Abstract: An extended-base tension leg substructure, an offshore platform supported on the substructure and a method for supporting an offshore platform on the substructure are disclosed, where the substructure includes a plurality of support columns disposed about a central axis of the substructure and interconnected by at least one pontoon. Each column comprises an above water and submerged portion. The substructure also includes a plurality of wings or arms radiating from the columns and/or the pontoons, each wing fixedly or removably securing at least one tendon extending from a wing to an anchor on the seabed. The substructure includes an open, wave transparent central zone for improved access to well-related equipment, conduits or the like and the wings minimize translational movement and rotational flex in the substructure reducing fatigue in the tendons and their connections.

Abstract: 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.