The Investigations Of Occurrence Probabilities Of Freak Wave In Multi-Directional Irregular Wave Groups

Freak wave is a special ocean wave with disasters. With the increase of human activities about ocean, damages of freak wave on ships and structures such as offshore platforms is more and more. And gradually it has been received greatly attention, but due to the complexity of freak waves. And the complete observation records are relatively little, so far its generation mechanism and properties, especially its probability of occurrence is still not very clear. Although many scholars have done a lot of research, and obtained a lot corresponding results. But most of them are based on two-dimensional wave, while actual wave is three-dimensional, not only composed of different frequencies, but also composed of different direction. Therefore this thesis will conduct the experimental study on the occurrence probability of freak wave and its effect factors in Multi-directional Irregular Wave GroupsThis paper has considered the different directional spreading and relative depth, and conducted a long-playing physical modeling for Multi-directional Irregular Wave Groups. The statistical parameters (such as significant height skewness、kurtosis and spectrum) of Wave Groups, and wave height、crest and trough of probability distributions are analyzed. And it studied the effects of wave steepness、directional spreading and relative depth for the statistical parameters of the wave train and probability distributions. The results show:First, it is found from the experiment for freak wave about the different directional spreading. As the wave tend to uni-direction, the more generation of freak wave is based on the wave instability from self-focusing. And as the directional spreading is wider, especially uniform distribution, the more generation of freak wave is based on the convergence between waves. Second, the directional spreading and relative depth has obvious effects for the statistical parameters of the wave train. For a relatively large depth,as the directional spreading is narrow, due to the instability of nonlinear wave, with the wave propagation, kurtosis has a tend to increase, and the frequency of spectrum appears down and high-frequency energy is reduced. As the relative depth is shallow, the third-order nonlinear is suppressed, with the wave propagation, kurtosis has a little change, and the frequency of spectrum even appears on. Third, the directional spreading and relative depth also affect the wave heights crest and trough of probability distributions. For a relatively large depth,as the directional spreading is narrow, the end of wave height、crest and trough of probability distributions increase, with the wave propagation and significantly increased. As the relative depth is shallow, the third-order nonlinear is suppressed, and second-order nonlinear play a major role. The change of the directional spreading has no obvious effects for wave height crest and trough of probability distributions.Through the analysis of the statistical parameters and probability distributions of the Irregular wave groups. The results show, for a relatively large depth,as the directional spreading is narrow, due to the wave instability, the probability of occurrence of the big wave height will increase. Therefore, freak wave is more likely to occur in such a sea state, in the design of the offshore structures, it should fully take into account the impact of sea conditions on the distribution of wave heights.