LNG tank ship having LNG circulating device

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.