Many approaches exist to build environmental contours, i.e. the ensemble of joint metocean parameters corresponding to a N-year return level. However, these contours may provide rather different curves according to their calculation process. In this work, the use of response meta-models is introduced for the inter-comparison of environmental contours for two application cases: the roll of a FPSO and the tension in a mooring line of a semi-submersible.
Some state-of-the-art methods available for modelling multivariate extremes are applied for two or more variables and in a directional context. Among the conditional models, joint parametric laws (such as Weibull, log-normal...), Heffernan and Tawn simulations and several copula are investigated. Two ways of contour building are considered in 2D and 3D: Inverse First Order Reliability Model (I-FORM) and physical-space Huseby contouring method.
The comparison between the resulting design points are presented for both applications. The combined wind, wave and current parameters statistics (including directions) are presented, in addition to the extreme loads obtained from the meta-models. The advantages and drawbacks of each method are reviewed. Some key findings are finally presented, pointing out the interest of a meta-model to determine the more realistic set of design points in function of the structure response of interest.
