This paper presents an original approach aiming to obtain the optimum configuration of an offshore wind farm (OWF). Thanks to cost models, we take into account the costs off all the parts of the electrical network. The optimization platform, based on a genetic algorithm, also allows us to evaluate the reliability of an OWF. This approach is used to compare the topologies obtained in different cases on a real OWF, the "Banc de Guerande." The selected design of the OWF is compared to the results obtained using different methods: cost optimization, or both cost and reliability optimization. The optimization results show that the ring topologies give interesting results if the total cost including the expected energy not supplied during the lifetime of the OWF are taken into account. The number of offshore substations and their positions are also considered in the optimization: we show that the introduction of more substations in order to obtain better performances is possible at a reasonable cost. The presented optimization tool can help to design such farms taking into account several constraints.