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Offshore structures are generally classified as small-scale structures or large-scale structures.Their wave forces are then estimated by Morison equation and diffraction/radiation theories,respectively.However,the classification criterion is not well quantified.In the present paper,a numerical wave flume is established to simulate the wave forces acting on a fixed and vertical surface-piercing circular cylinder under linear waves.By solving Navier-Stokes equation and Euler equation with free surface involved,respectively,the viscous force and inertia force are separated accurately.The variation of viscous force and inertia force with the cylinder diameter to wave length ratio is discussed in detail.The scale intervals for significant viscous and diffraction effects are given.The error caused by neglecting viscous and diffraction forces is quantitatively analyzed.Based on these analysis,the concept of medium-scale structure is proposed and the classification criteria for small-,medium-and large-scale structures are given.In the meantime,the estimation methods of wave forces for different scales of structures are suggested.