Linus Wrang

At the stage of design load analysis for offshore installations such as wave energy systems, a critical step is the determination of environmental cases to be evaluated for the definition of the characteristic design load. A commonly used methodology for load case selection, applied in multiple studies and recommended by technical specifications and guidelines, is the environmental contour approach. Here, 50-year environmental contours were generated for four study sites located in the North Sea, Skagerrak and the Baltic Sea by considering both observations and hindcast (model) data. For the construction of the contours, the well-established inverse first-order method (I-FORM) and a modified version using principal component analysis (PCA) were both examined. Furthermore, a 2-dimensional peaks-over-threshold (2D POT) method was evaluated. It was found that a version of the regular I-FORM was able to produce satisfactory contours which properly accounted for the highest waves. When using PCA, the dependency in the data was not properly captured by the probability functions under consideration. The 2D POT method, where applicable, was found to underestimate the extreme sea states. Comparisons between contours obtained from observations and hindcast data showed that the contours may differ substantially depending on the site and method, and thus care must be exercised when using hindcast data for such purposes.

Here, accessibility to near-shore and offshore marine sites is evaluated based on wave and ice conditions. High-resolution third-generation wave model results are used to examine the operation and maintenance conditions for renewable energy sources with a focus on wave energy. Special focus is given to the wave field and ice characteristics for areas within the Swedish Exclusive Economic Zone including analysis of return levels for extreme values for significant wave height, which provides guidance for dimensioning wave energy converters. It is shown that the number of weather windows and accessibility are influenced by distance from the coast and sea-ice conditions. The longest waiting periods for the closest weather window that is available for Operation and Maintenance (O&M) is in ice-free conditions shown to be strongly correlated with the fetch conditions. The sheltered Baltic Sea is shown to have very high accessibility if marine infrastructure and vessels are designed for access limits of significant wave height up to 3 m. In the northern basins, the waiting periods increase significantly, if and when the ice-conditions are found to be critical for the O&M activity considered. The ice-conditions are examined based on compiled operational sea-ice data over a climatic time period of 34 years. The results are location specific for the Swedish Exclusive Economic Zone, but the analysis methods are transferable and applicable to many other parts of the world, to facilitate assessment of the most promising areas in different regions.