Vessel comprising engine

1. A ship including an engine, the ship comprising:

a first self-heat exchanger performing heat exchange with respect to boil-off gas (BOG) discharged from a storage tank;

a multistage compressor performing multistage compression of the BOG discharged from the storage tank and having passed through the first self-heat exchanger;

a second self-heat exchanger precooling the BOG compressed by the multistage compressor;

a first decompressor expanding a first portion of a fluid cooled by the second self-heat exchanger and the first self-heat exchanger; and

a second decompressor expanding a second portion of the fluid cooled by the second self-heat exchanger and the first self-heat exchanger:

a line sending the BOG having passed through the first decompressor and the second self-heat exchanger to at least one of a generator and a low-pressure engine: and a heater disposed on the line,

wherein the first self-heat exchanger cools the BOG compressed by the multistage compressor and having passed through the second self-heat exchanger using the BOG discharged from the storage tank as a refrigerant, and

the second self-heat exchanger cools the BOG compressed by the multistage compressor using the fluid expanded by the first decompressor as a refrigerant.

2. The ship according to claim 1 , wherein the second portion of the fluid expanded by the second decompressor is sent to the storage tank.

3. The ship according to claim 1 , further comprising:

a gas/liquid separator disposed downstream of the second decompressor and separating liquefied natural gas and gaseous BOG from each other,

wherein the liquefied natural gas separated by the gas/liquid separator is sent to the storage tank and the gaseous BOG separated by the gas/liquid separator is sent to the first self-heat exchanger.

4. The ship according to claim 1 , wherein some of the BOG compressed by the multistage compressor is sent to a high-pressure engine.

5. A method comprising:

1) performing multistage compression with respect to boil-off gas (BOG) discharged from a storage tank;

2) precooling the BOG after the multistage compression through heat exchange;

3) cooling the fluid precooled in step 2) through heat exchange with the BOG discharged from the storage tank prior to the multistage compression;

4) expanding, by a first decompressor, a first portion of the fluid cooled in step 3);

5) using the fluid expanded in step 4) as a refrigerant for heat exchange in step 2);

6) expanding and reliquefying, by a second decompressor, a second portion of the fluid cooled in step 3);

7) heating, by a heater, the fluid that has expanded by the first decompressor and subsequently has precooled the BOG in step 2); and

8) sending the fluid heated by the heater to at least one of a generator and a low-pressure engine.

6. The method according to claim 5 , further comprising:

9) separating gaseous BOG and liquefied natural gas generated through partial reliquefaction of the BOG expanded in step 6) from each other; and

10) sending the liquefied natural gas separated in step 9) to the storage tank and joining the gaseous BOG gas separated in step 8) with the BOG discharged from the storage tank to be used as a refrigerant for heat exchange in step 2).

7. The method according to claim 5 , wherein some of the BOG after the multistage compression in step 1) is sent to a high-pressure engine.