Extreme response estimation for marine structures based on design contours and response surface methods

Design contour methods are frequently applied for large and complex structures in order to minimize the number of time-consuming structural analyses. These methods are based on first identifying extreme environmental conditions with a given probability of exceedance based on the relevant joint distribution function. These conditions are located along a level surface (i.e. "contour") in the space of load parameters. As the second step, response analyses are performed for a selection of extreme conditions which are located along this contour. This approach is based on a representation of the underlying stochastic processes in terms of a sequence of piecewise constant levels. In the present paper, the extreme environmental conditions which are obtained based on such a model are compared to those obtained by instead applying a continuous process model. Furthermore, various alternative and relevant definitions of design contours are compared for the two-component case. A procedure is subsequently introduced which allows the two process components to have different characteristic time scales. Extension of the contour concepts to higher dimensions and non-Gaussian processes is outlined. A particular example of a two-component wave-wind process is investigated. In addition to the design contours themselves, stress response levels obtained for a specific example is also considered.