Boil-off gas reliquefication apparatus and method for vessel

A method of operating a ship comprising a cargo storage tank storing liquefied cargo gas, an engine, a fuel tank storing liquefied fuel for the engine, and a fuel vaporizer for vaporizing liquefied fuel from the fuel tank for supplying to the engine,

• wherein the method comprises:
• compressing, at a multistage compressor, boil-off gas (BOG) discharged from the cargo storage tank to provide compressed BOG (CBOG);
• cooling, at a first heat exchanger, a flow of the CBOG from the multistage compressor by heat exchanging with a flow of the BOG flowing from the cargo storage tank to the multistage compressor to provide a flow of cooled CBOG;
• branching off, from the flow of cooled CBOG, a first flow of cooled CBOG and a second flow of cooled CBOG;
• expanding, at a first expansion unit, the first flow of cooled CBOG to further cool the first flow;
• cooling, in a second heat exchanger, the second flow of cooled CBOG by heat exchange with the first flow from the first expansion unit;
• cooling, at the fuel vaporizer, a cooled second flow from the second heat exchanger by heat exchange with liquefied fuel from the fuel tank to further cool the cooled second flow;
• and
• branching off, from the further cooled second flow flowing downstream the fuel vaporizer, a third flow and a fourth flow;
• expanding, at a second expansion unit, the third flow to cool the third flow;
• heat exchanging, at a third heat exchanger, the cooled third flow from the second expansion unit with the fourth flow to cool the fourth flow;
• expanding, at a third expansion unit, the cooled fourth flow from the third heat exchanger to re-liquefy at least portion of the cooled fourth flow for returning to the cargo storage tank,
• wherein at the fuel vaporizer, the cooled second flow from the second heat exchanger is further cooled by heat exchange with the liquefied fuel before the further cooled second flow is sent to the second expansion unit and the third heat exchanger whereas the liquefied fuel is heated and gasified by heat exchange with the cooled second flow from the second heat exchanger such that the gasified fuel is supplied to the engine,
• wherein the multistage compressor comprises a first compressor, a second compressor, a third compressor, a first cooler interposed between the first compressor and the second compressor and configure to cool compressed BOG from the first compressor, and a second cooler interposed between the second compressor and the third compressor and configure to cool compressed BOG from the second compressor,
• wherein the method further comprises:
• returning the first flow that cooled the second flow at the second heat exchanger, to a point of the multistage compressor between the second compressor and the second cooler;
• and
• returning the third flow that cooled the fourth flow at the third heat exchanger, to a point of the multistage compressor between the first compressor and the first cooler.