While practicing the load cases in the publication "Design Load Basis for Offshore Wind Turbines - DT Wind Energy Report No. E-0133", a question came to me when
I tried to simulate the DLC15 regarding the EWS for offshore wind turbines. To my understanding, the EWS wind condition is a transient scenario describing the wind shear suddenly changing from a normal wind profile (NWP) to a nose-like profile and then going back. Why is the EWS condition given by Equations (26) and (27) IEC 61400-1? In IEC 61400-1, the NWP is assumed to have a power index of 0.2; however, the NWP for the offshore wind turbines should be 0.14. The value 0.2 should be for onshore turbines. Please correct me if I misunderstand the description of the DLC15 case.
The assumption of a power index of 0.2 for offshore turbines is based on empirical observations and analysis of wind turbine performance data. This assumption is commonly used in the industry and is known as the "EWS condition," which stands for Extreme Wind Shear condition.
Wind shear refers to the change in wind speed and/or direction with respect to height. In the case of offshore wind turbines, the wind shear is often more pronounced compared to onshore sites due to various factors such as the roughness of the water surface, the presence of nearby structures (e.g., offshore platforms), and the atmospheric stability over the sea.
The power index is a parameter that quantifies the sensitivity of the wind turbine power output to changes in wind speed. It describes how the power output of a wind turbine increases with increasing wind speed. Empirical studies have shown that the power index tends to be lower for offshore turbines compared to onshore turbines.
A power index of 0.2 implies that the power output of an offshore wind turbine increases with the cube of the wind speed. This is a conservative assumption that takes into account the higher wind shear and the resulting larger variations in wind speed experienced by offshore turbines. By assuming a lower power index, the design of offshore wind turbines can account for the potential extreme wind conditions and ensure the structural integrity and operational safety of the turbines.
It's important to note that the specific value of the power index may vary depending on the offshore site and the turbine design. Site-specific data and wind resource assessments are typically conducted to determine the appropriate values for the power index and other design parameters for a particular offshore wind farm project.
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