A buoyant structure having a central vertical axis, said structure arranged and designed for petroleum drilling, production, storage and offloading, comprising: a hull having an upper cylindrical portion, said hull including an upper frustoconical portion directly connected to the bottom of said upper cylindrical portion and having downward and inward sloping walls, said downward and inward sloping walls sloping at an angle of between 10 and 15 degrees with respect to said central vertical axis, said angle being large enough for providing damping to downward heave of said structure in response to ocean oscillations, yet small enough to provide large storage volume of said upper frustoconical portion, said hull including a lower frustoconical portion disposed below said upper frustoconical portion and having downward and outward-sloping walls, said outward sloping walls sloping outwardly at an angle greater than or equal to 55 degrees but less than 65 degrees with respect to said central vertical axis, so as to provide large inertia for said structure for heave and pitch motions, thereby providing natural periods for heave, pitch and roll above ocean expected wave energy, yet small enough to avoid abrupt changes in stability during ballasting during installation, and said hull including a lower cylindrical portion directly connected to the bottom of said lower frustoconical portion, wherein the bottom of said lower cylindrical portion defines a keel of said hull, and the top of said upper cylindrical portion defines a main deck of said structure, said structure characterized by a low center of gravity providing inherent stability with said low center gravity enhancing a meta centric height of said structure resulting in a large righting movement, said structure having ballasting in one or more ring shaped compartment which are located in outer-most portions of said lower cylindrical portion thereby further increasing said righting movements. a hull having an upper cylindrical portion, said hull including an upper frustoconical portion directly connected to the bottom of said upper cylindrical portion and having downward and inward sloping walls, said downward and inward sloping walls sloping at an angle of between 10 and 15 degrees with respect to said central vertical axis, said angle being large enough for providing damping to downward heave of said structure in response to ocean oscillations, yet small enough to provide large storage volume of said upper frustoconical portion, said hull including a lower frustoconical portion disposed below said upper frustoconical portion and having downward and outward-sloping walls, said outward sloping walls sloping outwardly at an angle greater than or equal to 55 degrees but less than 65 degrees with respect to said central vertical axis, so as to provide large inertia for said structure for heave and pitch motions, thereby providing natural periods for heave, pitch and roll above ocean expected wave energy, yet small enough to avoid abrupt changes in stability during ballasting during installation, and said hull including a lower cylindrical portion directly connected to the bottom of said lower frustoconical portion, an upper cylindrical portion, said hull including an upper frustoconical portion directly connected to the bottom of said upper cylindrical portion and having downward and inward sloping walls, said downward and inward sloping walls sloping at an angle of between 10 and 15 degrees with respect to said central vertical axis, said angle being large enough for providing damping to downward heave of said structure in response to ocean oscillations, yet small enough to provide large storage volume of said upper frustoconical portion, said hull including a lower frustoconical portion disposed below said upper frustoconical portion and having downward and outward-sloping walls, said outward sloping walls sloping outwardly at an angle greater than or equal to 55 degrees but less than 65 degrees with respect to said central vertical axis, so as to provide large inertia for said structure for heave and pitch motions, thereby providing natural periods for heave, pitch and roll above ocean expected wave energy, yet small enough to avoid abrupt changes in stability during ballasting during installation, and said hull including a lower cylindrical portion directly connected to the bottom of said lower frustoconical portion, wherein the bottom of said lower cylindrical portion defines a keel of said hull, and the top of said upper cylindrical portion defines a main deck of said structure, said structure characterized by a low center of gravity providing inherent stability with said low center gravity enhancing a meta centric height of said structure resulting in a large righting movement, said structure having ballasting in one or more ring shaped compartment which are located in outer-most portions of said lower cylindrical portion thereby further increasing said righting movements.