For a semisubmersible operated in water at a depth of 1.5km, the sample data of the relative wave crests at different locations from an experiment were assumed to follow a twoparameter Weibull distribution and the method of LHmoments was used to estimate the parameters. Then, the exceedance probability distributions for different orders of LHmoments were obtained. The results show that as the order of LHmoments increases, the twoparameter Weibull distribution yields better trend of the large relative wave crests under the 1yearreturn sea state, and clearly a better fit of the large relative wave crests under the 100yearreturn sea state.