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For substructures of offshore wind turbines, the fatigue limit state is in most cases a decisive design factor. However, to calculate the fatigue lifetime of wind turbines, numerous time domain simulations for different load cases with changing environmental conditions are necessary. According to the state of the art, wind speed bins (of 2 m s −1 ) are employed, while keeping all other environmental states constant. However, assuming constant parameters in each wind speed bin is an unfounded simplification. Therefore, in this study, methodologies for fatigue assessment considering scattering environmental conditions are investigated by assuming statistical distributions for environmental conditions for all wind speeds that are derived using real data measured at the North Sea research platform FINO3. These statistical distributions are used to conduct time domain simulations of an OC3 monopile-with a 5-MW wind turbine-using the aero-servo-hydro-elastic simulation framework FAST. The fatigue lifetime is calculated, and its uncertainty due to finite sampling is assessed. It is shown that if scattering environmental states in each wind speed bin are applied, the uncertainty due to finite sampling is significant. Furthermore, only some wind speed bins contribute to the overall fatigue damage. Based on these findings, in a last step, different Monte Carlo sampling concepts are investigated to reduce the number of simulations needed to calculate the fatigue lifetime with a defined uncertainty. By combining several wind speed bins and by sampling according to the damage distribution, it is proved that the number of simulations can be reduced by more than 30% without increasing the uncertainty. KEYWORDSfatigue limit state, offshore wind energy, scattering environmental conditions, simulation error, uncertainty INTRODUCTIONThe share of offshore wind energy in overall energy production has grown over the last years. However, the cost of offshore wind energy is still high compared with other renewable energies. 1 As it is forecast that substructure improvements can potentially reduce the overall turbine cost by more than 5%, 2 an accurate and reliable simulation of offshore wind turbine substructures is beneficial. For substructures made of steel, mainly the fatigue limit state is decisive. 3 To calculate the fatigue lifetime of wind turbines, numerous time domain simulations are necessary. Current standards 4 define that these simulations should mirror the changing environmental conditions at the precise site of a wind turbine. However, for research purposes, these data are scarcely available, and even in industrial projects, the data quality is frequently poor because information on some parameters or long-term data is missing. This is why several research projects characterized environmental conditions at specific sites or entire areas and published statistical distributions as a reference. Examples for data bases of main conditions (wind speeds, wave height, wave period, and wind and wave directions) are the UPWIND desi...

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Abstract.For the design and optimisation of offshore wind turbines, the knowledge of realistic environmental conditions and utilisation of well-founded simulation constraints is very important, as both influence the structural behaviour and power output in numerical simulations. However, real high-quality data, especially for research purposes, are scarcely available. This is why, in this work, a comprehensive database of 13 environmental conditions at wind turbine locations in the North and Baltic Sea is derived using data of the FINO research platforms. For simulation constraints, like the simulation length and the time of initial simulation transients, wellfounded recommendations in the literature are also rare. Nevertheless, it is known that the choice of simulation lengths and times of initial transients fundamentally affects the quality and computing time of simulations. For this reason, studies of convergence for both parameters are conducted to determine adequate values depending on the type of substructure, the wind speed, and the considered loading (fatigue or ultimate). As the main purpose of both the database and the simulation constraints is to compromise realistic data for probabilistic design approaches and to serve as a guidance for further studies in order to enable more realistic and accurate simulations, all results are freely available and easy to apply.

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