Study On The Effect Of Asymmetric Load Considering Slamming On The Fatigue Strength Of Super Large Container Ship

Due to the characteristics of large opening,the super large container ship is often different from other ships in the verification of total longitudinal strength and fatigue strength.Compared with the total longitudinal strength,its horizontal and torsional strength are often weak.Therefore,in the calculation of hydroelastic response under wave action,not only the vertical bending strength,but also the horizontal and torsional strength should be considered.Especially when the natural frequency of the hull itself is close to the frequency of the wave,this strong hydroelastic response is more obvious.In bad sea conditions,super large container ships often have heeling angle or transverse velocity.At this time,when they are hit by water slamming,air classification and air circulation are very easy to occur.In addition,when the ship is actually sailing in the ocean,the heading angle is not always in the same direction.If it is an oblique wave,because there is a certain angle between the wave direction angle and the sailing direction of the ship,the load received by the ship is easy to be unevenly distributed and the load is asymmetric due to the action of the wave.If there is slamming load under high sea conditions,the asymmetric load caused at this time contains slamming component.The high-frequency asymmetric load will further increase the torsional flutter component of the hull structure and seriously reduce the fatigue life of the ship.Therefore,this paper studies the impact of the asymmetric load considering slamming on the fatigue damage of super large container ships,The main work completed is as follows:1.A suitable bay section is selected to combine the section load and three consecutive bay sections,thus simplifying the model of the whole ship.When calculating the example ship,the rules of the wave amplitude under the incidence angles are obeyed.The hazardous sea conditions below each wave are selected for stress analysis.The stress values of calculating points and their nearby joints are compared for the whole ship model and the partial model,thus verifying the partial model’s feasibility to simplify the whole ship model.2.The first three vertical modes and bending torsional coupling modes of the example ship are calculated by finite element method and transfer matrix method,and the errors and advantages and disadvantages of the two methods are analyzed.3.Based on the three-dimensional hydroelastic theory,the symmetrical load caused by slamming in waves: the bending moment time history of vertical section,and the asymmetric load caused by slamming in oblique waves: the horizontal bending moment time history and torque time history are calculated.The amplitude and frequency of several loads are analyzed to further illustrate the importance of asymmetric loads caused by slamming.4.Based on S-N curve and linear cumulative damage theory,the fatigue of hot spots at the corners of super large container ships is checked,and the stress response transfer function and fatigue cumulative damage degree results of fatigue hot spots under different working conditions are obtained.5.Based on the direct method of finite element transient response analysis method,the hot spots are calculated under the action of symmetrical load and asymmetric load of oblique wave during slamming,Calculation of stress response time history within 1800 s.According to the rain flow counting method,the damage corresponding to each hot spot is calculated.