—In this paper,the second-order perturbation method in frequency domain is used to calculateRAO and spectra of motion and mooring line tension of a turret-moored tanker in ballast condition.Thecalculated results are compared with corresponding experiment results.In the experiment the wave head-ing is 180°,and the wave spectra is the P-M spectrum and white noise spectrum.In the theoretical calcu-lations,the damping coefficient of slow oscillation of the tanker is determined on the basis of the dampingobtained from a test of irregular waves where the mooring system is replaced by a nonlinear spring withnonlinear stiffness similar to that of the mooring system.From the comparison between theoretical calcula-tions and experimental results,it can be found that the theoretical results obtained by the second-orderperturbation method in frequency domain are in good agreement with the experimental results,indicatingthat the damping coefficient of slow oscillation of the tanker required in frequency domain calcu -In this paper, the second-order perturbation method in frequency domain is used to calculateRAO and spectra of motion and mooring line tension of a turret-moored tanker in ballast condition .calculated results are compared with corresponding experiment results.In the experiment the wave head-ing is 180 °, and the wave spectra is the PM spectrum and white noise spectrum. In the theoretical calcu-lations, the damping coefficient of slow oscillation of the tank is determined on the basis of the damping obtention from a test of irregular waves where the mooring system is replaced by a nonlinear spring with nonlinearity stiff similar to that of the mooring system. Flash the comparison between theoretical calcula-tions and experimental results, it can be found that the theoretical results obtained by the second-order perturbation method in frequency domain are in good agreement with the experimental results, indicating that the damping coefficient of slow oscillation of the tanker required in fr equency domain calcu