A Study On The Hydroelastic Response Of Floating Structures Near Islands And Reefs

With the trend of larger scale of ships and the appearance of Very Large Floating Structures,the elastic intrinsic vibration frequency of floating structures becomes lower which is close to the wave frequency.Therefore the hydroelastisity theory concerned with the phenomena involving mutual interactions among inertial,hydrodynamic and elastic forces is proposed.This theory is an effective method dealing with the fluid-structure interaction problem with the motion response,structural strain and stress solved in a unified manner.With the combination of the 3D seakeeping theory and the 3D structural dynamic theory,the 3D hydroelastc theory was established by Wu Yousheng in the 80 s of the last century.Further studies were carried out by Du Shuangxing and Wang Dayun in the frequency and time domain respectively while the nonlinear hydroelastc theory was further developed by Chen Xujun and Tian Chao.A lot of improvements and developments have been achieved with the efforts and studies of many other scholars.The study and application of the Very Large Floating Structure receives more and more attentions due to its strong influence on the social,economic,political and military affairs.Researches of Very Large Floating Structures have been carried out by the countries such as the United States and Japan.The study of the Mobile Offshore Base was carried out by the United States from 1992 while the research institute of the Mega-Float was set up by seventeen Japanese companies in 1995.With the support from Chinese National Natural Science Foundation Council,Shanghai Jiao Tong University and China Ship Scientific Research Center were able to carry out a systematic research on the hydroelastic responses of pontoon-type very large floating structures.However,most of the above studies were carried out with the open sea and uniform water depth assumptions.The floating structures deployed near islands and reefs could be used as a floating harbor,logistic base and fishery processing for the development of the natural and tourism resources.The flow field would be strongly disturbed by the complex seabed bathymetry near islands and reefs due to the shallow and varying water depth.Therefore the seabed effect on the dynamic responses should be taken into account for the floating structures deployed near islands and reefs.Boussinesq equation is able to simulate a lot of complex phenomena such as reflection,diffraction,refraction,breaking and nonlinearity while the Green function method is able to calculate the wave loads of floating structures.A new method is established for the numerical simulation of the motion responses and connector loads of floating structures near islands and reefs with the combination of the Boussinesq equation and the Green function method.The main research work of this paper includes the following aspects:(1)A thorough literacture review was carried out.Three kinds of widely used water wave equations were introduced: the mild slope equation,the Boussinesq equation and the Green-Naghdi theory.The Boussinesq equation and its improvements which are used for the numerical simulation of the flow field in the newly built method were mainly reviewed.Further more,the development process of the 3D hydroelastc theory and the related research progress at home and abroad were reviewed.Three mathematical models dealing with the dynamic responses of floating structures near islands and reefs were reviewed.The Bingham method received more attention because of some common ideas with the new method built in this paper.The similarities and differences between the Bingham method and the new method built in this paper were further analyzed.(2)A new method was built for the hydroelastic responses of floating structures near islands and reefs with the combination of the Boussinesq equation and the 3D hydroelastic theory.The Boussinesq equation was applied for the simulation of the wave field near islands and reefs.Therefore the Froud-Krylov force and the body surface boundary condition of the diffraction potential could be obtained through the pressure and velocity field calculated by the Boussinesq equation with the effect of the complex seabed taken into account.The diffraction and radiation potentials were calculated by the Green function method with finite water depth and complex seabed treated as a fixed body.The corresponding program has been compiled according to the theory described above.(3)The velocity distribution along the water depth under complex seabed condition was studied by the Boussinesq equation and the water channel test.The good agreement between the numerical predictions and the model test results proved the capability of the Boussinesq equation in the aspect of the velocity field simulation.(4)A single module of the Very Large Floating Structure was taken as an example for the numerical study of the motion response under a typical seabed bathymetry near islands and reefs.The good agreement between the numerical predictions and the model test results showed the validity and the applicability of the mathematic model built in this paper.The influence of the complex seabed on the floating structures was preliminarily studied with the comparison of the motion responses under even seabed condition.(5)The motion responses and connector loads of the Very Large Floating Structure with eight modules were studied with the new method built in this paper and the Rigid Module Flexible Connector(RMFC)model.With the island sheltering effect,non-uniform seabed condition(2D slope)and the coupling effect of multiple modules taken into account,the numerical predictions were in good accordance with the model test results which again verified the applicability of the numerical method built in this paper.The influences of the island sheltering and non-uniform seabed effect on the motion responses and connector loads of floating structures were revealed through the study in this paper.