The energy required for the transportation of raw materials and the production of most manufactured goods depends on crude oil. For these reasons, FPSOs (Floating, Production, Storage, and Offloading) have become the primary production units of crude oil offshore. It is leading to an increase in the number and expanding of the production and storage facilities of the FPSOs. An increase in the oil production at the topside facilities of FPSOs will contain more gases, which leads to a rise in blow-by gas. Changes to the blanket gas system may be necessary as the flow rate of the blow-by gas is expected to increase. The purpose of this paper is to suggest a novel blanket gas system with a proper control method for controlling the cargo tank pressure when the blow-by gas is occurring. Unlike the existing system, in this proposal, the purge header that supplies the inert gas is possible for a use of the vent purpose in the situation where the blow-by gas is generated. By using the vent header and purge header for the purpose of venting, the pipe size can be drastically reduced. To quickly convert the purge header for the purpose of venting, the application of an appropriate control method is essential. A simulation was carried out for confirming the efficacy of the pressure control and the processible blow-by gas quantity compared to the existing system. In addition, as the amount of blow-by gas increased, a study on the possibility of installing large pipes used in the existing system configuration and the dual pipes suggested by this proposal was investigated. As a result of the simulation, this proposal presented better results in terms of both the pressure control performance of the cargo tanks and the arrangement of the piping compared to the existing system.