Occurrence of Wind-Wave Misalignment using FINO and OBLEX data
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Wind-wave interaction has been an important aspect of environmental conditions to be considered for offshore wind turbine performance, especially in deeper water at stable atmospheric conditions such as North Sea. It is generally understood that ocean waves are thought to be influenced by the surface wind that transfers momentum from the atmosphere to the waves. However, recent observations have suggested that when swell persist with faster propagation than the surface wind, momentum from waves can also transfer to the atmosphere. It is not uncommon for the wind and waves to be misaligned, and this situation can result in interesting features that may have an impact on the power production, turbulence generation, and fatigue damage to the wind turbine. In general, there are misalignments between the wind and waves at all wind speeds: small misalignments at large wind speeds and large misalignments at lower wind speeds. The largest misalignments are associated with stable atmospheric conditions (Bachynski et al., 2014). Furthermore, Bachynski et al. (2014) using observations from the North Sea suggest that misalignment of up to 30° is common, while misalignment larger than 60° occur less than 5% of the time. In this thesis, MATLAB analysis of 11 years of data from FINO 1 and 8 years data from FINO 3 has been conducted to determine the frequency of occurrence of wind wave misalignment under swell and wind wave conditions. Waves are usually aligned with the wind. However, swell do not always correlate to the direction of the wind. In this study, the frequency of occurrences of wind-wave misalignment are studied and the effects of the aligned and misaligned wind-wave on distribution of wind speed have also been checked. Total misalignment and opposing misalignment for platforms FINO 1 and FINO 3 were observed higher in FINO 1. In the other hand, wind-wave misalignment under swell were found to occur less than the ones under wind wave. Turbulence intensities in each site was analysed according to the classification of wind-wave misalignment and it was found that the turbulence intensity under opposing wind-wave was lower compared to the perpendicular and aligned conditions. Wind profile at FINO 1 was observed as well, taking into account the lower heights measurement at OBLEX campaign. A 1-month period between September and October 2015 was selected based on availability of data. Observation by plotting mean wind speed data from FINO 1 and OBLEX resulted in wind profiles as expected. However, an interesting point to see is that the mean wind speed at the lowest measurement 15 m LAT is higher than the wind speed at 20 m LAT which may well get an accelerated wind speed because of the position of sonic anemometer at 15 m height relative to the position of sonic anemometer at 20 m.
Master's thesis in Offshore technology: Marine and subsea technology