Floating offshore wind turbines (FOWT) have been increasingly deployed to harvest the sustained high-speed offshore wind in recent years. Under the complex offshore environments, FOWT could experience significantly different environmental conditions with various occurrence frequencies during their lifetimes. In the present study, the dependence structure of six wind and wave-related environmental parameters (wind direction, wind speed, mean wave direction, significant wave height, peak spectral wave period, and wave directional spread) is established using a C-vine (canonical vine) copula model. Different families of bivariate copula are used as the building blocks to model the interdependence for each pair of environmental parameters. Thereafter, the environmental contour method and Rosenblatt transformation, according to a prescribed 50-year return period, are used to determine the long-term design loads at critical locations of a spar-type FOWT. Fractile level and multiplication factor are obtained for five different structural responses to provide guidance in parameter selections for future designs.