Study On The Influence Of Welding Residual Stress Towards Fatigue Life Of The Key Joint On Semi-submersible Platform

With the development of welding technology and the improvement ofconstruction standard，high-strength steels are becoming more and more commonused on the key joints of offshore engineering structure. The usage of high-strengthsteels may have benefits on increasing structural strength. But the weldingdeformation and residual stress may on the contrary become more serious for thereason that the elasticity modulus of high-strength steels haven’t increased.Especially，the existence of welding residual stresses could have lots of impact tofatigue life. So it is significant to study the influence of welding residual stresstowards fatigue life.A typical semi-submersible offshore platform structure has been taken intoconsideration in the paper. Starting from the mechanism of welding residual stressproduction, this paper have done lots of work to analyze the distribution laws ofwelding residual stress and its influence to the fatigue life by using both finite elementmethod and test. In this paper, the typical loading conditions have been found after thehydrodynamic analysis, whose model was built by ANSYS software. Then thestructural model would be built to analyze the overall strength of the platform. Thefatigue hot spots confirmed. At last, this paper comes to some meaningful conclusionby the contrastive analysis of the impact of welding residual stress to the fatigue life.The major works of this paper summarized as following:(1) Carried out the study on the influence mechanism of welding residual stressto fatigue life based on its production mechanism. The analysis method and procedurehave been built up.(2) Carried out the study on welding residual stress based on the typical T-weldstructure by using ANSYS FEM analysis. The feature and distribution law of T-weldstructure were confirmed. Residual stresses would be taken into account aspre-stresses. Some T-weld specimens have been designed to conduct fatigue test.Further discussion of the impact of residual stress to fatigue life would come out bythe contrastive analysis of test and FEM results.(3) Built the hydrodynamic model and predicted the wave loads of the platformby using design wave theory. Then analyze the overall strength of semi-submersible platform under survival condition in which4typical dangerous wave loads wereconsidered. Then determine the high stress region as the key joint for later calculation.(4) Extracted the hot spot model from the structural model and refined the meshof the key joint with the refinement techniques. Then obtained its fatigue life based onS-N curve method and accumulated fatigue damage theory. Take the hot spot model asresearch object and study its welding residual stress distribution law. Put weldingresidual stress onto the model as pre-stress and calculated its fatigue life using S-Ncurve method. Discuss the influence of welding residual stress to the welded structure.