Dry tree semi-submersible platform for harsh environment and ultra deepwater applications

Column-stabilized floating offshore platform structures (10) having spaced apart buoyant main vertical columns (11) joined at lower ends by horizontal lower truss members (13) in a pin connection and joined at upper ends by a buoyant deck mount structure (14), and/or by horizontal truss members, to form a moment connection. A buoyant keel tank (15) having a central moon pool (15A) can be retracted and extended relative to the main columns between a retracted transport mode and an extended operating mode. Ballast of the columns and keel tank can be adjusted to raise or lower the center of gravity of the structure with respect to its center of buoyancy to stabilize the structure and compensate for variable or fixed loads, deck payloads, environmental conditions, and operational and installation stages. A three-sided deck mount allows on-site float-over deck installation.