Converging Profile Relationships for Offshore Wind Speed and Turbulence Intensity

Abstract. This paper reduces uncertainty in the quantification of offshore wind speed and turbulence intensity. A primary application is estimation of extreme wind speeds for design of wind energy generation systems, including turbines and fixed or floating support structures. Results of significance to normal winds, for resource assessment, engineering design or operations, can also be derived. This research is part of wider efforts to bring together the long established, but traditionally separate, onshore wind energy and metocean technical disciplines. A range of industry standard relationships, including those from International Electrotechnical Commission (IEC) and International Organization for Standardization (ISO), are compared with an extensive set of met mast data collected offshore Northwestern Europe over recent decades. Analysis initially focused on over 1000 independent storm peak events identified within the 10 minute mean wind speed time series. Time series and coherent vertical profiles were subjected to detailed scrutiny and analysis, considering wind speeds at various averaging periods, turbulence intensity and gust factors. Most peak events were associated with neutral atmospheric conditions, so were well represented by the ISO Frøya profile, with shear close to the IEC power law exponent of α = 0.11. A new pragmatic framework for classification of wind profiles in terms of relative shear is outlined, bringing together key elements of the IEC and ISO standards. This relative shear framework, which extends over the full range of measured wind speeds, is related to atmospheric stability and used to distinguish different classes of ambient turbulence intensity. New empirical relationships to quantify offshore wind ambient turbulence intensity are described. This study highlights the critical role of turbulence intensity in the estimation of gust factors, with a range of relevant relationships from IEC, ISO and other sources assessed using the extensive set of measured storm peak events. A simple generalised form of gust factor relationship is adopted, with a coefficient that varies with averaging period. A similar, but distinct, analysis yields recommendations for estimating peak 10 minute mean winds from peaks in 1 or 3 hour mean winds. Finally, a simplified workflow for the estimation of extreme offshore winds is outlined.