Abstract: A moored floating offshore wind semi-submersible platform with at least three columns characterized in that columns are supported on a T-shaped underwater hull made up of two elongated pontoons, where one pontoon is perpendicular to the other pontoon and a method that allow that the semi-submersible platform is constructed in hull-assemblies and blocks at a first location, transported efficiently to a second location close to the final offshore location where the hull-assemblies and blocks may be assembled quay-side while floating in the water. The platform will support at least one wind turbine on a supporting structure (tower) but may also support two turbines and in the latter case the platform will be moored offshore with a mooring turret to allow the platform to align in a favourable direction to the wind.