Joint Return Values Calculation Of Environmental Conditions With MaximuEntroy Distribution For Offshor E Platform Design

Safety and economics are contradicted when the offshore engineering is designed, which depend on the cognition of the ocean environmental conditions. How to correctly predict the occurrence probability and the intensity of the ocean environmental conditions is essential and difficult. At present, the univariate probability analysis method is commonly used in China, in which each environmental load is estimated independently by long-term statistics distribution model, which results in higher design parameters and construction cost.Based on the maximum entropy distribution, the equivalent multidimensional maximum entropy distribution model is proposed to estimate ocean environmental conditions, such as wind, wave and current. The designed lifetime of the ocean engineering is considered firstly to determine the joint ocean environment return periods. The corresponding joint return values are solved by the equivalent multidimensional maximum entropy distribution which properly lower the ocean environment design criteria and ensure the reliability of different structural types of offshore platforms in China.The methods of obtaining the data are introduced in this paper, such as ocean observation methods and hindcasting models. The hindcasting models are proved to fit the observations very well by analyzing the samples in some sea area, from which the data are reliable to estimate the joint return value.Deduced by the physical theory, the maximum entropy distribution is more believable than other distribution models. By introducing the location parameter, the maximum entropy distribution can transform into many common distributions, which can be predicted that the maximum entropy distribution has a broader applicability than others. There are four parameters in the maximum entropy distribution comparing with two or three parameters in Pearson-III distribution, Gumbel distribution and so on, which means that the maximum entropy can fit observed data better. The experience fitting method and L-fitting method are deduced to solve the parameters of the maximum entropy distribution, which are suitable for estimation the environment conditions in engineering applications.The compound distribution --Poisson-maximum entropy distribution is described and proposed here to compare with the maximum entropy distribution. It was initially developed to model storm, associated with typhoon, which is extended here to model also wave, which from area subjected to frequent storm, like the North Sea. Application examples are given to estimate extreme typhoon significant wave heights observed at Nakagusukuwan Observation Station in Japan and significant wave heights of the northern North Sea. In these two examples, the occurrence probability of typhoon and the frequency of significant wave heights larger than the thresholds are fitted by Poisson distribution, which are considered as a kind of“storm”. The Poisson-maximum entropy distribution is proved that has advantages over the maximum entropy distribution in fitting random storm environment.Based on the maximum entropy distribution and the trivariate equivalent maximum entropy distribution, four methods are compared and analyzed to estimate the joint return value of ocean environment conditions in Bohai sea area. These four methods are the traditional univariate probability analysis method, the load return value method, the conditional probability distribution method and the multidimensional extreme value method. The mean of the secondary environmental conditions joint probability in the conditional probability distribution method is proposed firstly in this paper.It can be seen that, after considering the relation among wave height, wind speed and current speed, the smaller return values are calculated by the methods which are based on the equivalent trivariate maximum entropy distributions than the traditional univariate probability analysis method, which is based on the maximum entropy distribution. The traditional environmental return period is always 50 years or 100years which is lack of theoretical analysis. The designed lifetime of offshore engineering is considered to determine the design joint return period. After that, the design joint return values can be determined by various design objects.