On the basis of hindcasted data, a Poisson Bivariate Log-Normal Distribution is proposed to calculate the joint probability of extreme wind velocity and wave height simultaneously occurred during the storm processes. The examples denote that the proposed method can show the combined effect of the environmental loads on marine structures. The estimated 100-year and 50-year return base shears respectively decrease about 8.9% and 10.0% of those by the traditional procedure. The new model is clear in the sense of probability and suitable to the statistical analysis of the storm processes in the Bohai Bay. Compared with traditional method, the joint design criteria of environmental loads calculated by PBLND are recommended to offshore structure design in Bohai Bay.
Reasonable determination of environmental conditions plays a crucial role in the design and the construction of ocean engineering. For example, the steel quantity in use for building some platform in the Bohai Bay, China will decrease by 50% as the design wave height reduces from 10m to 8m (Sun, 1989). The design force for many offshore platform types is dominated by waves and a 50 or 100 year return period wave height is a common design level condition. By extension, one could combine 100-year wind with 100-year wave and current. This traditional method is, however, unduly conservative since the largest wind, wave and current values in 50 or 100 years are not expected to occur simultaneously. The exploitation of oil and gas in the Bohai Bay shows that a large amount of oil fields are marginal. Overestimated design environmental conditions make some of them to be abandoned due to higher costs. At present, more and more researchers have concentrated on the study of combined design criteria of environmental conditions