Suitability of offshore wind turbine design standards for America, Asia and Europe

Offshore wind characteristics are not well studied for offshore wind turbine (OWT) design due to insufficient measurements at offshore locations. The current OWT design standard IEC (International Electrotechinical Commission) 61400-3 Design Requirement for Offshore Wind Turbines uses wind conditions observed on land and over European waters to define the wind models for OWT design. While discrepancies between these models and observations have been found in Europe, the models' suitability in other areas is not known. This thesis aims to test the suitability of the wind models in IEC 61400-3 for OWT design in East Asia, Europe and the US using offshore wind data from these three regions. Four aspects of the wind climate that dictate fatigue and extreme loading are evaluated: wind speed distribution, wind shear, turbulence intensity and extreme wind speeds. Results include: 1) For a constant mean wind speed, as the Weibull shape factor k increases, the OWT fatigue damage estimate from IEC 61400-3 becomes more conservative. For k>2 IEC 61400-3 is conservative. 2) Observed wind speed profiles exhibit a considerable number of large shear events (m>0.14), particularly in the speed range 5-10 ms-1. One deterministic wind shear may not be conservative for all sites. 3) I propose a new model for turbulence intensity and wind speed based on empirical equations of Monin-Obukhov similarity theory. The model shows a better agreement with the observed relationship between turbulence intensity and wind speed offshore than currently included in IEC 61400-3. 4) Relative to the extreme wind speed with 50-year return period from the Gumbel distribution, the IEC 61400-3 standard definition (V50e=5*V ave) is generally conservative offshore in the world. However, IEC 61400-3 is not conservative in tropical cyclone prone areas.