A Numerical Study Of Ship Manoeuvring In Regular Waves Based On Two-time Scale Method

In order to improve the navigation safety of ships,the International Maritime Organization(IMO)has issued the “Standards for Ship Manoeuvrability”,which puts forward quantitative requirements for ship maneuverability in calm water.These standards have not considered ship manoeuvrability in adverse environmental conditions such as winds,waves and currents,while in fact ships are often subjected to waves when navigating at seas or in the waters near coast.On one hand,the presence of waves will affect the ship’s sailing trajectory,which makes it more difficult for ships to alter course or avoid obstacles by performing the manoeuvres;on the other hand,the introduction of the Energy Efficiency Design Index(EEDI)by IMO in recent year restricts the greenhouse gas emissions of all new-built ships,resulting in the reduction of installed power of ships,which may lead to the insufficient engine power to maintain ship manoeuvring safely in waves.Therefore,it is of great practical significance and engineering application value to study the manoeuvrability of ships in waves.The study of ship manoeuvring in waves involves the coupling of ship motion in seakeeping problem and ship manoeuvring motion in manoeuvring problem.In the past,the studies on this coupling problem depend mainly on experimental methods,which however rely highly on experimental equipments,experimental techniques,and are also restrited by the experimental cost.With the development of the computer technology,the numerical method based on Computational Fluid Dynamics(CFD)technique provides a powerful means for investigating this problem.However,the direct CFD simulation for ship manoeuvring in waves has not only a high requirement on the hardware and software resources of computer,but also requires a very long computing time.Therefore,it cannot meet the practical requirement in engineering effectively.In order to find a more effective way that can meet the practical requirements in ship’s design stage,the two-time scale method is adopted in this thesis to numerically study the ship manoeuvring in regular waves.The coupled motion in waves including the manoeuvring motion and wave induced motion is divided into a superposition of high frequency wave induced motion and low frequency manoeuvring motion in calm water,where the second order wave forces and moments from the seakeeping module are transmitted as the inputs to the manoeuvring module while the kinematic parameters such as the ship’s speed and position are treated as the initial values in the seakeeping module.Then the numerical prediction of ship manoeuvring in waves is realized by solving the motion equations of ship manoeuvring in waves.Firstly,the mathematical model of ship maoneuvring motion in regular waves is established based on the three degrees of freedom(3-DOF)MMG(Manoeuvring Modeling Group)model in calm water incorporating with the second order wave drifting force and moment,and the hydrodynamic models for the hull,propeller,rudder and the wave force in the mathematical model are established respectively.As for hydrodynamic models with respect to the hull,propeller and rudder,the corresponding hydrodynamic derivatives and the hull-propeller-rudder interaction coefficients are obtained from the experimental data available in the literature or the numerical computation using CFD method.As for the wave force model,in the frame of potential flow theory,the initial boundary value problem is formulated by introducing the the trailing vortex model and considering the ship’s lateral speed and yaw rate.Then the B-spline function based time domain Rankine higher order panel method is utilised to solve the initial boundary value problem,and the first order wave force,hydrodynamic coefficients are computed based on the obtained velocity potential.On this basis,the near field pressure integration method is applied to compute the second order wave force.Finnaly,the two-time scale method is employed by using two different time steps for solving the wave induced motion and the manoeuvring motion respectively.To validate the reliability of the established mathematical model and the numerical method proposed in this thesis,ships advancing along the straight-line course in deep and shallow water in regular waves are firstly considered,and the first order wave force,hydrodynamic coefficients,wave-induced motions as well as the second order forces are computed.By comparing the numerical results with the experimental results or numerical results available in the literature,the correctness of the present numerical method is validated.Further,a ship obliquely moving with small drift angles in regular waves in deep water is taken as the example to study the influences of the trailing vortex model and ship’s lateral speed on the motion responses and the second order wave forces.The comparisons of the numerical results with the model test results available in the literature indicate the feasibility of the present method.On this basis,the turning performances of ships in regular waves in deep water are studied based on the two-time scale method,and the influences of wavelengths and incident wave angles on the turning performance are investigated.The numerical results are compared with the experimental results in the literature,which demonstrates the effectiveness of the present numerical method.Meanwhile,the wave-induced high frequency motions during turning are also investigated,and the changing characteristics are analyzed in detail.Finally,a preliminary study on the turning performance of a ship in regular waves in shallow water is carried out,and the influences of water depth,wavelength and incident wave angle on the turning performances are explored.The researches in this thesis are of great theoretical significance and engineering reference value for deepening the understanding of the coupling mechanism between wave induced motion and maneuvering motion in waves,evaluating ship manoeuvrability in waves,and improving ship’s navigation safety in waves.