Nonlinear thermal convection in a spherical fluid layer in the presence of spherically symmetric gravity, spherical Rayleigh-Benard convection, is investigated. At the onset of spherical Rayleigh-Benard convection, there exists the (2l+1)-fold degeneracy of the linear solution, where l is the degree of a spherical harmonics. Nonlinear convection is studied through fully three-dimensional numerical simulations. Several new spherical patterns of nonlinear convection are found. In particular, a steadily drifting pattern in the form of a single giant spiral roll covering the whole spherical surface without defects is discovered for various Prandtl numbers for the first time.