The results of the offshore integration of fossil and renewable energy sources (RESs) to produce an energy vector, namely methanol, are discussed. The methodology developed and hereby presented, allowed at first the selection of the best technology for offshore RESs exploitation, where a near-to-decommissioning platform is used as an energy hub. Based on the availability of natural gas from the depleted reservoir, the most suitable process for offshore methanol production was chosen among various alternative innovative processes. The optimal mix for energy production was then identified by varying the number of converters for each RES. Finally, the methanol production process was designed on the basis of the available electric power, including a backup system based on natural gas, to perform RESs valley filling and to achieve a steady state process.