Time-domain Numerical Study On The Interaction Between Waves And Moored Multiple Floating Bodies System

With the rapid development of offshore oil and gas resource development, the mobile platform FPSO which gathers production, storage and unloading of oil has been widely used. As an important part of FPSO in the process of unloading, the transport system which is consisted by FPSO and transport ship is more and more applied in the marine engineering and equipment. A time-domain numerical model is established by Cummins method to study the interaction between waves and nonlinear moored multiple floating bodies system. In view of the layout of external transport system, side-by-side and tandem floating bodies system are both considered to investigate the mutual effect of hydrodynamic, motion responses and cable force characteristics under the action of wave.According to the linear potential flow theory, the velocity potential boundary integral equation for multiple floating system is established by using the higher order boundary element method. The velocity potential in the flow field can be obtained by solving the velocity potential integral equations. Meanwhile, point sources are arranged on floating bodies" inner surface to remove the irregular frequency effects in solving the hydrodynamic coefficients of multiple floating bodies system. It can convert hydrodynamic coefficient and wave exciting force obtained by frequency domain model to time domain wave force, added mass and hysteresis function in motion response equation by using inverse Fourier transform method. Then the time domain motion equation can be established by considering nonlinear relation between mooring force and motion response. The motions of floating bodies moored by nonlinear cables and the mooring force can be obtained by the calculating of motion equation based on the four order Predictor-Corrector method. Through the comparing with the existing numerical results, it can be proved the present frequency domain theory based on irregular frequency method in analyzing hydrodynamic interaction of floating bodies is correct. In order to compare the results of motion response in frequency domain, the motion response of two side-by-side barges moored by linear elastic spring is computed by the time domain numerical model. The correctness and effectiveness of the time domain numerical model is confirmed by the comparison of the motion response in frequency and time domain.The hydrodynamic characteristics of multiple floating bodies have significant effect on the motion of moored multiple floating bodies, this paper analyzed the hydrodynamic characteristics in the frequency domain first. The influences of the space of floating bodies and wave direction on hydrodynamics and motion response are investigated. Then the effect of different mass matrix forms on the motion of multiple floating bodies in the global coordinate system is studied. The homogenous barges and FPSO-Shuttle system in the actual projects under side-by-side arrangement are considered, respectively. Finally, the time-domain numerical analysis model for motion response of multiple floating bodies system moored by nonlinear cables is used to study the motion and mooring force of side-by-side and tandem arrangement multiple floating bodies system. In view of a tandem arrangement of twin barges moored by nonlinear cables, the influences of the properties of mooring cable between floating bodies, wave amplitude, wave length and wave angle on the motion responses and the mooring forces are also studied.