Some offshore structures have configurations as uniform vertical cylinders of equal section. The interaction between the oscillation of them and fluid motion is of importance in many design cases. For those special cylindrical bodies, a new type of local disturbance source is presented in the paper, which, being distributed on the sectional contour of the cylinders together with the known wave source, can be used to estimate the three-dimensional hydrodynamic forces on those cylinders of arbitrary section oscillating in still waters. The solution method both for wave forces and radiation forces is also discussed. Some of the interaction structures of the uniform vertical cylinders of equal section. The interaction between the oscillation of them and fluid motion is of importance in many design cases. For those special cylindrical bodies, a new type of local disturbance source is presented in the paper, which, being distributed on the sectional contour of the cylinders together with the known wave source, can be used to estimate the three-dimensional hydrodynamic forces on those cylinders of arbitrary section oscillating in still waters. The solution method both for wave forces and radiation forces is also discussed.