This paper presents results from a case study of vortex-induced motions (VIM) of a floating, spar type offshore wind turbine. VIM response can lead to increased motions of the wind turbine tower as well as contribute to fatigue of moorings and cables. Reliable predictions of VIM response are therefore needed for safe and cost-effective design. One option is to extend the use of analysis methods developed for vortex-induced vibrations (VIV) of slender structures. Three different analysis methods are compared in the present work:

• Linear frequency domain analysis with coefficient-based VIV loads.

• Nonlinear time domain simulation with a harmonic load found using a VIM design curve.

• Nonlinear time domain simulation with the recently developed time domain VIV load model.

The nonlinear time domain simulations can include the dynamic wave and wind loads together with the VIM loads. Nonlinear interaction effects can then be included, and fatigue damage can be calculated from the total response instead of combining damages calculated separately.

Only the analysis with time domain VIV loads gives in-line VIM response. The results show that if in-line motions occur, they may give significant contributions to the mooring line tensions and fatigue.

The effects of the higher Reynolds number and the low length to diameter ratio of floating spars must be investigated. Comparisons with full-scale measurements and adjustment of load coefficients are necessary before a time domain VIM load model is established.

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