The Occurrence Probability Of Freak Waves In Uni-directional Irregular Wave

Freak wave is one of ocean disasters with great damage. It leads to damage of ships and offshore platforms and deaths. Recently the freak wave has been received greatly attention and there is a lot of research about its generation mechanism and properties. But there are also many shortcomings, especially its probability of occurrence and the effect factors are obscure. Therefore this thesis will conduct the experimental study on the occurrence probability of freak wave and its effect factors in Uni-directional Irregular Wave.This article first provide a brief introduction on the domestic and foreign research of freak wave. The Rayleigh distribution and other theoretical wave heights distribution are reviewed briefly. Some of these wave heights distribution models are the modification of Rayleigh and other distribution models are presented based on the second order approximation or considering the effects of high-order nonlinearities and comparative analysis of advantages and disadvantages.Secondly, considering the generation mechanism of freak wave, long time series of wave trains are experimentally simulated on the occurrence probability of freak wave based on Benjiamin-Feir instability. The Benjiamin-Feir index (BFI=2^1/2ε/（2Δf/fp） is an important index and it reflects the instability of wave trains. The irregular wave with different instability can be acquired in the experiment through adjust different value of wave steepness ε, spectral bandwidthΔ f, the peak frequency of spectrum fp. The statistical properties of random train are investigated, such as the evolution of the significant wave height, Skewness and Kurtosis. Experimental results show that the Benjiamin-Feir instability is a main generation mechanism of freak wave. The results show that the kurtosis is significantly dependent on BFI and it increases obviously along the flume when BFI is larger. The freak waves are observed in random wave trains. They occur frequently than expected especially for wave trains with larger BFI. The generation and development process of freak wave in random wave are studied. The development and evolution of the statistical characteristics of wave train are analyzed as the freak wave propagate along the tank, such as spectrum, freak wave profile, characteristic wave period and wave height.Finally, the occurrence probability of freak wave and the relationship between freak wave occurrence probability and characteristic parameters are especially analyzed. The distribution of experimental wave heights are analyzed and are compared with some representative theoretical wave heights distribution. The results show that BFI have a significant impact on the distribution of big wave. The bigger for the BFI the greater probability of the freak wave happens. The spectral period and spectral peak factor do not much affect the occurrence probability of freak waves.The paper reaches a conclusion that the BFI is an important parameter that affects the occurrence of freak waves. The strong unstability obviously change the statistical parameters of wave train during wave transmitting and are reflected in the increasing of kurtosis and the occurrence probability of freak wave.At the same time, it can be seen that it has a strong correlation between the occurrence probability of freak wave and kurtosis. The existing probability distribution of wave height have shortcomings to analyze the extreme waves. Thus the influence of kurtosis and BFI must be taken into account for the accurate extreme wave height distribution model.