Research On The Vertical Bending Moment And Its Effect On The Fatigue Strength Of Wide-flat Structures

With the fast development of ocean resources,large and very large floating structures are more and more popular for most near sea countries;meanwhile,because of the transportation demands,some vessels sailing from river to sea,or in the shallow channel among islands or coastal,have the trends to be large-scale but their drafts are limited.These structures are commonly of wide-flat feature,which lead to lower vertical stiffness,and for some special ships,their molded lines of stems and sterns may vary severer than normal ones.Therefore,wave loads suffered by these structures may have their own characteristics.In addition,sailing under some harsh sea conditions,with the vary wave excitation,wave-induced vibration like springing and whipping may occur frequently.Under the combined effect of these factors,the stress ranges and their cycle numbers suffered by wide-flat structures may increase significantly,and thus have a greater impact on fatigue damage.In this thesis,a wide-flat vessel is taken as example to make research on the wave-induced vertical bending moment(referred to as VBM)in mid-ship section,and the fatigue damage caused by them,besides the influence of factors like speed and high frequency vibrations on the fatigue damage was discussed.The main contents are concluded as following:(1)Based on three-dimension(3D)potential theory,the VBM on the mid-ship section under different regular waves and different speeds were evaluated with frequency response function;the time histories of VBM for the wide-flat vessel under selected random sea conditions were obtained.Results show that the influence of speed effect causes the VBM increase up to 1.37 times than that of zero speed,while the nonlinearity effect may lead to a 5% to 10% increase in VBM.(2)The wet and dry modes of the vessel were calculated respectively by the finite element method;then,based on the 3D hydroelastic theory,the frequency response functions of VBM considering the hydroelastic effects were acquired;finally,the VBM results of the 3D potential theory,3D hydroelastic theory and segmented model tests were compared.From the comparison,it can be seen that there is a significant difference among the three results of VBM due to the easily occurred whipping and nonlinear springing.(3)Hot spot stress concentration factor(SCF)of one selected typical joint on the mid-ship section was calculated by the finite element method.According to the beam theory,VBM time histories,which obtained from simulation and segmented model tests,are transferred into hot spot stress time histories with consideration of the calculated SCF;based on linear cumulative damage rule,using the results of rain flow counting and S-N curve,hourly cumulative fatigue damage caused by the VBM were obtained for different sea states.Results show that fatigue damage can increase by 3 times when speeds rise from 12 kn to 18kn;considering the nonlinearity,fatigue damage can increase by 10~25%,while it can increase amazingly by more than 30 times when considering the high frequency vibrations,.