LNG tank ship having LNG circulating device

1. An LNG tank ship for transporting LNG, the LNG tank ship comprising:

an LNG tank containing liquid phase (LNG) and gaseous phase (boil-off gas);

a safety valve configured to release part of the boil-off gas from the tank when a vapor pressure within the tank exceeds a cut-off pressure of the safety valve, wherein the safety valve is configured to adjust the cut-off pressure within a range between a first value and a second value, the first value being 0.25 bar (gauge pressure) or below, the second value being between about 0.25 bar (gauge pressure) and about 2 bar (gauge pressure); and

a liquid volume computing module configured to compute a current volume of LNG contained in the LNG tank;

wherein the cut-off pressure of the safety valve is adjusted at least based on the current volume.

2. The LNG tank ship of claim 1 , further comprising a boil-off gas circulating device comprising a boil-off gas pathway and configured to suction boil-off gas of LNG from the LNG tank, to flow the boil-off gas through the boil-off gas pathway and to return the boil-off gas to the LNG tank without liquefying the boil-off gas.

3. The LNG tank ship of claim 2 , wherein the boil-off gas circulating device further comprises a compressor configured to compress at least part of the boil-off gas that flows through the boil-off gas pathway.

4. The LNG tank ship of claim 2 , wherein the boil-off gas is suctioned through an intake port located at or near a top surface of the LNG tank.

5. The LNG tank ship of claim 2 , wherein the intake port is substantially distanced from an interior surface of the LNG tank.

6. The LNG tank ship of claim 2 , wherein the boil-off gas circulating device further comprises a branch pathway branching from the boil-off gas pathway.

7. The LNG tank ship of claim 2 , wherein the boil-off gas returns to the LNG tank through a plurality of orifices.

8. The LNG tank ship of claim 1 , wherein the boil-off gas pathway comprises a portion located outside the LNG tank.

9. The LNG tank ship of claim 1 , wherein the boil-off gas pathway throughout is located inside the LNG tank.

10. The LNG tank ship of claim 1 , wherein the LNG tank is integrated with a body of the ship.

11. A method of processing LNG contained in an LNG tank ship, the method comprising:

providing the LNG tank ship of claim 2 ;

computing a current volume of LNG contained in the LNG tank;

adjusting the cut-off pressure of the safety valve at least based on the computed volume;

suctioning, through an intake port, boil-off gas from an upper portion of the LNG tank;

flowing, through the boil-off gas pathway, boil-off gas from the intake port toward the discharge port; and

discharging, through a discharge port, boil-off gas from the boil-off gas pathway into a lower portion of the LNG tank.

12. The method of claim 11 , further comprising:

compressing at least part of boil-off gas flowing in the boil-off gas pathway toward the discharge port; and

flowing compressed boil-off gas toward the discharge port.

13. The method of claim 11 , further comprising:

loading LNG into the LNG tank at a first time, wherein the cut-off pressure is set at a first value during loading;

subsequently consuming or releasing at least part of the boil-off gas from the LNG tank; and

adjusting the cut-off pressure to a second value that is higher than the first value.

14. The method of claim 11 , wherein substantially no liquid phase LNG is suctioned through the intake port.

15. The method of claim 11 , wherein the discharge port is submerged in liquid phase LNG.

16. The method of claim 11 , further comprising:

unloading liquid phase LNG from the LNG tank;

gasifying the liquid phase LNG to gaseous phase LNG; and

supplying the gaseous phase LNG to an LNG infrastructure for supplying LNG to consumers.