MARINE ENVIRONMENTAL DESIGN PARAMETER ESTIMATION WITH PLACKETT COPULA DISTRIBUTION FOR OCEAN PLATFORM

The bivariate and trivariate distributions of wave height, wind velocity and current velocity are constructed with Plackett Copula, respectively. Different load combinations for these three variables are obtained in three ways: single variable design method, conditional distribution method (including both mode and mean value cases) and the trivariate joint distribution method. The base shearing force of a jacket platform is calculated and the joint return periods of different combinations are estimated. Results show that the computed base shear forces by the first method are obviously larger than others, and the corresponding return periods are larger than the joint return period. The results by the second method change with the choice of the predominant variable. Wave height contributes much more than both wind velocity and current velocity to the product of the platform base shear force. In the cases of considering current velocity or wind velocity as a predominant factor, the platform base shear is relatively smaller. The trivariate distribution constructed with Plackett Copula, considering the correlations among variables, can be used to estimate the design parameters of ocean environmental conditions. Compared with the single variable design method, the estimated design parameters can decrease and lead to lower platform construction cost, which will be suitable for the exploitation of a marginal oil field.