Investigation On Hydrodynamic And Motion Interaction Between Side-by-side Mooring System

With the continuous development of shipping and ocean engineering industry,the exploration and exploitation of offshore oil and natural gas gradually deepens.A single floating structure is often unable to complete operations independently,and multiplefloating-bodies system operations are beginning to appear.For example,FPSO and shuttle tanker work together to transmit oil and gas.To study the complex hydrodynamic interaction between side-by-side mooring ships,the hydrodynamic performances of the system are investigated based on the conventional potential flow theory.A time-domain numerical model for solving the hydrodynamic interaction between waves and side-by-side mooring system is established by Cummins method.This numerical model is used to perform parametric analysis on the motion responses of the floating-bodies system and the force characteristics of the mooring lines and fenders under the action of waves.To study the hydrodynamic performances of side-by-side ships,hydrodynamic coefficients,1st wave exciting forces and motion responses under different wave directions and gaps are calculated in the frequency domain.The calculation results show that the hydrodynamic interaction between the double-floating bodies makes the self-induced additional mass appear negative on sway,heave and yaw motions.As the gap decreases,the frequencies of the extremum of the self-induced additional mass and damping coefficient move to high frequencies.Because of the shielding effect,in the beam sea condition,1st wave exciting forces of sway,heave,roll and yaw motions of the ship on the weather side are larger than those of the ship on the lee side.The difference in size makes shielding effect acted on the smaller ship more obvious.In the head sea condition,the 1st wave exciting forces and motion responses of sway,roll and yaw of the two ships are larger than the single-ship case.By using inverse Fourier transform method,hydrodynamic coefficient and 1st wave exciting forces in the frequency domain can be converted to time-domain added mass,1st wave exciting forces and hysteresis function.Then the time-domain equation for solving the interaction between waves and side-by-side mooring system can be established.The influence of wave directions and amplitude on the motion responses,mooring forces and fender forces are systematically studied,and the influence of hydrodynamic interaction between side-by-side ships is analyzed.The calculation results show that a good identity exists in the surge,sway and yaw motions of two ships.In the beam sea condition,the hydrodynamic interaction between ships makes the amplitude of sway,roll and yaw motions smaller,and those of the ship on the weather side decrease more.Instead,the amplitude of heave and pitch motions are larger.In the head sea condition,the hydrodynamic interaction makes the amplitude of heave,pitch and yaw motions smaller,and the ones of sway and roll motions larger.The model test verifies the numerical model established in this paper.