Abstract: The invention relates to a spar based vessel for accessing offshore installations and vessels, including wind farm turbines, in which the centre of gravity of the vessel is positioned below the centre of buoyancy, which is positioned below the operational waterline, the operational waterline occurs at low cross-sectional area vertical struts, and the vertical struts support a topside structure for passengers. Active ballast control system and location of the propulsive elements permits the vessel to travel in spar orientation by positioning the vector of propulsion to lie in the same horizontal plane as the transverse centre of drag of the vessel. A docking system permits safe connection of the vessel to offshore installations, including wind turbines of generic design.