Ultra Large Container Ships' Dynamic Response Analysis Under Springing Coupling Horisontal And Torsional Vibration And The Effect On The Fatigue Damage

In recent years,the competition in the world shipping industry has become increasingly fierce,and container ships have continued to develop at a high speed and large scale.The resulting hull stiffness reduction results in a significant springing effect.At the same time,due to the large opening characteristics of the deck,horizontal bending and torsional coupled vibration are prone to occur,and the fatigue strength of the position such as the hatch angle has become a hot issue in the industry.Previous methods by rigid body and simplified beam theory to calculate hull wave load and structural response have not been able to meet the needs of such problems.Therefore,for the ultra large container ships,it is necessary to establish a set of bends to check the fatigue strength considering the horizontal bending and torsional coupled vibration.In this paper,the structural response calculation methods under horizontal-torsional coupled elastic vibrations and the effects on fatigue strength will be explored.Firstly in this paper,the modal analysis is quickly calculated based on the transfer matrix method with the horizontal bending and torsional coupled modes,and is compared with the modal modes results by finite element method.It proves that the transfer matrix modal modes analysis program can improve the computational efficiency with the reliable results.Then,the springing wave loads of a 21000 TEU container ship is carried out based on the three-dimensional linear hydroelastic theory.In order to check the fatigue strength of ultra large container ships,this paper carries out the stress RAO through the finite element quasi-static response analysis based on the spectral analysis method,respectively by rigid body and springing loading.And then estimates the ships' fatigue damage of typical structures such as the bulk corners,so as to explore the impact of springing on the ultra-large container ships fatigue strength by the comparative analysis of the two methods.Through the research and comparison,it is found that the fatigue damage caused by the springing response of the quasi-static method is significantly larger than the rigid body theory,and the fatigue life is reduced by about 10%to 40%,even as high as 45%.and the contribution to the fatigue strength of the springing effect cannot be ignored.Further,with reference to the rules of Chinese Classification Society,the ships' fatigue strength considering only the vertical vibration effect is calculated,and the overall springing response' contribution to the fatigue is compared.Finally,the contribution of bending-torsional coupling component to the fatigue strength in springing response is obtained,and it can be predicted that it is necessary to consider the horizontal bending and torsional coupled vibration.For a linear dynamic system such as the target ship,in fact,the stress response will have a dynamic magnification effect relative to the static result when considering the springing effect.In order to fully explore the reliability of the method,this paper uses the frequency response analysis method to analyze the dynamic response of the target ship.By comparing with the results of the quasi-static method,it is found that in the wave frequency range,the stress results of the dynamic analysis show two distinct resonance magnification regions,and the dynamic magnification factor is about 2-6 with different wave angles.Further comparing the structural fatigue damage and fatigue life of the two stress analysis methods,it is found that for the ultra-large container ship,the resonance magnification area is still in the range of wave frequency with high energy due to the lower frequency of the first-order torsional vibration.It shows that the fatigue life by frequency response analysis method is reduced by about 20%-40%compared with the quasi-static method.It can be seen that,when checking the fatigue strength of ultra-large container ships,it is necessary to consider the springing effect with horizontal bending and torsional coupled vibration,and when considering the amplification effect of the dynamic response,the fatigue damage is obviously increased.