NOVELTY - Producing synthetic fuel using gas to liquids (GTL)-floating, production, storage and offloading (FPSO) system for offshore oil/stranded gas fields, involves separating crude oil and gas at an FPSO facility and storing the separated crude oil in a crude oil storage; pretreating by saturation and desulfurization; reforming the saturated and desulfurized gas with carbon dioxide and steam; providing the separation of hydrogen; chemically reacting the hydrogen with carbon monoxide; producing electric power by operating a fuel cell using the synthetic gas or hydrogen; and producing steam.    USE - For producing synthetic fuel using a gas to liquids (GTL)-floating, production, storage and offloading (FPSO) system for offshore oil/stranded gas fields (claimed).    ADVANTAGE - The associated gas from oil fields and natural gas from stranded gas fields may be converted into liquid hydrocarbon by the GTL-FPSO process without emission of CO2 into the atmosphere. In addition, since surplus CO2 resulting from the GTL process may be recycled to the reforming reactor or stored in the subsea storage, the cost of CO2 disposal may be minimized. Since surplus hydrogen or synthetic gas from the GTL-FPSO process may be utilized to produce electric power, the power required for the GTL-FPSO process for producing clean synthetic fuel may be produced offshore without environmental pollution. Since water or carbon dioxide produced at the fuel cell during the production of electric energy may be recycled to the reforming reactor, the efficiency of the GTL process is improved. The CO2 produced from the Fischer-Tropsch synthesis and the fuel cell may be recycled to the reforming reactor as a carbon source for preparation of the synthetic fuel, and water-gas (WGS) reaction or reverse WGS reaction may be performed prior to the Fischer-Tropsch synthesis in order to adjust the composition of the synthetic gas required for the Fischer-Tropsch synthesis. The yield of liquid hydrocarbon may be improved by separating CO2 produced during the reforming and recycling it to the reforming reactor. Since CO2 produced from the Fischer-Tropsch synthesis and the fuel cell may be used as a carbon source of the synthetic fuel and surplus CO2 may be stored in the subsea storage, climatic change caused by CO2 emission can be prevented. The compact fixed bed type reactor, specifically the microchannel type reactor, provides improvement in aspects of space and cost. Further, the designing of the GTL-FPSO system may be optimized in consideration of safety issue related with the wave motion of the FP facility.    DETAILED DESCRIPTION - Producing synthetic fuel using a gas to liquids (GTL)-floating, production, storage and offloading (FPSO) system for offshore oil/stranded gas fields, involves separating crude oil and gas at an FPSO facility and storing the separated crude oil in a crude oil storage; pretreating the separated gas by saturation and desulfurization; reforming the saturated and desulfurized gas with carbon dioxide and steam to produce a synthetic gas comprising carbon monoxide and hydrogen; providing the separation of hydrogen from the modified synthetic gas; providing the separation of hydrogen to an upgrading reactor, and removing carbon dioxide from the remaining synthetic gas and returning the removed carbon dioxide to be used as a reaction source of the reforming; chemically reacting the hydrogen with carbon monoxide to produce liquid hydrocarbon, while separating water produced during the process and using the same as cooling water to remove reaction heat thus producing steam, the steam being recycled to be used for the reforming; upgrading the liquid hydrocarbon with the hydrogen provided to the upgrading reactor; producing electric power by operating a fuel cell using the synthetic gas or hydrogen, with water and carbon dioxide being produced during the process; and producing steam using the water produced by the fuel cell as cooling water and returning the same to a reforming reactor along with the produced carbon dioxide.