Hydroelastic Analysis Of A Supper Large Floating Structure Edged With Perforated Anti-motion Plates

Recently, the exploration of ocean spaces and resources expands quickly to meet population and urban development in countries with long coastlines. When faced with some natural conditions and environmental consequences such as water depth, seabed, cost, time and so on, Very large floating structure (VLFS) is usually regarded as an alternative option for birthing land from sea. For a VLFS, the structural length to the vertical length ratio as well as the structural length to wavelength ratio both is larger than unity. Thus the deformation will become dominant over the rigid, and the fluid-structure interaction problem should be considered. Meanwhile, being one of the essential design considerations for the VLFS, the methods in reducing hydroelastic response still need further investigate. Based on review, analysis and comparison of various ways for mitigating hydroelastic response of VLFS, the submerged, perforated, inclined anti-motion plates attached to both ends of the VLFS are proposed to be an alternative solution for reducing the motion of VLFS. In this dissertation, the response amplitude operator of the VLFS edged with anti-motion plates in the frequency domain, the motion analysis of the of the VLFS edged with anti-motion plates in the time domain, the physical model design of the sandwich-type VLFS and the perforated anti-motion devices, the transient behavior of a VLFS during unsteady external loads in wave condition, the hydroelastic response analysis of the VLFS with elastic edged restraint and mooring line based on the catenary’s theory in deepwater depth are carried out. The main works of this dissertation are as follow:(1) In frequency domain analysis, the hydroelastic responses of VLFS with the perforated, non-perforated or the dual submerged plates are calculated and the seakeeping ability of the VLFS with various anti-motion plates is compared. The boundary element method (BEM) solutions based on Kelvin sources are used for the hydrodynamic diffraction and radiation forces, in which the viscous effect of the perforated anti-motion plate are taken into consideration through the Darcy’s law. The numerical solutions are validated against a series of experimental tests in3D water basin, from which the empirical relationship between the porosity and the porous parameter is developed by using the least-squares fitting scheme. The effectiveness of the perforated plates in reducing the motion of VLFS, can be significantly enhanced by selecting optimal wave and plate parameters, and the optimal cases are obtained through a systematic parametric study.(2) A powerful direct time domain modal expansion method is applied to compute the time history of motion of VLFS edged with various anti-motion devices subjected to regular wave loads or irregular wave loads. To reduce the CPU time and memory, A quarter of numerical model is built based on the symmetry of flow field and structure in hydrodynamic forces, and special care is paid to the rapid and accurate evaluation of time domain free-surface Green functions and its spatial derivatives in finite water depth by using interpolation-tabulation method. The numerical solutions with different porous parameter of anti-motion plates are then compared with corresponding experimental results. The measured data and simulation results correlate well each other. The response-reduction efficiency of a pair of perforated plates and dual perforated plates is both numerically and experimentally assessed(3) The direct time domain modal expansion method is applied to obtain the transient response of the VLFS subjected simultaneously to incident wave and external loads including a huge mass drop, and landing or takeoff load of an aircraft. This method is described for elastic motion to arbitrary time-dependent external loads and the diffraction and radiation problems. These results under external loads such as a huge mass impact onto the structure and moving loads of an airplane are verified with existing numerical results and experimental test. Then, the developed numerical tools are used in the study of the combined action taking into account of the mass drop/airplane landing or takeoff as well as forward or reverse incident wave action. The deflections of the runway, the time history of vertical positions and the trajectory of the airplane are also presented through a systematic time-domain simulation.(4) As water depth increases in ocean environment, the traditional mooring system of rubber fender-dolphin is no longer applicable and the chain mooring system is adopted necessarily. The wave-induced responses of VLFS edged elastic linearly restricted system under wave action are investigated. And the expression of generalized stiffness matrix under considering moored boundary conditions is presented. The relationship between the spring stiffness and the vertical deflections of VLFS is analyzed and the BEM is applied to obtain the deflection and bending moment of VLFS in deepewater conditions for various wave and spring parameters based on the modal expansion approach.(5) Finally, geometry and mechanics characteristics of the mooring are added in the time domain motion equation of VLFS, and the time domain hydroelastic responses of the moored VLFS are investigated in deepwater. Considering the gravity, tension, current force and mooring line extension, the piecewise extrapolating method is employed to the static analysis of the multi-component mooring line, and the relationship curve between excursion and mooring force can be obtained by using B-Spline functions. Thus the motions including elastic deflections and rigid motion can be obtained by solving simultaneously the elastic motion and rigid motion equation. The wave-induced responses of the moored VLFS are computed in regular and irregular waves, and the parameter analysis of wavelength, waterdepth is also implemented.