Investigation Of The Effect Of Springing And Whipping Responses On The Fatigue Strength Of Ultra Large Container Ships

In recent years,in order to reduce the cost of maritime transportation and satisfy the energy conservation and environment protection requirements,the size of container ships is becoming larger and larger.The increase of scale and deadweight has resulted in significant decrease in the natural frequency of ultra large container ships.Meanwhile,due to the high speed of container ships and the large bow flares,the bow structures may suffer severe slamming forces.The springing and whipping may therefore occur frequently.Both the springing and whipping responses increase the fatigue loading significantly and have a great effect on the fatigue strength of ultra large container ships.In this thesis,the numerical methods for predicting the springing and whipping responses of ultra large container ships are investigated,and a method for assessing the effect of springing and whipping on the fatigue strength of ultra large container ships is established.The research contents are as follows:(1)According to the thin-walled beam theory,the section bending and torsiona property parameters accounting for warping are calculated;Dry vertical and coupled horizontal and torsional modes are analyzed by the transfer matrix method,and the results are compared with those obtained with a commercial finite element code.An automatic mesh generation technique for elastic hulls with the modal displacements is presented.(2)The research on the method for predicting the springing and whipping load of ultra large container ships is carried out.Sectional loads including springing of ultra large container ships are calculated by a three-dimensional linear hydroelasticity method in the frequency domain.In order to avoid the complex time-domain hydrodynamic analysis,a simplified and effective method is used to predict the whipping load.The motions and sectional loads in the frequency domain are transformed to the time domain,and the bowflare slamming force is taking into account,the nonlinear forced vibration equation of the hull in the time domain is established.Then,the sectional loads including whipping are calculated by a modal superposition method.(3)Based on the springing and whipping load predication method,the fatigue damage calculation method accounting for the springing and whipping responses is investigated.Taking into account the springing-induced bimodal stress spectra,a theoretical model of spectrum-based fatigue analysis for springing is proposed and verified.The fatigue damage including whipping is calculated by the rain flow counting method combined with the cumulative damage theory.Based on the results of wave-induce fatigue damage including springing and whipping responses,a correction factor of the wave-induced stress range of ship hull girder is obtained,and a method for assessing the effect of springing and whipping on the fatigue strength of ultra large container ships is established.Then,a comparative analysis of ABS,CCS and DNVGL rules about fatigue assessment including springing and whipping is carried out.(4)The method established above is applied to study the fatigue damage due to springing and whipping of a 9200 TEU container ship.A thorough analysis of the effects of speed,load condition,wave heading and sea state etc.is carried out to provide criteria for determining the calculation parameters in fatigue assessment accounting for springing and whipping.Finally,by combining the correction factor with the fatigue assessment rule,the fatigue damage of the midship longitudinal is calculated based on simplified stress analysis method.