Fatigue due to environmental conditions is studied on a top-tensioned riser. The fatigue is due to two factors. First, the waves produce a displacement of the top of the riser, which excites the structure. Secondly, currents create vortices behind the structures. The phenomenon is then referred to as vortex-induced vibrations (VIV), whereby the vortices can lock onto a structural mode through the shedding frequency. In the present paper, we have two objectives. The first is to compare the fatigue estimates given either by a modal analysis or by Navier-Stokes calculations for a riser in a current. The second is to determine if studying the wave and current effects separately produces conservative results or if they must be studied together.

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