The application of stability checks in simulated offshore wind structures is performed through tools that are established in the offshore wind industry. In particular, the structure should fulfill certain strength and fatigue criteria among which Fatigue Limit States (FLSs) checks are critical. In terms of the process, fatigue assessment can be carried out in both time (TD) and frequency domain (FD), with the former being more popular in the offshore wind energy sector. Nevertheless, simplified tools in the FD have been suggested, since they yield results similar to those of the TD analysis. In order to decide upon the choice of tool in performing the FLS estimations, a relative comparison should be implemented. Particularly, fatigue assessment in the TD, fatigue assessment in the FD and a simplified type of fatigue assessment in the FD are examined. These types of fatigue assessment are conducted in three respective tools, with the TD type conducted in NREL’s FAST v8 and both the FD types simulated within MATLAB tools. The tools are judged upon the desirable levels of Modelling & Simulation (M&S) fidelity. A set of criteria are defined for the particular use case, in an attempt to express that fidelity. The criteria’s metrics are additionally provided in the proposed methodology and associated measurements are taken during simulations. Only after conducting further multi-criteria decision analyses (MCDA), the eventual levels of fidelity for the three types of fatigue assessment result to a consequent ranking among the tools. Therefore, the measured criteria lead to the quantification of these levels of fidelity for all three tools and the application of MCDA results to their ranking. The proposed fidelity framework is applied in a case study in order to evaluate the proposed methodology, as well as to select the type of fatigue assessment and consequent tool that should be used within an early design stage. The results indicate that the simplified fatigue assessment in the FD should be preferred over conventional fatigue assessment in both TD & FD. In addition, the criteria included in the fidelity framework seem to provide a multifaceted approach, since none of the tools is favored in all categories. Finally, similarities in results between relevant types of conducted comparisons as well as between different types of MCDA, enhance the consistency of the proposed fidelity methodology and increase its credibility.