A Research On Numerical Prediction Of Ship Maneuverability In Regular Waves

Ship maneuverability is commonly analyzed in calm water.In reality,however,a sea going vessel is affected by environmental forces,notably wind,waves and currents.Wave effects usually have the most important influence on ship maneuvering.As a ship steers in waves,wave-induced ship motions occur,and the maneuvering trajectory is changed due to the wave drift forces.The maneuverability of the ship may therefore be significantly different in waves than in calm water.In fact,it is important to understand the maneuverability of a ship in waves for ship navigation safety.Numerical prediction of ship maneuverability in regular waves is considered in the present study.A mathematical model of the ship maneuvering in regular waves is proposed.Since a ship will undertake 6 degrees of freedom(DOF)wave-induced motions while maneuvering in regular waves,in the mathematical model the total ship motion is divided into the low frequency maneuvering motions and the high frequency wave-induced motions.The low frequency motions are determined by the MMG model which takes into account the mean second-order wave drift forces and yaw moment.The high frequency motions are governed by another set of 6 DOF motion equations.Due to the difference of the motion frequency,two time scales are used in solving the high and low frequency motion equations.Potential flow theory is used to calculate the high frequency hydrodynamic force and the wave drift forces.To this end,the boundary value problem of the classical seakeeping theory is extended to that includes the transverse speed and the yaw rate of the ship.The boundary value problem is solved via a time domain Rankine panel method.In this method,the unknown quantities are discreted by B-spline basis function,and explicit and implicit Euler schemes are used for the time derivatives in the linearized kinematic and dynamic free-surface boundary conditions,respectively.The time domain analysis of the high frequency motion is performed by interatively solving the discrete integrate equation,the discrete kinematic and dynamic boundary conditions on the free surface,and the motion equations.The Wigley I hull and the S-175 container ship are used to validate the time domain Rankine panel method.The comparison of the numerical results with the model test data shows that the present approach can accurately predict the 6 DOF motions of a ship under various wave directions.Second-order wave drift forces and moments are determined via near-field pressure integration of the time-domain linear velocity potential.Numerical computations are conducted for the Wigley I hull in head sea and the S-175 container ship in head and beam seas.The computed mean second-order wave forces and moments are compared with experimental data.The comparison shows that the present method can give reasonable predictions.With regard to the low frequency hydrodynamic forces,the MMG maneuvering model is employed,and the hydrodynamic forces related to the ship hull,the propeller and the rudder are modeled.The hydrodynamic forces on the oblique towed and steady turning ship hull are computed based on the theory for wings of low aspect ratio,which show that the predictions are not accurate enough when the drift angle is large.Hence,the available PMM model test data of the hydrodynamic derivatives and other hull-propeller-rudder interaction coefficients are used in the following maneuvering simulations.The maneuvering simulations are carried out for the S-175 container ship.Turning and zigzag maneuvers in calm water are tested firstly,and the numerical results show good agreements with the model test data.Then,the proposed mathematical model is applied to simulate the turning and zigzag maneuvers of the ship in regular waves with different wave lengths and wave directions.The numerical results are compared with experimental measurements,which demonstrates that the proposed mathematical model and the associated numerical methods can reasonably predict the maneuvering motion of a ship in regular waves.Overall,a new method of evaluating the ship maneuvering in regular waves is proposed in the present study,by incorporating the theories of maneuvering and seakeeping.The work of this thesis is of some value to improve the hydrodynamic performance and navigation safety for a ship during voyaging at sea.