The Research On Predicting Method On Ship Nonlinear Stochastic Motion Response In Longitude Wave

The analysis and research on ship motion response and control has always been a frontier and hot topic in the field of ship engineering.It is of great significance to ensure the ship safety when the ship navigats in the sea.The ship motion response in waves is closely related to the stability and whether it is capsized.So far,a large number of in-depth research have been obtained on the stability of ships in crosswind and beam waves.However,through the observation and statistics of complete shipwreck accidents,even if the design and production process of the ship fully meets the domestic and international stability standards,the ship sailing in longitudinal or oblique waves may also have large-scale roll motion,and even cause capsizing accident.With the development of the second generation intact stability criteria,the research and control of the dynamic characteristics of the ship motion in longitudinal waves has become a research hotspot in the world.Based on the second generation intact stability criteria,the nonlinear stochastic dynamics method was mainly used to study the parametric roll,surf-riding and broaching of ship in random wavs.The probability density function and extreme value distribution of ship in random waves were studied,and how to reduce the parametric roll motion of the ship was investigated.In order to verify the effectiveness of the rudder roll stabilization,the influence of the different parameters of the rudder control were studied through the rudder anti-roll tests and the numerical simulation.The control problem of fin stabilizer was also studied in the thiese.Finally,the ship parameteric roll motion under special wave trains,including wave group and freak wave were studied.The research in this thesis help us to improve the understanding of the mechanism of different failure modes of ship stability in random waves.And a feasible control method was designed in this thesis to control the ship motion response.It is of great theoretical and engineering significance to improve design standard of ships and to ensure the stability and safety of ships.The main contents and conclusions of this thesis includes:(1)The probability density function of parametric roll under random wave was studied.The differential equation of ship parametric roll motion was established,and the stochastic averaging method was used to study the steady state probability density function of roll motion.Then the program was developed.The results show that under random waves,the ship was more likely to occure the large parametric roll motion in head waves than in following wave.As the heading angle increased,wave forcing was the main cause of the large roll motion.With the same wavelength,as the ship speed increases,the range in which the roll angle exceeds a certain angle becomes smaller.(2)The improved PID fin stabilizer control system was investigated.The fin stabilizer can effectively reduce the roll amplitude of the ship,but little research has been done on the effect of the fin control method on the anti-rolling effect with the consideration of parametric roll motion at the same time.In order to reduce the parametric roll mot ion of the ship,the fin control system was designed to improve the anti-roll performance.This study demonstrated PID parameters optimization method based on neural network algorithm.The SIMULINK simulation calculation program was developed to study the improved fin stabilizer control system to adapt to the nonlinear environmental condition.Through the Matlab Simulink s imulation to verify the modified PID control strategy provides an effective way to reduce the roll motion.It is found that the artificial network optimized PID control system performs better in nonlinear anti-roll system.Compared with the conventional PID control system,the controller optimized by neural network can improve the anti-roll effect.It has a better performance in nonlinear conditions.(3)The ship parameteric roll motion under special wave trains,including wave group and freak wave were studied.The parametric roll motion of the ship is a slow generation process,which is affected by the effective duration of the wave.Therefore,it is important to study the parametric roll of the ship in the wave group.And the group height and the group length of the wave are controlled by two parameters,which are group height factor and group length factor.In this study,a simulation system was developed to simulate the ship parametric roll motion in wave group.The research show that the group length factor increases,the stochastic wave contains more continuous high waves.And the parametric roll motion of ship was obviously changed when the ship under the wave group.The group length factor has more obvious effects on parametric roll.The phase modulation method was adopted to simulate the freak wave,and the parametric roll motion of ship with single-degree-of freedom under freak wave was studied.The results show that the phase modulation method can simulate the freak wave at the corresponding time.Although the wave height increased significantly,since the wave force and the encounter frequency were related to the ship parametric roll motion.For freak wave,the maximum roll angle increases comparing with the maximum roll angle under normal wave trains,but it does not occur at the moment when the wave height is the maximum.(4)In order to describe ship surf-riding in random longitude waves,the one-degree-of-freedom surge motion equation was established.Based on the stochastic Melinkov analytical method,the probability of ship surf-riding in random wave was solved and the parameter sensitivity analys is was studied.With the calculation and analysis,it is found that the wave load of the ship was increased with the increase of the significant wave height,and the probability of sruf-riding was increased.The probability of sruf-riding was decreased with the increase of the wave characteristic period;decreased with the increase of the heading angle;and increased with the increase of the propeller revolution.The effective measure to reduce the probability of surf-riding was to reduce the propeller revolution,so that the ship speed is as far as possible from the wave speed.(5)Based on the mechanism of cumulative-type broaching,the model of ship broaching in random waves was established by building one degree of freedom of ship yaw motion.The stochastic averaging method was used to solve the steady state probability density function and the probability distribution function of yaw motion.Then the parameter sensitivity analys is on the ship broaching was studied.The results show that the ship is more likely to occure the broaching with the increase of the significant wave height and the chracteristic period.(6)The model test on rudder roll stabilization to restrain the parametric roll motion of a container ship under parametric roll motion has been studied.Model test is an important method to study the seakeeping performance and maneuverability of ships.With the model test,the effectiveness of the rudder roll stablizaiton and the influence of the rudder on the ship’s parametric roll motion is studied,including the largest allowed rudder angle,the control trigger condition,wave length and ship speed.The results of the tests would provide valuable reference for ship rudder roll stabilization.The speed of the ship has a great influence on the roll damping coefficients of the ship.As the speed increases,there was a significant increase in the linear roll damping coefficient of the vessel.When the ratio of the encounter frequency to the natural frequency of the roll is approximately equal to 2 and the ship navigates heading in the wave,the parametric roll motion is observed.The large amplitude roll motion of the ship can be controlled with the rudder anti-roll module.The better anti-rolling effect can be obtained while the rudder anti-roll module is turned on earlier,the angle of the rudder and the control parameter are as large as possible.Through the model test,the parametric roll of the ship was effectively suppressed.