This paper investigates the dynamic response of a fully nonlinear model of a DeepCWind floating offshore wind turbine (FOWT) after one of its three-catenary mooring systems is broken. The drift area of the platform, pitch motion of the wind turbine, and tension on the two ends of the mooring line are the main dynamic response foci; in addition, a single mathematical formula is provided in this study to predict the maximum drift in surge direction. After the platform reaches the new equilibrium position maintained by the remaining two mooring lines, the tower pitch exceeds 20 degrees. The tension change is closely related to the drift motion, necessitating an increase in the minimum breaking load (MBL) of the mooring line components. The mathematical forecast of the maximum surge shows good agreement with the numerical results, even with different water depths