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The procedure for predicting the second-order biharmonic response and tether forces of a tension-leg platform in regular waves is presented. This consists of evaluating the second-order wave exciting forces based on the second-order wave diffraction theory, and solution of the equation of motion including the effect of the second-order mooring tether forces. Model tests are carried out to measure the wave exciting forces on the fixed circular dock, as well as the response and tether forces of the tension-moored circular dock. Satisfactory agreement is observed between the theoretical and experimental predictions, confirming the validity of the proposed method. The conclusions arising from the present study are summarised as follows : 1) The vertical-plane motion and tether forces of the tension-leg platform in regular waves include the second-order biharmonic components which are comparable in magnitude with the first-order wave-frequency components. 2) The nonlinear sum-frequency wave exciting forces provide an important source of excitation of such oscillation. These exciting forces can be predicted accurately based on the second-order wave diffraction theory. 3) The contribution of the second-order velocity potential to the second-order wave exciting force is significant, in particular to the heave and pitch exciting forces. The simplified method neglecting this contribution is found to underestimate the forces drastically. 4) The theoretical method proposed in this study provides an effective means for predicting the second-order sum-frequency response and tether forces of the tension-leg platform.