Nonlinear Hydroelastic Analysis Method Of Very Large Floating Structures In Inhomogeneous Sea Conditions

A kilometer-scale VLFS differs from the traditional marine structures,due to the elastic deformation and the rigid body displacement with the same order of magnitude.Therefore,the coupling effect of the wave motion and structure deformation should be considered to investigate the hydroelastic behaviors of the VLFS’.For VLFS’ located near coast/islands or floating bridges connecting the fjords,the sea bottom is not uneven,and the sea condition will vary in the longitudinal direction of the structure owing to the varying sea bottom topography and the islands/shores.In this circumstances,it is necessary to consider the effect of the spatially varying wave conditions on the hydroelastic behaviors of the VLFS’.The thesis first adopts a new method to deal with the fluid-structure interaction problem,in which the continuous VLFS is discretized into rigid modules connected by elastic beams.Then,based on the multi-body hydrodynamic theory and the beam bending theory,a frequency-domain hydroelastic method for VLFS’ in waves is proposed,which is validated with the existing experiment results.Further,based on the developed method,the nonlinear hydroelastic method is developed,considering the effect of the inhomogeneous wave conditions,the second-order nonlinear wave excitation forces and the nonlinear connectors.Finally,adopting the proposed method,the linear and nonlinear hydroelastic behaviors of a designed floating bridge with the length of 4 kilometers located in fjord are investigated considering the inhomogeneous wave effect.The dissertation includes the following parts:1)The basic hydroelasticity theories are briefly reviewed;the applicability and application of the existing numerical methods on the hydroelasticity of VLFS’ are summarized.2)The main theories referred to the discrete-modules-based hydroelasticity method are introduced and derived in detail;the reliability and accuracy of the frequency-domain hydroelasticity method are verified by the comparisons with the experimental data.3)On the basis of the Cummins’ equation and the proposed hydroelasticity method,the time-domain hydroelastic equations of motion are established and the corresponding program suit is compiled;the feasibility of the time-domain method is validated with the commercial software and the traditional three-dimensional hydroelasticity theory.4)Based on the proposed time-domain method,the spatial-varying inhomogeneous wave field is divided into different regions;then the time-domain wave excitation forces on the modules in different regions are solved by the inhomogeneous wave conditions,and the time-domain method for the hydroelastic analysis of VLFS’ in inhomogeneous wave conditions is developed.The results show that the inhomogeneity of the waves has a significant effect on the bending moments,shear forces and torsional moments of the structure,especially with a wave direction of 90°,in which larger forces than those due to homogeneous wave conditions may be induced.Inhomogeneity will influence not only the value of the maximum forces but also the location of the maximum forces along the structure.Therefore,the inhomogeneous effect should be considered in the structural design.5)Combining the present method with the second-order nonlinear theory,a time-domain method for nonlinear hydroelasticity of a moored VLFS in homogeneous and inhomogeneous wave condition is developed.In comparison with the linear hydroelastic results in homogeneous wave conditions,the effect of the inhomogeneity and the nonlinearity on the hydroelastic behaviors of the VLFS is discussed.It is found that the second-order nonlinear wave excitation forces have no obvious impact on the vertical displacement and structure forces,but amplify the horizontal displacement and dynamic tensions of the mooring lines greatly.Especially in the 90° inhomogeneous wave condition,different from the homogeneous results,the mooring forces present obviously symmetrical character,which should be pay attention in the design stage.6)Based on the proposed time-domain method and considering the effect of the nonlinear connectors and the moving loads,the thesis presents a hydroelastic approach for nonlinear connected elastic floating structures subjected to combined effect of waves and moving loads.A hydroelastic response analysis about the effect of different moving loads velocity,connection gaps,wave amplitudes on the dynamic behaviors of the structure and connectors,is carried out numerically.Results indicate that the nonlinear characteristics of the connectors play an important role in the bending moment of the floating structural and the force of the connectors,and may cause fatigue problems,which should be consider in the structural dynamic analysis.7)The linear and nonlinear hydroelastic behaviors of a designed floating bridge located in fjords,is numerically simulated in the inhomogeneous wave conditions,adopting the established theories and methods.The numerical simulations will be employed as fundamental theoretical basis in the future design of that kind of floating bridges.