sequence has different effects on the stress in the three directions of xyz,and there is no relationship between the increase and decrease.The equivalent post-weld residual stress obtained by using the same welding sequence for all weld bead on the weld surface is the largest.(4)The fatigue strength of the T-shaped structures with four different groove types,including fillet weld,groove fillet weld,full penetration,and full penetration with C-grinding,was analyzed to investigate the effect of the groove form on the fatigue life.The results show that the fatigue life of the fillet weld structure is obviously lower than that of the other three structures regardless of whether the fillet weld is loaded on the web or loaded on the panel.The groove fillet welding structure has the smallest stress peak value and the longest fatigue life when loaded on the web.Although it is inferior to the full penetration with C-grinding when the panel is loaded,it is not much different from the full penetration.The C-grinding structure after the full penetration is inferior to the full-penetration structure as compared to the pure full penetration structure.It is inferior to the full penetration safety when the web is loaded,but it shows a great advantage when the panel is loaded.All the studies carried out in this paper are to pave the way to solve the fatigue problems of the complex nodes of the semi submersible offshore platform and provide technical support for the complex structure design of the new generation of offshore platforms.