Pitch-roll Stabilization Of Ships On The Basis Of On-line Predictions Of Hydrodynamics

Six degree-of-freedom ship motions occur due to the ocean disturbances such as wind,ocean waves and current,etc.Which bring adverb effects on ship maritime operation safety.Ship motion reduction plays an important role in improving the safety,comfortability and operability of a ship.Therefore,it has been widely focused on in the research of sea-keeping performance.In order to overcome the adaptive problem and deficiency in estimating the ship hydrodynamic that exists in the conventional control approaches such PID method,a ship motion control tool was developed based on the on-line prediction of ship hydrodynamic.Wave induced forces or moments acting on the ship were predicted real-time as control inputs by using a wave effect auto-regressive(WEAR)model and a hydrodynamic estimation model base on dynamic equavilent balance equations.The heaving and pitching were coupled in the hydrodynamic estimation model where the memory effect represented by the impulse response function was also taken into consideration.Controllers for roll stabilization,pitch stabilization and the pitch-roll stabilization on a ship with two pairs of active fins were designed based on the ship motion equations and on-line prediction of ship hydrodynamics.Numerical simulation and model tests were carried out to demonstrate the feasibility and analyze the performance of the control approach in ship motions stabilization.The dissertation was organized as follows.Firstly,prediction of large amplitude ship response on waves was investigated.Accurate ship hydrodynamic estimation is the key role in ship design and the fudamental part of ship motion control.As the numerical simulation of pitch-roll stabilization require relative high efficiency and accuracy,time domain prediction methods for ship motions were investigated based on a nonlinear approach.Nonlinearity due to the wetted surface was considered in the incident excitation wave force and hydrostatic restoring force which were estimated on the instantaneous wetted surface.The radiation and diffraction forces were calculated by applying Fourier Transform to the frequency hydrodynamic coefficients.In addition,to improve the accuracy in forecasting the nonlinear ship response,contributions of various wetted surface processing techniques and hydrodynamic coefficients were analyzed.Thus,an improved nonlinear 2D method for predicting high speed ship hydrodynamic problems was developed.Secondly,as the wave forces estimation is the key part of ship motion control,an on-line prediction method for ship hydrodynamics was proposed base on an adaptive wave effect auto-regressive(WEAR)model and the dynamic equavilibrium between the external forces and ship motions.Numerical simulation results of a cruise ship demonstrate that the proposed method was effective and adaptive in forecasting the ship hydrodynamics on-line at both sea state 5 and sea state 6.Results of pitching and rolling moments indicate relatively high accuracy of the prediction method.Thirdly,controllers for roll stabilization,pitch stabilization and the pitch-roll stabilization on a ship with two pairs of active fins base on the on-line prediction of ship hydrodynamics were designed.Numerical simulations were carried out to demonstrate the feasibility and analyze the performance of the control approach.The PID control method was also studied for comparative purpose.Simulation results suggest that the proposed control approach was effective in pitch stabilization,roll statbilization and the pitch-roll stabilization.And results also indicate that the proposed method shows higher efficiency than the PID method.The above numerical simulation results provide theoretical supports in model testing design.Finally,the pitch stabilization and pitch-roll stabilization experiment were carried out.The experimental works provide both further demonstrations of the feasiblity in practical application and performance evaluations of the proposed control method.The experimental results show the proposed control approach was effective in both pitch stabilization and the pitch-roll stabilization.The heave motion was also slightly control.In heading waves,the reduction efficiency exceeds 33% in irregular waves and more than 40% in regular waves for pitch stabilization.In oblique waves,the roll was reduced by more than 40% while the pitch was reduced by more than 30% when the pitch-roll stabization control was implemented.