In the fast-growing interest of floating offshore wind turbines (FOWTs), design standards related to the met-ocean conditions have become a critical aspect to minimize uncertainties in the design of FOWTs. The widely used standards for offshore wind turbines available are the IEC 61400-3, 2009 and DNV-OS-J101, 2014. Not until recently, IEC has published the latest design requirements specific for FOWT, the IEC 61400-3-2, 2019. Even so, within the authors' present knowledge, this most current standard does not change much in content, particularly true for the design turbulent wind models, which is the focus of our study. The current available standards do not elaborate a thorough prescription on turbulent wind model suitable for offshore environments, although some turbulent wind models are recommended. In the IEC 61400-1, two turbulent wind models are given: Mann uniform shear model and Kaimal spectra and exponential coherence model. Both models were derived to fit neutral atmospheric stability conditions and have equal spectral energy content of the along wind velocity component (). The distinct characteristic between the two model is that how the coherence of the wind components are defined. A previous study found that the two models given in the IEC standards resulted in significant difference of yaw motions on a spar wind turbine, which leads to the hypothesis that the wind coherences were the main cause for this difference. Based on this finding, we simulated similar case with variation in the input parameters of the two turbulent wind models on a spar wind turbine. We found that by using the two models, the different loads and motions response of the spar wind turbine are influenced by wind coherence to some extent but not significant. Furthermore, we also observed that another important factor affecting the uncertainty in loads and motions of the spar wind turbine is the energy content of the along wind velocity component which resulted a 60% difference tower top yaw loads between the maximum and minimum load case.

Another issue addressed with the use of turbulent wind model given in the current standards is the absence of atmospheric stability conditions consideration, as it is generally known that unstable conditions are of importance when one consider offshore wind environment. From stable to neutral to unstable conditions, increases, especially at low frequencies. The fact that unstable conditions have higher compared to neutral conditions would add uncertainty in the loads and motions prediction of a FOWT. To further investigate the influence of on the loads and motions of a spar wind turbine, we studied and simulated Højstrup spectra. The Højstrup spectra was derived to fit unstable atmospheric stability conditions by adding low-frequency component to the Kaimal spectra. From the study, we observed two major findings: first, that the was proven to increase progressively as the stability conditions move toward more unstable. Secondly, there is an increasing trend in loads and motions of the spar wind turbine from neutral to very unstable stability conditions, given that the same input for wind coherence was applied. Up to 60% difference tower top yaw loads were noted between neutral and very unstable conditions.

From our present studies, we concluded that both and wind coherence have significant influence on a spar wind turbine loads and motions, especially for tower top yaw loads. Although the influence of latter is less, it is still being investigated in our on-going study.