- An interface-preserving level-set numerical method has been incorporated into a Reynolds-Averaged Navier-Stokes (RANS) method for the simulation of wave runup and greenwater effect on offshore structures. In this method, the free surface flows are modeled as immiscible air-water two-phase flows and the free surface itself is represented by the zero level-set function. Calculations were performed first for two- and three-dimensional problems for dam-breaking flow and free jet problems involving violent free surface motions. The levelset RANS method was then employed for the simulation of greenwater on offshore structures and slamming on platform deck. The numerical results clearly demonstrated the capability of the level-set method to deal with complex free surface flows involving breaking waves, water droplets and trapped air bubbles. Copyright 2006 by The International Society of Offshore and Polar Engineers.