Method of constructing and launching an offshore semi-submersible platform and an offshore semi-submersible platform thus constructed

A method of constructing and launching an offshore semi-submersible platform, wherein the offshore semi-submersible platform comprises a plurality of floating columns, each of which is connected to at least one other of said floating columns by means of at least one lower structural connection arm which is placed connecting between the two columns near their bases, said method being characterised in that it comprises the following operating steps:

• a) making said semi-submersible platform in a dry environment by dividing it into a plurality of sub-assemblies each comprising at least one of the floating columns and at least one semi-arm of the respective lower structural connection arm which semi-arm is already structurally integrated in the column itself and extends cantilevered therefrom with a respective free end;
• b) providing each sub-assembly in a dry environment with at least one temporary thrust box, which is positioned below said at least one semi-arm and is provided with a housing seat of the respective semi-arm and at least one ballast chamber;
• c) separately launching into the water the individual sub-assemblies which float independently by virtue of the respective floating column;
• d) adjusting for each sub-assembly the ballast of the respective temporary thrust box so as to obtain a balanced floatation of the sub-assembly which allows the free end of said at least one semi-arm to be positioned at the same height as the free end of the semi-arm of the sub-assembly intended to assume an adjacent position;
• e) bringing the sub-assemblies at the free ends of the respective semi-arms close to each other two-by-two until they are aligned, with the aid of axial alignment means previously arranged at the free ends of the semi-arms;
• f) connecting the temporary thrust boxes to each other two-by-two;
• g) welding the free ends of the semi-arms together so as to create a structural connection between the columns;
• h) removing the temporary thrust boxes.