A vertically installed Spar-type floater for offshore wind turbine and related construction method are provided. The floating system is a gravity stabilized deep-draft floater including a plurality of vertically extending columns, each column containing a ballast material; a ballast tank coupled to the lower end of each of the columns; a top deck having a plurality of through-bores approximate its periphery coupled to the upper end of each of the columns; a wind turbine assembly supported by the top deck at the center; a plurality of mooring lines linking the floating system to the sea floor. The floating system has a temporary vertically towing configuration which allows the entire floating wind turbine system to be assembled at a quayside and towed vertically to an offshore site, and self-installed into operating configuration.