The series expression of random wave drag forces is studied in this paper by means of nonlinear spectral analysis. A new method is proposed in this study since the derivative of the wave drag force is discontinuous. The characteristics of random process are used to obtain the coefficients of the series in this method. Numerical verification indicates that the series expression given in the paper approaches the original expression satisfactory. The maximum error is less than 4% when measured in the peak values of the time series and less 3% when measured in the root mean square values. That is to say, the series expression given in the study has reasonable accuracy and can be used in engineering computations. The application example indicates that the spectral densities of wave forces, as well as the root mean squares of wave forces, can be easily obtained without lose much accuracy using the series expression of wave drag forces.

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