A Study Of Numerical Simulation Method Of Ship Maneuvering Prediction In Waves

With the continuous acceleration of globalization,trade between countries has become more frequent.As an indispensable means of transportation in bulk commodity trade,ships are becoming larger,faster and more intelligent.Meanwhile,sea routes are becoming more and more intensive and the number of ships is increasing.However,when a ship is operating in the actual marine environment,the influence of waves cannot be ignored.Wave action will lead to poor ship maneuvering performance,lower propulsion efficiency,and even have a significant impact on ship navigation safety,which puts forward higher requirements for ship maneuvering performance.In addition,in order to improve the energy efficiency of shipping and reduce greenhouse gas emissions,the international maritime organization(IMO)proposed to introduce the energy efficiency design index(EEDI).In order to meet this requirement,the most direct way is to reduce the ship’s main engine power.However,this may lead to the lack of sufficient propulsion and maneuvering performance of the ship in severe sea conditions,thus causing major safety problems.Under the above background,this paper studies the numerical simulation of ship maneuvering in calm water and waves.First of all,the research progress and current status of maneuvering mathematical models,maneuvering in calm water and waves are introduced.Then aim at the maneuvering study of full ships,a standard KVLCC2 tanker is chosen as an object,and the MMG standard maneuvering model is used in the present study.Three kinds of captive model tests(oblique towing test,circular motion tests and rudder angle test)are simulated with the help of CFD technique,and the hydrodynamic forces and moment acting on the hull including surge force,sway force and yaw moment and rudder normal force are calculated.The maneuvering hydrodynamic derivatives in the MMG standard model are obtained by the method of polynomial fitting according to hydrodynamic forces and moment acting on the hull in various speeds and drift angles.All the linear hydrodynamic derivatives and some nonlinear derivatives have a good agreement with the experimental results.However,there is remarkable difference between CFD results and experimental results for a few nonlinear derivatives.The hydrodynamic coefficients representing the interference between hull and rudder are calculated according to the relationship between hull forces and rudder force of rudder angle tests.Based on the hydrodynamic coefficients obtained from CFD method,the MMG standard maneuvering motion equations are programed using Fortran language,and the fourth order Runge-Kutta method is used to solve the differential equations of motion.Turning and zigzag maneuver in still water are simulated.The turning trajectories and heading angles of zigzag motions are compared with the free running model tests results of published paper available,and a favourable agreement is obtained,indicating that the predication of ship maneuvering characteristics using CFD method has an acceptable precision.The influence of the hydrodynamic coefficients on the maneuvering indicators including advance,tactical diameter,the first and second overshoot angles is studied.Above all the hydrodynamic derivatives,the longitudinal resistance coefficient(Xuu’),linear hydrodynamic derivatives in the sway and heading directions(Yv’、Yr’、Nv’、Nr’)have greater influences than other nonlinear derivatives.The advance AD and tactical diameter DT decrease with the increase of propeller thrust deduction fraction tp,while increase with the propeller wake fraction wP0.And these two coefficients are more sensitive under the condition of high speed than low speed.The advance AD and tactical diameter DT decrease with the increase of rudder force deduction fraction tR and straightening coefficient γR.The deduction fraction tR is more sensitive under the condition of large rudder angle,while the straightening coefficient γR has opposite result.Potential and viscous flow coupling method is used to study the ship’s maneuvering motion in regular and irregular waves.Considering the influence of the second-order average drift force,the Taylor expansion boundary element method(TEBEM)based on the potential flow source-pair mixed distribution is used to calculate the average wave force of ships sailing in different wave directions and frequencies at various speeds.In order to add wave load to the right side of the MMG equation,real-time interpolation was performed for the wave forces according to the changes of ship speed and encountering wave direction and frequency,so as to solve the ship’s maneuvering motion in the wave environment.In the case of irregular waves,in addition to the sea state information and wave propagation direction given in advance,the time difference and wave height between the instantaneous equilibrium positions are approximately regarded as the semi-period and wave height of regular waves,which serve as the basis for wave force interpolation.Numerical simulations of the turning motion in regular and irregular waves are studied at different speeds and sea states,and the turning trajectories are compared with the free running tests available in the literature.The results show that the present method has acceptable accuracy,and can be used to evaluate ship maneuvering performance in waves quickly as a kind of practical method.