The computational fluid dynamic (CFD) toolbox OpenFOAM is used to assess the applicability of Reynolds-averaged Navier–Stokes (RANS) solvers to the simulation of oscillating wave surge converters (OWSC) in significant waves. Simulation of these flap type devices requires the solution of the equations of motion and the representation of the OWSC׳s motion in a moving mesh. A new way to simulate the sea floor inside a section of the moving mesh with a moving dissipation zone is presented. To assess the accuracy of the new solver, experiments are conducted in regular and irregular wave traces for a full three dimensional model. Results for acceleration and flow features are presented for numerical and experimental data. It is found that the new numerical model reproduces experimental results within the bounds of experimental accuracy.