Freak Wave Simulation And Its Interaction Characteristics With Marine Structure

To satisfy the increasing demand of oil resources,exploitation of oil and gas resources in deep water is receiving increasing attentions from all over the world.As a result,a variety of floating structures designed for the deep sea environment have been constructed.Thus,it becomes an important issue for researchers in ocean engineering field to ensure the safety of these structures and the workers on them.Note that extreme waves,such as freak waves,could probably result in extremely large loads on marine structures,thus representing great threats to the safety of marine structures.In the present paper,for the safety of marine structures in extreme sea states,investigations are conducted on the simulations of freak waves,nonlinear freak wave forces,and motion and dynamic responses of floating structures caused by freak waves.Generation models of freak waves are built on the basis of different hypotheses regarding freak wave formation and are implemented in both physical wave flume and numerical wave tank.To improve the generated freak waves,phase iteration is successfully implemented to narrow the gap between the generated waves and the target wave trains.An optimization method for the focal position is introduced to adjust the focal position to the target location.The wavelet transform method is then adopted to reveal the variations of wave energy constructions during the formations of freak waves.Furthermore,the particle velocities are measured using the particle image volocimetry(PIV)technique.Predictions on the wave velocity profiles based on various wave kinematics models are conducted and compared with the measurements.Fully nonlinear numerical wave tank(NWT)is established to obtain abundant information on the flow fields of freak waves.The influences of the mesh scheme on the simulation of freak waves are deeply investigated.On this basis,the reflection superposition model of freak wave is proposed and implemented in the NWT.Wave reflection effects on the freak wave formation are investigated,revealing that the wave reflection could probably be a factor of freak wave formation,especially in harbor,shallow water region.A series of experiments on freak wave forces are performed to investigate their nonlinear characteristics.Available prediction methods on wave force are proved to be insufficient in predicting the wave forces due to freak waves.Dimensionless analysis on the peak forces of freak waves is performed.A wavelet-based synchronization method between the calibrated wave and wave force train is introduced in the absence of reference information.Wavelet-based transfer functions are proposed to reveal the relationship between wave forces and freak waves during transient processes.The wavelet spectra and the wavelet auto-bicoherence spectra are adopted in studying the global and local energy constructions of freak wave forces,clarifying the contributions of high-frequency components to the peak forces and the sources of high-frequency components.On the basis of the Morison equation,various types of wave force prediction models are introduced by combining different wave kinematics models and inertia force coefficient schemes.The effects of viscous force,diffraction,and transient wave elevation on the nonlinear wave forces are examined and the contributions and sources of nonlinear wave forces are demonstrated.The prediction model based on the modified Wheeler model and frequency-dependent inertia coefficients is recommended for the predictions of freak wave forces.The effects of freak waves on the motion and dynamic responses of a moored semi-submersible are experimentally investigated.Two mooring systems are employed to reveal the effect of mooring stiffness.Above all,this paper achieves the generation and optimization of freak waves based on different hypotheses of freak wave and proposed a new reflection focusing model for freak wave formation.Moreover,a series of analysis approaches are developed based on wavelet transform and the wave force prediction model for freak wave was achieved.The effects of freak waves on the motion and dynamic responses of a moored semi-submersible were investigated and the relevant influences were clarified.These findings and achievements are of significant value to both the academic and industrial circles.