This thesis tests a lumping block method on accuracy in broad and narrow wave spectra. The method aims to construct a robust and efficient alternative for the computationally expensive time domain fatigue calculations of mooring lines in the early design stage of a floating offshore wind turbine (FOWT). A robust way of modelling must be applicable and reliable in all possible sea states, including extreme wavelengths, nonlinear waves, and dynamic amplification caused by the resonance frequencies. The method’s reliability for different wave spectra is validated by comparing the proposed numerical lumped time-domain simulations with the original numerical time-domain simulations.

The lumping block method gives accurate damage results for narrow wave spectra. However, the model’s reliability decreases when the broadening of a wave spectrum causes damage-increasing conditions, like outliers in wave periods and nonlinear waves. The lumped broad wave spectrum simulations underestimate a mooring line’s structural damage, especially for wave periods above resonance. Moreover, the non-Gaussian wave-induced surge responses are the most critical parameter for reliable mooring line fatigue calculations. They must be considered for simplifying fatigue mooring line calculations in general. Furthermore, the non-Gaussian responses cause the underestimation of the proposed lumping block method for the broad wave spectra.

Nevertheless, the proposed method yields accurate but slightly conservative damage results and is robust and efficient in fatigue calculations for sea states corresponding to a narrow spectrum. Hence the lumping block method is safe and usable for significant parts of the North Sea since only middle or narrow spectra occur.