| Abstract: | Three dimensional analytical procedures are developed for determining the wave-exciting and motion-induced hydrodynamic forces for fixed and floating ocean structures in a fluid of finite or infinite depth. The effects of structural flexibility are considered, and the solution of the hydroelastic problem is obtained by employing the direct boundary-element method for the fluid and the finite-element method for the structure. The fluid potential is expressed by means of a Helmholtz integral, which involves the normal fluid velocities at the fluid-structure interface. Upon discretizing the integral equation by using constant or linear elements and by enforcing the velocity continuity condition at the discrete points of the interface, the fluid potential is coupled with the elastic and inertia properties of the structure. Solution of this set of complex linear simultaneous equations then yields the potential function at the interface nodal points. All fluid and structural motions then can be readily calculated. A computer program based on this approach is developed to calculate the wave-exciting and motion-induced hydrodynamic forces for fixed or floating structures. First, it is applied to rigid structures. Comparisons of the results with those obtained by several previous investigators reveal good agreements. The relationship between the fluid-structure interaction force and the stiffness of the structure then is by examining the hydrodynamic forces and vibrational characteristics of a box-type structure with a flexible bottom plate in an infinite fluid. As expected, the interaction force increases with decreasing flexibility or increasing exciting frequency. Natural frequencies of the bottom plate with heaving motion are determined. Results show that the hydrodynamic load affects natural frequencies of the structure in both the fundamental and higher modes. |
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