Research On 7DOF Mathematical Model Of Rescue Ship Motion

Both practical applications in Naval Architecture and Ocean Engineering and the development of ship maneuvering simulators have driven the research of ship maneuvering in waves.To improve the precision of the rescue ship motion simulation by exsiting simulators in the background of ship rescues in rough sea(the thesis specifically refers to sea state 6 and above),four aspects are further investigated:the unified model of maneuvering and seakeeping,the anti-rolling issue in rough sea,the solutions of wave loads in seakeeping,and motion simulatons in real time.The final purpose is to acheive better accuracy of mathematical models to describe motions of rescue ships,to analyze ship motion characteristics in rough sea,and to provide the theoretical support for the behavioural realism of simulators.(1)In the aspect of the unified model,the impact of environmental loads on ship motions is involved comprehensively,and a complete 6 degrees of freedom(DOF)coupled mathematical modular model is established systematically based on the Cummins unified model.The modules include the hull hydrodynamic parameters,resistance-propulsion,restoring forces,rudder forces,wind and wave loads,and also the effects of the current flow and immersions of rudder and propeller in waves are considered.The main modules are verified one by one and the maximum errors in calculations of roll damping and propeller revolution are about 5.0%and 10.9%,respectively.(2)In the aspect of the anti-roll tank modelling,a general form of mathematical model of anti-roll tank is established using Hamiltonian dynamics to deal with the issue of rescue ship rolling in rough sea.Then,models of a U-shaped anti-roll tank and a optimal phase PD control of gas valves on each reservoir are derived.Finally,the 7DOF ship-tank coupled mathematical model are established by combining the tank model with the 6DOF model.To verify the accuracy and validaty of the anti-roll tank model and PD control model,only motions of ship roll and tank liquid are considered,and comparisons between experiments and simulations in regular waves are performed.The results show that passive and controllable passive anti-roll tanks can indeed reduce roll motion effectively by 46.5%and 66.9%,respectively.(3)In the aspect of numerical calculations of wave loads,the effects of both first-order wave excitation loads and second-order mean drift loads on ship motions are involved.Firstly,the perturbation expansion method is used to establish a linear boundary value problem(BVP)considering deep water and ship speed in the frequency domain.Taking the hull geometry into account,a 3D Green function source method is utilized to solve first-order radiation and diffraction problems numerically.Based on the principle of momentum conservation,a far-field expression of drift loads with fast computation and convergence is derived,and programs of loads calculations are developed.Numerical calculations for a simple geometric hemisphere,hulls of Wigley-I,S175 with a bow flare and Mariner are carried out and results show that present methods are able to guarantee the accuracy of first-order problems and predict drift loads effectively.Moreover,in the region of short wavelength of ?/L<0.5,results of the thesis give a better prodiction of aymptotic values of drift loads compared with numerical results of other scholars.Secondly,in the time domain,the maneuvering and seakeeping analyses of 4 real ships(NHJ111 with transom stern,Mariner,S175 with bow flare,and YuKun)are investigated to verify the accuracy and validaty of the 7DOF modular model and wave loads models.The turning motions in calm water are performed,the relative error is approximately 7.0%by comparing calculated results with experimental data,and methods presented in the thesis give better prodictions than empirical methods and the 2D strip theory.Meanwhile,motions and wave loads of Mariner and S175 in short-crested irregular waves in sea state 5 are reported.Compared with published theoretical results in regular waves,the calculated trajectories and motion quantities give satisfactory predictions.The calculated drift loads are consistent with theoretical results in general,and slightly smaller at local peaks and troughs where exhibit some stochastic oscillation characteristics.Moreover,taking combined actions of winds,currents and waves into account,maneuvering motions of YuKun is excuted.The calculated turning trajectory agrees well with experimental data,which shows that established models are able to predict ship motions in actual sea conditions sufficiently.Finally,the validated 7DOF model is utilized to perform motions of NHJ111 in sea state 6,including maneuvering simulations under actions of winds and currents in calm water,immersions of rudder and propeller and anti-roll tank in short-crested irregular waves.(4)The 7DOF model is then applied to the rescue ship simulator,and a simulation test platform of ship-tank coupled motion system is developed independently.From the perspective of visual perception,the synchronous interpolation method is conceived to achieve real-time calculations of loads in short-crested irregular waves and thus increase the frame rate from 7 frames per second(FPS)to 20 FPS,meeting the visualization minimum requirement of 15 FPS,achieving a balance between "accuracy" and "real-time".From the perspective of physical perception,the purpose of simulating rescue ship oscillating motions in rough sea is achieved by using the 6DOF Stewart platform based on the algorithms of washout filtering and dynamic virtual assembly in both tests and simulations,which lays the foundations for the research of the rescue ship simulator with the Stewart platform and the formulation of relevant standards.