A buoyant structure for at least one of: petroleum drilling, production, storage, and offloading, the buoyant structure including: a. a hull defining a vertical axis, whereby the hull has an outer hull side connected to an inner hull side, and the hull has an upper hull diameter (D 2); b. a planar keel defining a lower hull diameter (D 1); c. a lower cylindrical portion connected to the planar keel; d. a lower frustoconical portion disposed above the lower cylindrical portion; e. an upper frustoconical portion directly connected to the lower frustoconical portion; f. a main deck connected over the upper frustoconical portion, whereby the outer hull side has an outer hull side shape selected from the group: circular, elliptoid, and geodesic in horizontal cross-sections at all elevations; the inner hull side has a shape selected from the group: circular, elliptoid, and geodesic; an lower cylindrical portion diameter is identical to the lower hull diameter (D 1), and the lower hull diameter (D 1) is the largest diameter of the hull, and further the lower cylindrical portion diameter is from 105 percent to 130 percent of the upper hull diameter (D 2); the lower frustoconical portion is formed with an inwardly sloping wall at a first angle ranging from 50 degrees to 70 degrees with respect to the vertical axis; the upper frustoconical portion is with an outwardly sloping wall sloping at a second angle with respect to the vertical axis from 3 degrees to 45 degrees, and the lower frustoconical portion with the inwardly sloping wall, abuts the outwardly sloping wall forming a hull neck with a hull neck diameter (D 3); and the buoyant structure further comprises: g. a moon pool formed by the inner hull side, the moon pool having a first moon pool diameter proximate the main deck which increases to a second moon pool diameter proximate the planar keel wherein the second moon pool diameter is less than the upper hull diameter (D 2); and h. at least one ballast tank in communication with a control center in the hull, the at least one ballast tank is for ballasting and deballasting the hull; and wherein the buoyant structure defines a center of gravity below a center of buoyancy in the moon pool. a. a hull defining a vertical axis, whereby the hull has an outer hull side connected to an inner hull side, and the hull has an upper hull diameter (D 2); b. a planar keel defining a lower hull diameter (D 1); c. a lower cylindrical portion connected to the planar keel; d. a lower frustoconical portion disposed above the lower cylindrical portion; e. an upper frustoconical portion directly connected to the lower frustoconical portion; f. a main deck connected over the upper frustoconical portion, whereby the outer hull side has an outer hull side shape selected from the group: circular, elliptoid, and geodesic in horizontal cross-sections at all elevations; g. a moon pool formed by the inner hull side, the moon pool having a first moon pool diameter proximate the main deck which increases to a second moon pool diameter proximate the planar keel wherein the second moon pool diameter is less than the upper hull diameter (D 2); and h. at least one ballast tank in communication with a control center in the hull, the at least one ballast tank is for ballasting and deballasting the hull; and wherein the buoyant structure defines a center of gravity below a center of buoyancy in the moon pool.