Advanced analysis of wave data for long-term statistics and wave energy exploitation

The thesis deals with wave data analysis for the long-term statistics of sea storms and wave energy resources estimation. The correct evaluation of extreme values of significant wave height is one of the most important areas of scientific interest in maritime engineering because of its relevant contribution to the design stage of maritime structures and wave energy devices. The thesis gives an overview of the various methodologies employed in extreme values analysis of wave height focusing on the “Equivalent Storm Models”. Belonging to this category are the Equivalent Triangular Storm (ETS) and Equivalent Power Storm (EPS) models. They enable us to analyze storms by representing them through a simpler geometric shape associating to each actual storm a statistically equivalent one, defined by means of two parameters: the first representative of storm intensity and assumed equal to the maximum significant wave height in the actual storm, the latter representative of storm duration, determined assuming that the maximum expected wave heights in the actual and equivalent storms are the same. They provide a solution for the return period R(Hs>h) of a storm whose maximum significant wave height is greater than a fixed threshold h, and for the mean persistence Dm(h) above h. In the case of ETS model the solution of both R(Hs>h) and Dm(h) are achieved in a closed form, while the solutions for EPS model have to be solved numerically. The EPS model gives a more realistic representation of storms affording more conservative predictions. In the thesis a new model called Equivalent Exponential Storm Model is developed in order to combine all together the advantages of the previous ones. Several interesting results are obtained applying the new model. Furthermore the thesis presents various original analyses performed applying the three “Equivalent Storms Models” and processing different kinds of wave data. The variability of parameters of intensity and duration of equivalent storms with the assumption of different time interval between two consecutive records is investigated and its influence on long-term predictions is evaluated. A sensitivity analysis of return values to storm threshold is performed applying both the EES and Peak Over Threshold methods. Furthermore, considering the importance of wave direction when an angle dependent structure or device has to be designed, the thesis addresses the problem of directional analysis, providing a criterion to classify storms as “directional storms” with wave direction pertaining to a given directional sector. The introduced methodology concerns also the identification of the appropriate center and width of the sector. Another analysis performed in the present work deals about the long term statistics of ocean storms starting from time series of partitioned sea states, considering separated wind and swell seas. The ETS model is applied for long term predictions to time series of significant wave height calculated considering both contributions, and the wind sea only, in order to evaluate variability of return values due to having neglected swell contribution. Concerning the estimation of wave energy resource, in the thesis a simplified formula for the calculation of average wave power in deep water is applied for wave energy mapping of Mediterranean Sea in parallel to extreme values mapping, showing how the conjunction of these two information is fundamental at the design stage of wave energy device.