Investigation On Ship Motion Response Characteristics In Irregular Waves

Waves are the main external loads experienced by ships when they are sailing at sea,which directly affect the navigation status and working performance of ships and marine structures at sea.The motion response of ships in waves has always been a hot issue in ship hydrodynamics and experimental research.For a long time,people have used regular waves to study the seakeeping issues of ships.However,the seakeeping problem of ships is a dynamics problem.The method of simplifying the random waves of real sea conditions to regular waves is simplifying the wave spectrum with a certain frequency distribution to the Dirac function spectrum,ignoring a large amount of dynamic effects.Therefore,it is necessary to use irregular wave theory to simulate to more accurately describe the ship’s motion response in real sea conditions.Predecessors have done a lot of work on the simulation of irregular waves,but most of the current research is limited to the simulation of irregular waves,most of which only give the ship motion duration curve,and there is a lack of in-depth research on the statistical law of ship motion under the action of irregular waves.In fact,the statistical law of ship motion response is very important to predict the seakeeping of actual ships.Based on the above background,this paper will use numerical methods to study the ship’s motion response under irregular waves.Firstly,based on potential flow theory and boundary element method,the motion responses of Wigley ship in irregular waves are investigated systematically by Hydro Star.The distribution of the amplitude and response period of random motion in irregular waves is discussed by statistical method,the spectra of ship motion are also obtained by spectral calculation.Under the action of irregular waves,the amplitude of ship motion presents a state of irregular wave motion,and the probability density of its motion time history curve conforms to the Gaussian distribution.The numerical results indicate that there are significant differences of the response features between the random motion in the rolling direction,heaving direction and pitching direction.For heave and pitch motion,because the peak frequency of wave spectrum is far away from the natural frequency,the dispersion of the period corresponding to large-amplitude motion is small,which is basically stable near the peak period of the wave spectrum.While the dispersion of the period distribution corresponding to small-amplitude motion is large.And Fourier analysis indicates that the response frequency of heave and pitch motion has obvious peak values near the peak frequency of wave spectrum.For rolling motion,since the peak frequency of wave spectrum is coupled with the natural frequency,the response period of rolling motion with different amplitudes is stable near the natural vibration period and the period dispersion is small.The spectrum analysis also shows that the response spectrum of rolling motion is mainly concentrated near the ship’s natural frequency.Considering the influence of fluid viscosity and wave nonlinearity,using the finite volume method and the VOF free surface capture method,a numerical wave flume is constructed in Open FOAM,an open source platform.The correctness of the numerical model is verified by simulating regular waves and the motion response of structures under regular waves.Based on the long-crested irregular wave theory,the more accurate JONSWAP spectrum and white noise spectrum of irregular waves are simulated by the method of linear wave superposition.The generated irregular wave was applied in the three-dimensional wave flume to simulate the heave and pitch motion of the ship.The statistical method was also used to analyze the time-domain motion response characteristics of the ship in the viscous flow.The results show that heave and pitch motion still have small difference of large-amplitude motion period and large dispersion of small-amplitude motion period,which indicates that potential flow model can predict the general law of ship motion under the action of irregular waves.Further comparing the significant amplitude of ship motion with the motion response spectrum between the potential flow results and the viscous flow results,it is found that the motion amplitude was over-predicted by the potential flow method than the viscous results,and the dispersion of the response period is also larger,indicating the influence of fluid viscosity will not only affect the amplitude of ship motion under the action of irregular waves,but also affect the response period.