A simple approach to account for seasonality in the description of extreme ocean environments

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Environmental contours are often applied in structural reliability assessment and design of ships and other marine structures. Such contours are a practical tool for identifying joint extreme values of environmental variables to determine critical design conditions for the environmental loads. This may be used in the analysis of extreme structural responses and is particularly useful for structures with complicated response behaviour where full long-term analyses are not feasible. Environmental contours are typically calculated based on a joint probability distribution function established for the relevant input parameters, for example significant wave height and wave period, and stationary conditions are often assumed when fitting such models. However, non-stationarities due to, e.g., seasonality or directionality will often be important and this paper presents a simple approach for accounting for such non-stationary ocean environments in the construction of environmental contours. The usefulness of this simple approach is demonstrated by an example, where seasonally varying environmental contours are constructed. Different environmental contours are estimated for every week of the year and it is shown that these contours vary considerably between weeks. Contours corresponding to some average annual condition can also be made and compared to contours established without regard to seasonal variation.