Research On Ringing Of Offshore Structures Under Extreme Waves By Numerical Wave Tank

With the development of harbor, coastal and offshore engineering, more and more coastal and offshore structures are being or will be constructed in deep water. These offshore structures may encounter rough sea climate when extreme waves act on the offshore structures and may induce high frequency resonance response – Ringing. This response has been observed in offshore systems, it may destroy the structures. By far the mechanism of Ringing has not been fully understood yet. The study of ringing phenomenon involves the security, duration and design criterion of coastal and offshore structures, so it is valuable to carry out this research. In this thesis, systematic study, from the generation of extreme waves to the simulation of ringing response of offshore structures due to extreme waves, is carried out through simulation in numerical wave tank. The main work and results are as follows:1. A two-dimensional BEM-based numerical wave tank is set up. Then, the wave focusing method is adopted to generate an extreme wave in this numerical wave tank. The characteristics of extreme waves are systematically analyzed. The numerical results are in good agreement with the test results at laboratory. By comparing with the freak waves in the field, it is showed that extreme waves in real sea can also be simulated in the numerical wave tank. 2. To predict the extreme wave forces, slender body theory is applied to calculate the wave moment on small diameter cylinder in extreme wave. The Fourier spectrum of the wave moment shows that high frequency components will appear and they may cause ringing.3. A simplified oscillation model of small diameter cylinder is set up. Ringing responses of small diameter cylinder under the action of extreme waves are simulated and analyzed. The analysis of the simulation result indicates that the extreme waves are strong nonlinear and asymmetry, and they include high frequency components, so extreme waves can induce high frequency resonance response.4. At last, properties of the extreme waves and the response of small diameter cylinder are analyzed by using wavelet analysis method. The ringing mechanism is disclosed through the spatial and temporal transformation of wave energy and the spectrum of cylinder response. It is showed that in the evolution process of extreme waves, besides wave energy convergence due to wave focusing, high frequency energy is generated due to wave-wave nonlinear interaction.