An exemplary floatable vessel is described having an upper tower section with a reduced diameter or width and an affixed lower base section having an enlarged diameter or width. The lower section contains weighted ballast distributed upon its lower floor. The lower section also includes flotation tanks which can be filled with air to raise the vessel in the water such that portions of the lower section are raised above the water line. Alternatively, the flotation tanks can be flooded to dispose the lower section and a portion of the upper section below the water level. The upper tower section of the vessel supports a deck structure upon which rig components can be constructed or secured. The tower section includes flotation tanks as well. In preferred embodiments, these tanks are variable tanks that can be partially filled with air and partially flooded with water. The vessel defines a central chamber within which drilling risers are contained and suspended from the deck structure of the vessel downward toward the sea floor. One or more pneumatic supports are provided which assist in securing the riser and absorbing energy from movement of the platform. The floating vessel can be constructed and transported in a upright or vertical orientation so that it does not need to be upended prior to mooring at its intended location. In addition, structures such as rig components may be placed atop the tower portion prior to or during transportation of the vessel. During transportation by towing, the flotation tanks of the lower section are filled with air so that the lower section is partially raised above the surface of the water. The vessel is placed into its installed position by flooding the flotation tanks of the lower section to cause the lower section to become submerged.