Two caissons with complex geometry need to be towed to an installation site in a straight with potentially strong currents, fairly strong wind and moderate wave conditions. In order to design the mooring system, current, wind and wave forces need to be assessed. Both time-averaged and oscillating components are important. A methodology in which the degree of complexity is gradually increased has been conceived. In this methodology, first an assessment of the order of magnitude of each force is determined, after which it is decided whether in-depth computation of higher accuracy is required. This led to a methodology in which wind forces have been determined analytically, current forces and Vortex Induced Motions (VIM) have been determined using CFD and wave forces by means of a panel method. The resulting forces have been combined in a numerical mooring analysis. The focus of the presented paper is on the CFD model applied to determine VIM and a comparison with resonance frequencies determined analytically.