In the present paper, a complete and consistent second-order computation of airgap for a semi-submersible is presented. The second-order sum and difference frequency upwell will be calculated and combined with the linear upwell statistically to find the short-term extremes. The phasing of the low frequency surface elevation with respect to the low frequency vertical motion will be considered. The asymmetric characteristics of the surface elevation will be taken care of by the sum frequency surface elevation. By comparing the maximum and minimum extremes of surface elevation in a given sea state at specified off-body points, the “asymmetry factor” up to the second order can be obtained.