Radiation problems of sway and roll of an axi-symmetrical body has been solved by means of Bessho's variational approach. Calculation results have been in good agreement with Ohkusu's calculations of a vertical cylinder and Kudos experiments of a sphere. Hydrodynamic coefficients and wave exciting forces of bulbous bodies have been calculated systematically for use of actual design. Motions of a semisubmersible offshore platform of footing type have been calculated directly using above hydrodynamic coefficients and wave exciting forces. Added mass coefficients calculated by variational method can be used for inertia coefficients applied in Morison's wave force equation.