In very rapid current, a spar buoy with circular cross section is significantly inclined due to steady drag force and often oscillates due to asymmetrical vortex-shedding phenomena. In order to increase its operational time, the inclination and vortex-induced oscillation should be reduced. In the present study, spar buoys with wing type cross-section and with several spheres, which are developed to reduce the inclination and oscillation, are experimentally investigated. The wing-cross-section reduces the steady inclination of the spar buoy due to the low drag characteristics. However, it is confirmed experimentally and theoretically that the buoy sometimes oscillates violently due to selfexciting- vibration in rapid current. To avoid such a violent motion, a motion control system is needed. Instead of the wing cross-section, several spheres are added to a cylindrical spar buoy to reduce vortexinduced- oscillation by its three-dimensional flow characteristics. Moreover, since the drag coefficient of a sphere is much smaller than that of a circular cylinder, the steady inclination of it is also reduced.
A spar-buoy drilling derrick is used for geological survey for construction of bridges, port facilities and many kinds of offshore structures (Fukutomi et. al.(1995), Otsuka et. al. (1997)). In rapid current, the derrick is inclined due to large drag force and oscillates due to shedding vortices. In order to increase operational time of it, reductions of the inclination and motion are needed. Two types of spar buoys designed to reduce the inclination and motion, a spar-buoy with wing cross section and a cylindrical spar-buoy with multiple spheres, are experimentally investigated by model experiments using their scale models in a circulating water channel of Osaka Prefecture University.
The most common way to reduce the drag force in rapid flow may be to use wing-type or streamline-type cross-sections.
