Offshore wind energy is expected to be the first source of energy in the future thanks to its numerous advantages. However, its maintenance is a complex and costly activity, which is increasingly attracting the attention of researchers and industrials. This paper proposes a simulation-optimization approach for the routing and the scheduling of maintenance for offshore wind farms in order to minimize cost while keeping a high availability of wind turbines. A multi-agent based modeling and simulation is introduced to deal with the complexity of the system. Ant Colony System (ACS) algorithm is used to optimize maintenance tasks routing. In order to make the proposed approach more realistic, we have considered several parameters and constraints such as weather conditions, resources cost, maintenance duration. Several scenarios are experimented to demonstrate the approach efficiency during all the life cycle of the farm. The obtained results show the improvement of cost, energy generation and turbine availability.