This work is a part of a project aiming to study the impact of biofouling on turbine performances. It describes the fluid-structure calculation applied to a Darrieus turbine using the Openfoam code in order to take the mass variation of the solid into account. Forced and induced rotation simulations are compared for water and air. For both fluids, the results show that the flow field in the rotor’s wake is similar for the two types of movement. Therefore the solid solver does not change fluid results. Blue mussels are fixed to the blades, which leads to a supplementary mass that has been decorrelated from hydrodynamics to understand its effects. For the chosen implantation (small), the only observable modification is a delay between light and heavier rotors. However, the coarse mesh used in this study is not sufficient to compute the flow around blades with accuracy. This has a strong impact on calculation of forces and by extension on the rotor’s dynamics. A mesh convergence procedure is planned to reach the necessary mesh precision and laboratory experiments should be made in order to validate dynamical results with low flow speed.