Ocean stratification may cause surface wave and inner waves concurring in flow region

which will lead to multicolored wave diffraction effects

including second order low-frequency forces possibly resulting in body resonance

on large scale offshore structures.Based on stratified ocean wave theory

second order drift forces and second order difference-frequency forces on vertical circular cylinders in two layer ocean had been analyzed.The formulas of second-order low-frequency force of multicolored wave diffraction were derived

and the second-order drift force and difference-frequency force on a single vertical circular cylinder were evaluated.The result shows that the variation of ocean condition and structure parameter have some effects on low-frequency forces and in some case the magnitude of difference-frequency force is close to that of second-order drift forces.