Wind turbine installation method

The present invention relates to a method for installing an offshore wind turbine at a target location at sea with an installation vessel, the vessel comprising: - a nacelle support structure for temporarily supporting a nacelle comprising a hub having a plurality of root end connectors to which the root ends of the blades are to be connected, the nacelle support structure comprising: o a support tower extending upwardly from a deck of the installation vessel, o a support platform configured to temporarily support the nacelle, - one or more lifting devices configured for: o lifting the nacelle onto the support platform, o lifting a nacelle assembly including the blades onto a wind turbine mast located adjacent the vessel, wherein the method comprises: a) lifting the nacelle onto the support platform, and securing the nacelle to the support platform, b) orienting a root end connector of the hub of the nacelle in a direction facing a guide path of the blade moving system, c) connecting the root end of the first blade to the corresponding first root end connector of the hub, d) repeating steps b) and c) for subsequent blades and root end connectors until all blades are connected to the hub of the nacelle, thereby providing a RNA, e) lifting the RNA from the nacelle support structure and positioning the RNA onto a wind turbine mast located adjacent the vessel.