Research On Fatigue Strength Assessment Of Very Large Containership Considering Springing And Whipping Effects

In recent years in order to improve economy of large container ship,and satisfy the increasingly stringent requirements for energy saving and environmental protection,the design of container ships is developing in the direction of large scale,the increased ship size,big flare and the high speed make the hydroelastic effects more significant.Springing and whipping can significantly increase the fatigue damage of the ship,but at present in the assessment procesures of the structural load and fatigue strength,the springing and whipping effects are rarely considered.This thesis deals with the relevant research based on a large container ship,the calculation procedures of springing and whipping were discussed detailedly,and the practical method to meet the engineering requirements was summarized.The ship is suffering continuous wave loads when sailing in waves,and the structure is subject to dynamic response as a flexible hull.The seakeeping response was computed with a 3-D Rankine spring method and a strip theory method,in the time domain calculation the nonlinear loads and transient response were included considering the nonlinear Froude-Krylov force and the hydroelastic response,the reflection and radiation force stayed linear,which were computed in frequency domain.The whipping transient response was obtained from hydroelasticity calculation program,the springing response was calculated with a strip theory method,in the process the symmetric response in head seas and the asymmetric in oblique seas were considered.Considering the computational efficiency and reliability,slamming calculation is completed by two reliable 2-D method.The first is Generalised Wagner Model(GWM),the other is Modified Logvinovich Model(MLM),and the latter method is widely used because of the high computational efficiency.In every time step,slamming force is computed based on the relative motions,so in the motion calculation process the slamming force must be considered.The global ship hull response could be obtained from modal analysis and seakeeping calculation,which is completed by transferring results computed from the hydrodynamic model onto the structure FEM model,and the local structure response can be analyzed by the FEM,with these procedures the stresses on the nodes can be obtained directly from fine finite element model,so as to provide valid data to complete the strength and fatigue analysis.In the natural vibration calculation process through 3-D FEM,the ship is simulated as free floating state in the water,the nodes are not constrained,the boundary condition is free,in the wet vibration mode calculation the added mass should be considered,the added water mass was computed automatically through definition of the wet elements and the draught level,the basic principle is solving the fluid motion Laplace equation.The fatigue calculation consists two parts,the first part is simplified calculation using ABS rules including slamming,whipping and springing calculations.In order to definite the contribution to the fatigue damage,firstly,the ship should be regarded as a rigid body to calculate the bending moment and stress response,the two node vibration was not considered,secondly,the total bending moment and stress response were calculated considering the two node vibration,thus the total fatigue damage was obtained,at last the springing and whipping contribution to fatigue was identified.In the second part the direct calculation method was applied,at first the springing effect was not considered,the local structure fatigue damage was calculated using spectral analysis method and design wave method,in this step the linear wave loads were computed and transferred to FEM,from which the stresses responses were obtained directly,by mean of calculation in unite regular waves to get the stress transferred function,according to the wave spectral possibility distribution to calculate the fatigue damage.Design wave is a simplified method of spectral analysis,it's only need to define a control load at every heading angle,when this load becomes the maximum value the wave height and period is defined as the design wave,based on which the fatigue damage can be calculated,this method is more convenient and faster than spectral analysis method.When the fatigue damage is calculated considering the springing effect,spectral analysis method can be applied in the same way,in every short term process the springing and whipping are calculated to get the time history of loads,the effective stress cycles are counted by rain flow accounting method,in accordance with S-N curves the cumulative fatigue damage degree is calculated,at last compared with the fatigue damage without considering springing effect,the springing contribution to total fatigue damage can be obtained finally.Through the above procesures,the fatigue calculation of a very large containership was completed,the results show that the fatigue damage is serious in the ballast voyage due to low draught,and springing and whipping effects cause the fatigue damage increase further.According to North Atlantic wave spectral the increase is more than 40%in ballast voyage,and to global wave sprectral the increase is more than 30%,the combination increase is more than 20%too,and the in full load voyage the increase to fatigue damage is about 13%.For large containership the contribution of wave induced vibration to fatigue damage is very serious,in the assessment of structure strength and fatigue damage this effect should be considered carefully.