AbstractFloating offshore wind energy is a new form of marine renewable energy which is attracting a great deal of attention worldwide. However, the concepts of floating offshore wind turbines (FOWTs) are still in early stages of development and their failure properties are not yet fully understood. Compared to bottom-fixed wind turbines, FOWTs are subject to more extreme environmental conditions and significant mechanical stresses which may cause a higher degradation rate and shorter mean-time-to-failure for components/structures. To fill the research gap, this paper aims to conduct qualitative and quantitative failure studies on an OC3 spar-type FOWT platform with 3 catenary mooring lines. The failure analyses are performed based on two well-established reliability engineering methodologies, namely, fault tree analysis (FTA) and failure mode and effects analysis (FMEA). The most critical FOWT components are prioritized according to their failure likelihood as well as the risk-priority-number. Our results show a good agreement between the two methods with regard to failure criticality rankings. However, some differences between the results are also observed that are attributed to the difference between FTA and FMEA methodologies as the former incorporates the causes of various failure modes into analysis, whereas the latter is mainly adopted for a single random failure analysis. The results obtained from the FMEA study for the FOWT system will also be compared with those reported for bottom-fixed offshore wind turbines and some interesting conclusions are derived.