The current design of offshore wind turbines follows mainly the IEC 61400-3 standard. The list of Design Load Cases (DLCs) implied for this standard is comprehensive and the resulting number of time domain simulations is computationally prohibitive. The aim of this paper is to systematically analyse a subset of ultimate limit state load cases proposed by the IEC 61400-3, and understand the relative severity among the load cases to identify the most critical among them. For this study, attention is focused on power production and parked load cases. The analysis is based on the NREL 5 MW prototype turbine model, mounted on a monopile with a rigid foundation. The mudline overturning moment, as well as the blade root in-plane and out-of-plane moments are taken as metrics to compare among the load cases. The simulations are carried out using the aero-hydro-servo-elastic simulator, FAST, and the key observations are thoroughly discussed. The DLC 1.6a is shown to be the most onerous load case. Although the considered load cases are limited to power production and idling regimes, the obtained results will be extremely useful for the substructure (monopile) design and for efficient reliability analysis subsequently, as is also shown partially by some previous studies. (C) 2016 Elsevier Ltd. All rights reserved.