- Greenwater damage to offshore structures results from high pressures and dynamic loads that occur when wave crests inundate the structure far above the waterline in areas not designed to withstand such pressures. In the present study, a Navier-Stokes numerical method has been employed for the prediction of impact loads of both 3D short-crested and 2D long-crest waves on a fixed platform and its topside equipments. The violent free surface flow generated by large hurricane waves that reach into the platform deck is resolved by an interface-capturing level-set method. The Navier-Stokes equations are formulated in curvilinear coordinate system and discretized using the finite-analytic method. The level set equations of interface evolution and re-initialization were solved using an implicit 5th-order WENO (weighted essentially non-oscillatory) scheme. An overset grid system is employed to facilitate the simulation of complex flow around a fixed platform with 8 platform legs, an array of 20 risers, and 32 rectangular pegs representing major topside equipments. A new and very efficient grid-interpolation program has been developed to provide the required interpolation data for this complex chimera grid system. Time-domain simulation were performed for both the uni-directional long-crested and directional short-crested waves based on the directional wave spectra of Hurricane Katrina. The simulation results successfully captured the evolution of the wave crest as it propagates through the platform decks while exerting impact loads on the topside equipments in its path. A detailed comparison of the wave impacts and loading difference between the 2D uni-directional waves and 3D directional short-crested wave clearly indicates that the impact loads from short-crested waves differ drastically from those of long-crested waves. Truly big 3D short-crested waves occur in very rare occasions when all wave components of the directional wave spectrum are in phase. However, these extreme storm waves are capable of producing severe local damages to platform decks and topside equpments. The simulation results provide useful guidance for designing new platforms as well as assessing the continued reliability of existing structures. 2010 by The International Society of Offshore and Polar Engineers (ISOPE).