The fatigue limit state (FLS) of fixed offshore wind turbine structures is critical and difficult to handle. As it is the most common design driving criteria for offshore structures, the simulation and calculation of this phenomenon must be as accurate as possible. Research is needed to improve the current design. There are mainly two design approaches available: Integrated design approach (IDA) and Sequential design approach (SDA). The IDA, described in this paper, considers the coupled structural analysis of a whole wind turbine system exposed to wind- and wave-induced loads in an aero-hydro-elastic solver. The results given by solver are loads series, which are afterwards used for obtaining the stress series with stress concentration factors (SCF) included. The stresses are processed in terms of rainflow counting and finally, fatigue damage of a critical K-joint is obtained externally, to avoid the use of damage equivalent loads (DEL) as by default in the solver, but to calculate it by means of the Efthymiou principle. The whole procedure with methods is explained in this paper.