Research On Nonlinear Wave Loads' Time-domain Hydroelastic Analysis Method And Application Of Ships With Large Bow Flare

The ships with large bow flare,such as ultra-large containerships,large destroyers and aircraft carriers,can encounter severe slamming phenomenon due to large relative motion between ship bow and incoming wave under high sea state.The resulting high frequency whipping response leads to the increase of the ship hull nonlinear wave loads' amplitude and the total number of cycles,which threaten the longitudinal strength of this kind of ships,had drawn large attention of maritime world.IACS uniform regulation for longitudinal strength of container ships UR S 11A considers the influence of slamming and whipping on hull structure strength partially,the classi-fication societies,such as ABS,DNVGL and CCS,put forward the class guideline or guide notes of influence on hull structure strength including slamming and whipping.However,most of these requirements or guide notes only describe the calcula-tion requirements and calculating procedure,no algorithm in details and valid calculating tools are provided.Besides,the design wave loads' detennination of ships with large bow flare in military rules is based mainly on empirical formula,which cannot fully reflect the characteristics of wave loads and structures for specific type of ship.In order to consider the influence of slamning and whipping on wave loads response and structxire safety rationally,in the design and assessment of ships with large bow flare,it is needed to research on the wave loads calculating method including slamming and whipping,and thus developing nu-merical simulating software with engineering practical computational efficiency and stability.Based on the above goals,in this thesis,the two and three dimensional hybrid nonlinear time-domain hydroelastic calculating method for ships with forward speed is proposed,and re-search on wave loads model test of large flare ships is carried out,the determination meth-od of nonlinear design wave loads on ships with large bow flare is put forward,and the struc-ture strength assessment method considering ship hull elastic effects is presented.The fol-lowing re-search work has been carried out mainly:1)Based on the classical three-dimensional structure dynamics and potential flow theory,ac-cording to the proportion of the hydrodynamic components and the advantages of the two-and three-dimensional potential flow calculation,the fundamental three-dimensional time-domain hydroelastic theory is extended,and time-domain nonlinear hydroelastic theory calculation method for a ship with forward speed is established.On the three-dimensional in-stantaneous wet surface,the incident wave pressure and the fluid static pressure after the non-linear correction are calculated.Through the relationship between the hydrodynamic coeffi-cient of the frequency domain and the time domain hydrodynamic coefficient,including the memory effect of the contribution of the different wavelets in the irregular waves,with non-linear correction of the radiation and diffraction force in any wave form,considering the prac-tical slamming and green water wave load calculation method which takes large relative mo-tion between ship bow and wave into account,the efficiency of the time simulation is im-proved by the modification of the time domain algorithm,and the lateral motion divergence under the condition of the oblique wave is controlled.Accordingly,the corresponding numer-ical calculation program is developed.The numerical verification on 4400TEU containership in Whipping Benchmark Study is carried out,and through the comparison between theoretical calculation and experimental data,the "harmonics," is discovered and the correctness and ap-plicability of nonlinear hydroelastic theory on ships with large flares is verified.Considering the sensitivity of nonlinear theoretical calculation to the parameter selection,in order to im-prove the applicability and stability,the parameter selection is discussed in details.2)The similarity relation of ship model tests considering structural dynamic similarity,the classification of the model tests are introduced in detail.Classification and comparison between domestic and overseas hydroelastic model test methods are conducted which ana-lysed the advantages and disadvantages of mainstream test methods,the segmented keel beam model with variable sections is selected,with detailed introduction of the precautions,method and steps in the design and the preparation before the test.Besides,it is described that the considerations and suggestions for multi load components measurement of container ships.With the recomnendation of the data processing algorithm in the experiment,the correctness and effectiveness of the theoretical method and the numerical calculation program are verified against the model test results,which includes the experiments in the thesis and the interna-tional public ship model test results,the comparison of the response time history and the spectrum analysis using the two and three dimensional hybrid time-domain nonlinear hydro-dynamic theory was conducted.The phenomenon of "harmonics",the variation of nonlinear wave bending moment with wave height and speed,the statistical characteristics of load am-plitude in irregular waves,and the influence of damping on high frequency bending moment are discussed.3)Basing on time-domain nonlinear hydroelastic theory calculation method,for the giv-en target ship,the design load calculation method based on nonlinear long and short term pre-diction,nonlinear design wave method and nonlinear design sea condition method are pro-posed to determine the design load.The practical method of sea condition screening and the method for determining the design of sea condition are given.The results of the three methods are compared with the rules' value and the test value,and the practical conclusions are ob-tained.4)The basic methods and principles of fatigue damage calculation are introduced,with the international research hot spots,Based on the results of three-dimensional linear frequen-cy domain hydroelastic calculation and spectral analysis,the calculation method of fatigue damage correction with the characteristics of double-peak wide spectrum is discussed in detail,considering the effect of wave excited vibration.The fatigue damage of slamming and springing&slamming is considered on the basis of time-domain nonlinear hydrodynamic theory and rain flow counting method.Based on the relevant provisions in the regulations of multiple classification societies,the effects of speed,wave direction,loading and sea condi-tion on the overall fatigue calculation results are calculated,investigated and analyzed,which can be simplified in some calculations.The conclusions and die calculation suggestions ob-tained by the comparison of the direct calculation results and rules can provide theoretical and technical support for the fatigue strength evaluation of large container ships.