Study On Navigation Control And Gap Fluid Motion In The Process Of Ship-to-ship Transfer Operation

With the development of large-scale ships,their tonnage and draft are increasing,and some ports can no longer meet the berthing requirements of large ships.In order to improve the efficiency and reduce the risk of large ships operating in port,ship-toship transfer has developed rapidly.At present,the ship navigation generally depends on manual control,but the manual course adjustment lacks the ability of selfregulation to the ship motion and the change of the external environment,and is also susceptible to subjective factors,so it is very necessary to realize the automatic control of the ship navigation.In addition,the gap between the two ships is relatively small during the transfer operation.In some marine environment conditions,the hydrodynamic interaction between the two ships may be intensified with the resonant motion of the fluid in the gap of the two ships,which will lead to the increase of the ship’s motion and force,thus limiting the operability of the equipment on board.Therefore,the hydrodynamic characteristics of the transfer system and motion mechanism of gap fluid between two ships are systematically analyzed.In this paper,the mathematical model of four-degree-of-freedom maneuverability is established,and the automatic control of the berthing ship navigation is realized by compiling the motion control system.The course control effect of berthing ship in static water,regular wave and irregular wave is simulated.The results show that the control strategy presented in this paper has a good ability of rudder steering and correcting heading deviation,and shows good control effect under various environmental conditions.Based on the potential flow theory,the hydrodynamic coefficients of the two ships under different transverse spacing and the motion response,wave force and wave surface rise of the two ships under different waves are calculated and analyzed.The numerical wave tank is established based on CFD theory and its correctness of wave generation is verified.By analyzing the free surface,velocity vector distribution of the flow field near the two ships,the rise of the gap wave surface and wave force,the fluid response characteristics and mechanism in the gap of the two ships transfer system are studied,and the effects of different transverse spacing and wave direction are analyzed.The comparison between the CFD calculation results and the potential flow theory shows that the inviscid assumption will lead to the distortion of the calculation results near the resonance frequency.The theory of potential flow is modified by applying viscous damping lid on the free water surface between two ships,and the effects of different wave directions and different transverse distances on the modified damping coefficients are studied.The results show that the damping correction coefficient of 90° transverse wave is larger than that of 180° wave direction when the transverse spacing is constant,and the damping correction coefficient decreases with the increase of the distance between two ships.The time domain results of ship-toship transfer system with and without correction are compared and analyzed.The results show that the calculation results of the motion response of each degree of freedom of the two ships and the force of each mooring or fender without viscosity correction are higher than the corrected results,it is very necessary to consider the correction of fluid viscosity.