Liquefied gas storage tank and liquefied gas carrier ship

A liquefied gas carrier ship comprising:

• a liquefied gas storage tank comprising:
• an inner shell storing a liquefied gas;
• an outer shell forming a vacuum space between the inner shell and the outer shell;
• a fail-safe thermal insulating layer covering an outer side surface of the outer shell and including a layer made of a foamable material;
• and
• a vacuum area thermal insulation layer spaced apart from an inner side surface of the outer shell and covering an outer side surface of the inner shell;
• a vacuum degree detector, which detects a degree of vacuum in the vacuum space between the inner shell and the outer shell of the liquefied gas storage tank;
• an indicator, which notifies whether the degree of vacuum detected by the vacuum degree detector is in a voyage possible pressure range or a port-call necessary pressure range;
• a relief passage, which leads a boil off gas generated in the inner shell to outside;
• and
• a relief valve, which is provided on the relief passage and opens when a pressure in the inner shell has become higher than or equal to a predetermined pressure,
• wherein the predetermined pressure, at or above which the relief valve opens, is a pressure when the degree of vacuum in the vacuum space is in the port-call necessary pressure range.