Wind load assessment in marine and offshore engineering standards

Wind is a major environmental load component on offshore engineering structures. It is particularly relevant in the case of offshore wind turbines. These complex structures are commonly designed according to relevant international marine and offshore engineering standards. The calculation procedure recommended in these standards is currently based on various approximative methods, which may yield substantial underestimations in wind load assessment and structural response. In the current standards, wind is recommended to be taken as quasi-steady, represented by a simple empiric power-law approximation of the mean wind velocity profile without taking into account a myriad of relevant parameters representing atmospheric turbulence as it appears in nature. A critical perspective on approaches to wind load assessment recommended by current international marine and offshore engineering standards is thus provided in the present study. The standard, rule-based procedures, recommended by relevant international marine and offshore engineering standards, and realistic wind data were used as inflow conditions to calculate environmental loads (wind, wave, current) acting on a standard 5 MW monopile-supported offshore wind turbine fixed to the bottom of the sea. The structural response of this wind turbine was calculated using both these approaches. Substantial differences in the results were observed, particularly in the case of the axial displacements of the tower top as well as the forces and moments. These differences are reflected in load magnitudes, standard deviations, and variances. The largest discrepancies of nearly 60% are in the downwind displacements of the tower top and the pitching moment, while their standard deviations differ by more than 550% and 205%, respectively. Significant errors in the calculation of the wind turbine limit state and fatigue life may thus be expected when designing offshore wind turbines according to the provisions of the current international marine and offshore engineering standards. This indicates a clear need for further improvements and a revisiting of these standards in order to provide accurate recommendations regarding wind load assessment of offshore engineering structures.