A floating offshore fluid storage caisson platform has a large diameter vertically oriented buoyant column or caisson, or multiple caissons, defining a storage chamber, and a telescopic keel tank disposed at the bottom end thereof, and may have deck on top of the caisson to support storage, drilling and production structures, equipment, and quarters. The structure can be transported horizontally either dry on a transporting vessel or towed with its keel tank in a fully retracted position. At the field of operation, the structure initially floats horizontally. The keel tank is extended and then slowly flooded to move the center of gravity of the structure toward the keel tank and with the heavier tank, the structure tilts upright to assume an operating vertical position with the telescopic keel tank extended downward with respect to the caisson, and thereafter as the storage chamber is filled with fluid, the relative position of the keel tank is adjustably tuned to raise or lower the center of gravity of the entire mass of the structure with respect to its center of buoyancy and maintain the center of gravity of the structure below its center of buoyancy and stabilize the structure vertically at a desired draft according to ballast and variable or fixed loads, and to compensate for different operational, environmental and installation stages of the structure.