Accumulated Fatigue Damage Research On Hull Structure Considering Springing Effect

With the improvement of modern ship-building technology,the service life of ships is gradually prolonged,which exposed the seriousness of structure fatigue problem.Spectral Fatigue Analysis,which could take sea states in whole fatigue life cycle into consideration,is widely used in fatigue assessment of naval architecture and ocean engineering.However,each sea state would be calculated in spectral analysis,which consumes large amount of computation and brings many inconveniences.On the other hand,due to the increase of ship dimensions and application of high strength steel,structure stiffness is too low to get resonance with encounter wave frequency.High frequency stress cycles induced by springing effect occurred in structure response,which aggravated fatigue problem and its evaluation.In this paper,a cumulative fatigue damage method based on equivalent design wave was proposed,which took springing effect into the consideration,aiming at an efficiency and reliable approach for ship fatigue life assessment.The main contents are concluded as following:(1)Based on spectral fatigue analysis,the response of six hotspots on a 260-meterlong container ship was evaluated by response amplitude operator,and their accumulative fatigue damages under the sea states of voyage cycle were obtained.Through the comparison between low-frequency wave bending moment and composite wave bending moment,the influence of springing effects on vertical bending moment and fatigue damage were discussed.(2)Elastic-segment model test in towing tank was carried out.A new segment divide approach based on vertical mode shape was proposed and the results showed that this approach could provide a new line for segment model test;Simulation forecasting results was compared with test to analyze difference and the characteristic of springing effect.(3)A accumulative fatigue damage method based on equivalent design wave was proposed.After the verification of correlation between other wave parameters(wave direction and wave frequency)and fatigue damage,the wave frequency was designed as the linear springing effect occurred,the wave direction was designed same as test: heading and the design wave amplitude was revised to ameliorated the difference between simulation and test.By comparing the six hotspots' accumulative fatigue damage between spectral method,equivalent design wave method and test,the applicability of this approach was discussed.