An expression for the nonlinear wave force on a vertical, circular cylindrical member whose diameter is small compared to the incident wavelength is derived from an exact solution to the second-order diffraction problem. A comparison is then made with the inertial force estimates obtained using Stokes’ second and fifth-order wave theories in Morison’s equation with a mass coefficient derived from linear potential theory, as is currently recommended in the offshore design codes. Results indicate that in water depths of the order of 1/16 of a wavelength, nonlinear free-surface effects contribute significantly to the total inertia force on a slender cylindrical member.

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