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ABSTRACT Cao, D.P.; Jian, W.; Huang, Z.H., and Lo, E.Y.M., 2018. Prediction of wave runup on columns of two semisubmersibles models. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 1031–1035. Coconut Creek (Florida), ISSN 0749-0208. Water wave runup on columns of semi-submersibles has important implications on negative air-gaps created and slamming forces exerted on the underside of such platforms and their safe operations in rough seas. Many prediction formulas have been developed for wave runup on single vertical columns though none could be directly used for runup predictions on the columns of semi-submersibles due to the more complicated geometrical and diffraction effects. In the present study, small-scale experimental tests and limited numerical simulations using the linear diffraction and CFD solvers were conducted on two typical semi-submersible models, a twin-pontoon and a ringpontoon platform, to determine the runup on the columns. The experimental tests have the semi-models tested in a fixed configuration under regular and irregular waves and at different heading angles. Wave gauges and video recordings were used to capture the wave field and the runup on the columns. Key features observed were that sloshing between the front and rear columns created localized breaking over the pontoon, and that the maximum wave runup occurred at the downstream columns. A recently developed predictive runup equation (Cao et al., Coast Eng Jour. 59(3), 2017) is extended through by introducing an equivalent cylinder radius. The extension accounted for possible wave diffraction effects which led to higher runups than seen for single columns. The results from the limited numerical simulations were in quantitative agreement with experiments. [ABSTRACT FROM AUTHOR]