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 first decompressor expanding a first portion of the BOG having passed through the first self-heat exchanger after compression by the multistage compressor;

a second decompressor expanding a second portion of the BOG having passed through the first self-heat exchanger after compression by the multistage compressor; and

a second self-heat exchanger cooling some of the BOG compressed by the multistage compressor through heat exchange using the fluid expanded by the first decompressor as a refrigerant,

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 other BOG compressed by the multistage compressor using the BOG discharged from the storage tank as a refrigerant.

2. The ship according to claim 1 , wherein the second portion of the BOG 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 gas generated through reliquefication of the second portion of the BOG, and gaseous BOG, from each other,

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

4. The ship according to claim 1 , wherein the first portion of the BOG after compression 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) cooling a first portion of the BOG after the multistage compression through heat exchange with the BOG discharged from the storage tank;

3) cooling a second portion of the BOG after the multistage compression through heat exchange with a fluid expanded by a first decompressor,

4) joining the first portion of the BOG cooled in step 2) with the second portion of the BOG cooled in step 3) to provide a combined fluid;

5) reliquefying a first portion of the combined fluid joined in step 4) through expansion by a second decompressor;

6) using a second portion of the combined fluid joined in step 4) as a refrigerant in step 3) after expansion by the first decompressor;

7) heating, by a heater, the second portion of the combined fluid expanded by the first compressor and having been used as a refrigerant for exchange step 6); 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 gas generated through partial reliquefaction of the second portion of the combined fluid expanded in step 6) from each other; and

10) sending the liquefied gas separated in step 9) to the storage tank and joining the gaseous BOG gas separated in step 9) 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.

8. The method according to claim 6 , wherein some of the BOG after the multistage compression in step 1) is sent to a high-pressure engine.