One of the main mechanisms for driving down the cost of offshore wind energy is to install ever larger wind turbines in larger wind farms. At the same time, these turbines are placed further offshore in deeper waters. As a result traditional monopile foundations are not always feasible and multi-membered foundations, such as jackets and tripods, are required.Typically thousands of load cases need to be simulated for the design and certification of offshore wind turbines. As models of such foundations are significantly larger than their monopile counterparts, model reduction is often applied to limit the computational costs. Additionally, the foundation design is generally done by a specialized company, which bases its design on the results of the load simulations. Hence, an accurate estimation of the stresses in load simulation is essential to predict the integrity and the lifetime of different designs. The effect on the load accuracy of both the model reduction as well as the post-processing method used by foundation designers are investigated in this paper. A case study is performed on a jacket-based wind turbine model to verify and quantify the findings.Firstly, it is observed that the effect of the reduced foundation model on the wind turbine loads is negligible. However, both the reduction method and the post-processing method applied by the foundation designer have a large influence on the fatigue loading in the jacket. It is shown that the popular Guyan reduction results in significant errors on the fatigue damage and that a static post-processing analysis leads to serious underestimations of the fatigue loads. Finally, an outlook is given into future developments in the field of load calculations for offshore wind turbine foundation design. Nomenclature B Signed Boolean operator C Damping matrix D el i Fatigue damage in element i f Array of external forces g Array of connection forces Journal of Computational and Nonlinear Dynamics. Downloaded From: http://computationalnonlinear.asmedigitalcollection.asme.org/ on 02/20/2015 Terms of Use: http://asme.org/terms A c c e p t e d M a n u s c r i p t N o t C o p y e d i t e dH s Significant wave height K Stiffness matrix L Boolean assembly operator M Mass matrix p Array of nonlinear internal damping and elastic forces q Array of reduced degrees of freedom R Reduction matrix r Array of residual forces T Orthogonal projector u Array of displacement degrees of freedom v a Average wind speeds α Array of modal force amplitudes η Array of modal amplitudes λ Array of Lagrange multipliers φ Normal mode ψ C Constraint mode ω Frequency [i] Associated to internal DoF [b] Associated to boundary DoF [W T ] Associated to the wind turbine substructure [F] Associated to the foundation substructurė First time derivativë Second time derivativē Primal assembled Reduced variable CB Craig-Bampton method CMS Component mode synthesis DC Displacement controlled DEL Damage equivalent load DLC Design load case DoF Degree of freedom FC Force controlled FD Foundation designer GR Guyan red...