There are several challenges facing the design of mooring system of floating offshore wind turbines (FOWTs), including installation costs, stability of lightweight minimalistic platforms, and shallow water depths (50–300m). For station keeping of FOWTs, a proper mooring system is required in order to maintain the translational motions in surge and sway and the rotational motions in yaw of the platform within an adequate range. A combination of light pre-tension, shallow water depth and large platform motions in response to a survival storm condition can result in snap-type impact events on mooring lines, thus increasing the line tension dramatically. In this paper, we present a new snap load criterion applicable to a catenary mooring system and compare it with Det Norske Veritas’ criterion for marine operations. As a case study, we examine the extreme tension on a catenary mooring system of a semi-submersible FOWT exposed to a 100-year storm condition. The software OrcaFlex was used for numerical simulations of the mooring system. NREL’s FAST software was coupled to OrcaFlex to obtain aerodynamic loads along with hydrodynamic loads for FOWT analyses. Snap-type impact events were observed in the numerical simulations and were characterized by two criteria. Tension maxima were fitted using composite Weibull distributions (CWDs) and comparisons of probability exceedance were made for the two different snap load criteria.