A long-term joint probability model for metocean circular and linear characteristics

Structural performance analysis of offshore structures extremely depends on the smart selection of design environment. In this paper, a joint probability model is developed to provide the designers with the joint probability of occurrence of all possible sea states including their circular and linear variables. The presented Joint PDF consists of a marginal PDF for circular variable and two conditional PDFs for linear variables. The marginal PDF for wave direction as a circular variable is a mixture of von-Mises Fisher distribution and the conditional PDFs are assumed to be a conditional Weibull distribution for wave height and a conditional lognormal distribution for wave period. An extreme value analysis is also applied to make sure that the model covers all possible ranges of wave circular and linear parameters in the desired lifetime. Design scenarios can be chosen from the highest probable combinations of wave height, period and direction. The proposed method was applied on Caspian Sea 45-years and Persian Gulf 11-years hindcast data. The results indicate the acceptable accuracy and practicality of this methodology for the directionality dependent design approaches.