Natural heave period exceeding over 21 sec is not the only important design criteria for the design of an ultra deepwater semi-submersible for dry-tree support application. The key design features that would enhance the applications of the semi-submersible design for ultra deepwater are vessel motion characteristic, heave natural period, high air-gap, large deck-area, large deck payload, reduced mooring load, large riser pull-down load, feasible for dynamic positioning, feasible for dry-tow, feasible for wet- tow, feasible for self-installation, feasible for quay-side top side integration, feasible for float-over installation. This paper presents the design innovations of the semi-submersible that has reached the above listed desired key factors. The designed vessel supports dry-tree with large riser loads resulted from ultra deepwater which is decoupled and is also independent of the designed top side pay load. The design is streamlined to withstand harsh environment. The vessel is designed to favor both options with quay-side deck integration or float-over deck installation. Extreme low accelerations in extreme wave environment on top of the above desired features make this semi-submersible suitable for harsh environment and ultra deepwater applications. A brief summary of hydrodynamic response of the vessel is added in this paper. Copyright © 2009 by ASME.