Offshore wind energy is of special importance in order to meet the ambitious goals to produce climate-neutral energy. Therefore, an accelerated installation of offshore wind turbines is required. The design is to be achieved with respect to standards and guidelines. Especially probabilistic design methods allow an accurate and economic structural design.Not only the environmental conditions vary during the lifetime, but the short-term loads are also subject of random scattering. For the design of offshore wind turbines, the required load simulations are usually carried out in time domain. In comparison, it is less time-consuming to obtain loads by means of frequency-domain analysis. This is very beneficial for the probabilistic design which requires significantly more load simulations in time domain. However, non-linearities and time-variant behaviour of offshore wind turbines cannot be represented well during the load simulation in frequency domain. Extreme loads and fatigue loads can be calculated by means of frequency-domain analysis.The determination of the distribution functions of extreme values is well established on a theoretical background. As for the fatigue design, different empirical models exist which describe the distribution function of fatigue loads on the basis of frequency-domain analysis. In this thesis, a new model is introduced which leads to more accurate results.Since frequency-domain analysis is not always suitable, the transformation of signals given in frequency domain is required to generate respective random time series. As for the design of offshore wind turbines, only limited recommendations are stated in standards on how to carry out this transformation. Detailed analysis shows that accurate results with respect to wave-induced loads are also obtained for coarser discretisation of spectra. The resulting loads and their statistical properties are still accurate, while the numerical effort can be reduced in comparison to the stated recommendations.On the basis of theoretical findings, time series from load simulations of offshore wind turbine are analysed regarding their spectral properties. Investigations are carried out to evaluate the agreement between the extreme load and fatigue loads which are either simulated or calculated on the basis of the spectral properties. It is also shown that currents within sea states lead to increased fatigue loads.