Semi-submersible offshore structure having storage tanks for liquefied gas

A semi-submersible offshore structure configured to be anchored at sea and for storing liquefied gas, comprising:

• a plurality of liquefied gas storage tanks configured to store liquefied natural gas, wherein each of said liquefied gas storage tanks is a membrane type storage tank and has a shape selected from a cylindrical shape and a rectangular parallelepiped shape;
• a plurality of columns configured to be partially submerged below sea level and standing perpendicular to sea level, wherein each of said plurality of columns is provided therein with at least one of said liquefied gas storage tanks;
• pontoons configured to adjust ballast and each pontoon connecting lower sides of the columns to each other;
• an upper deck located on the plurality of columns and configured to connect the columns to each other;
• and
• a column bridge provided to the columns to bridge a liquefied natural gas (LNG) carrier to the semi-submersible offshore structure, wherein said column bridge is provided in parallel to the pontoons to connect upper portions of the columns to each other under the upper deck, wherein said column bridge connects columns which are connected by each of the pontoons, and wherein each pontoon is configured to reduce fluctuation of the semi-submersible offshore structure and to adjust ballast.