Displacing fossil fuels with renewables and increasing sustainable food and chemicals production are among the major challenges facing the world in the coming decades. Integrating climatological oceanographic data with a metabolism and growth rate model of the green marine macroalga from Ulva genus, we analyze the potential of offshore biorefineries to provide for biomass, ethanol, butanol, acetone, methane and protein, globally and in 13 newly defined offshore provinces. We show that for optimum fresh weight stocking density of 4 kg m-2 the total potential of offshore cultivated Ulva biomass is of the order of 1011 dry weight (DW) ton year-1, over a surface area of ~108 km2. We found that the distance of the offshore cultivation site to the processing facility is limited to 114-689 km, depending on cargo moisture content. The near-future technologically deployable areas, associated with up to 100 m water installation depth, and 400 km distance from the shore, can provide for 109 DW ton year-1, which is equivalent to ~18 EJ. This has the potential to displace entirely the use of fossil fuels in the transportation sector or provide for 5-24% of predicted plant proteins demand in 2054. In addition, we modeled the potential production of ethanol, butanol, acetone and methane from the offshore produced biomass. Finally, we analyzed the environmental risks and benefits of large-scale offshore macroalgal cultivation. These results are important as they show for the first time the potential of offshore biomass cultivation to reduce the use fossil fuels and arable land to provide for food, chemicals and fuels required for the society.