Second Order Nonlinear Hydroelastic Analysis Of Floating Systems

Hydroelasticity theories of ships and floating marine structures, which embody the full complexities of the dynamics of the structure concerned and the fluid around it, provide more consistent and more rational approaches for the assessment of the overall behavior of a flexible marine structure. The existing theories have been developed for more than 20 years, however mainly focus on linear problems (Bishop and Price, 1979; Wu, 1984; Du, 1996; Wang, 1996). The linear theories are suitable for investigating a floating body with small amplitude of motion or an elastic structure in waves with resonant frequencies within or higher than the range of the wave frequencies. When responding to high waves with large amplitude, the floating structure may apparently suffer from non-linear responses due to the second order hydrodynamic forces induced by the rigid body rotations of motion and the effect of the instantaneous wetted surface. For a marine structure with the scale much larger than a conventional ship, its resonant frequencies in water will be very low, and its behavior in waves will be quite different than a usual vessel. In these circumstances it is obviously necessary to investigate the influence of the second and higher order hydrodynamic actions on the hydroelastic responses of a floating structure. The first objective of the present thesis is therefore to establish a second order hydroelasticity theory and the corresponding numerical method for investigating the effect of the above mentioned second order wave forces on the structural behavior of the floating body. Most of the floating marine structures are kept stationary by a mooring system. However there has still been a lack of deepening study about the interaction between a æ…ºlexible?structure in waves and its mooring system. The second objective of the present thesis is to develop the second order hydroelastic analysis method and the corresponding computer program for predicting the responses of a moored flexible floating structure to waves. To achieve these two objectives, the following efforts have been made and the results are presented in the present thesis. These include: (1) The existing two and three dimensional hydroelasticity theories and the analysis methods for the second order wave forces are briefly reviewed. (2) The fundamental contents and the analysis methods of three-dimensional hydroelasticity theories are briefly introduced. (3) Under the condition that only the contributions of the first order velocity potentials and responses to the second order hydrodynamic forces are considered in the analysis, a frequency domain non-linear three-dimensional hydroelasticity theory for a floating body either traveling or stationary in waves is presented. The general formulations of the second order hydrodynamic coefficients appearing in the non-linear three-dimensional hydroelastic equations of motion of the floating body are also included. (4) The non-linear characteristics of the mooring system are discussed. The linearized III Abstract representations of the mooring effects, together with the stiffness matrices respectively for symmetrically disposed tensional and gravitational mooring systems are introduced. (5) The linear and non-linear three-dimensional hydroelastic equations of motion of the moored floating system in frequency domain are presented. Based on the computer program suite of linear three-dimens...