Improved environmental contour methods based on an optimization of hybrid models

Abstract

Estimation of the extreme values of ship and offshore platform responses such as bending moment and mooring tension is important in terms of structural design. Among several such methods, long-term and short-term analyses normally have been used in the industry. In general, long-term relative to short-term analysis is more precise but more time-consuming, because it takes into account all observed sea states. The environmental contour method, which performs a short-term analysis under a few important environmental conditions with an accuracy similar to that of long-term analysis, has been used as an alternative approach. There are several joint distribution models of significant wave height and peak period for environmental contour lines, but it is not known exactly which model most accurately represents actual environmental conditions. A marginal distribution of Hs and a conditional distribution of Tp, which generally are recommended in the rules and standards, have difficulties in drawing contour lines depending on the field. For the three-parameter Weibull distribution of Hs, a proper parameter estimation method should be selected among the several methods according to the observed data, and the distribution parameter of Tp has too little data to fit the given model. Another marginal distribution model is a bivariate lonowe (LOgNOrmal and WEibull) hybrid model of Hs with two distributions that allow for easy and unique estimation of parameters from any data [1]. The process of obtaining the distribution parameters from the existing hybrid model is clarified through the optimization process presented in this paper. And outlier technique is adopted to reduce the influence of some atypical data caused by small samples and, thereby, improve the accuracy of the Tp fitted result. The contour line through this proposed procedure predicts the extreme value similarly to the three-parameter Weibull distribution model, but it can be rather easily drawn. It is shown that the procedure proposed in this paper can be an effective method for drawing of environmental contour lines using any observed sea states.