An improved floating circular hull construction arrangement. The hull is divided into sections by watertight flats. In each section, longitudinal stiffeners spaced around the inside of the outer shell terminate adjacent the flats and do not penetrate the flats. The longitudinal stiffeners are supported by full rings that are concentric with the center of the hull. The rings are received over the longitudinal stiffeners, extend radially inward and are laterally supported with members configured to act in tension in either axial direction and which are parallel to the longitudinal axis of the hull. Rings on the inner shell extend radially outward toward the outer shell and may be laterally supported similarly to the lateral supports of the outer shell. The flats are stiffened with angles or bulb tees curved to form concentric circles that are in turn supported with radial girders spaced around the flats and spanning between the inner and outer shells. The compartments are assembled with the circular sections in a vertical orientation to minimize self-weight distortion during erection. The completed circular sections are rotated to the horizontal to be joined to the other sections to form a complete cylinder.