Offshore wind is an important source of renewable energy and is steadier and stronger than onshore wind. Offshore areas not only have strong winds but also contain other potential renewable energy sources, such as ocean waves and tidal currents. Therefore, it is interesting to investigate the possibility to utilise these energy potentials simultaneously, particularly the combination of wind and ocean wave energy due to their natural correlation. For this reason, previous researchers have examined the use of a floating wind turbine (FWT) and a wave energy converter (WEC) on a single platform (Aubault et al., 2011; Peiffer et al., 2011; Soulard and Babarit, 2012). In this paper, a combined concept involving a spar-type FWT and an axi-symmetric two-body WEC is considered and denoted as STC. With respect to operational conditions, a previous study (Muliawan et al., 2013) indicates that the STC not only reduces the total capital cost but also increases the total power production compared to the use of segregated FWT and WEC concepts. As with other floating systems, the STC must be designed to ensure serviceability and survivability during its entire service life. One of the design criteria is the ultimate limit state (ULS), which ensures that the entire STC system will have adequate strength to withstand the load effects imposed by extreme environmental actions. Therefore, in the present study, coupled (wave- and wind-induced response mooring) analysis is performed using SIMO/TDHMILL in the time domain to investigate such responses of the STC system as mooring tension, spar-tower interface bending moment, end stop force, and contact force at the Spar-Torus interface under extreme conditions. Environmental conditions that pertain to the northern North Sea metocean data are selected and include operational, survival and 50-year conditions. Finally, the ULS level responses that are capital cost indicators for both FWT alone and for the STC system are estimated and compared.