Abstract: The present paper considers the prediction of wave loading on large offshore structures under steep shallow wave conditions for which severe nonlinearities may be presented. A numerical nonlinear diffraction method developed recently has been made more efficient in various ways so that forces can be calculated economically. This method is compared with other approximate prediction methods and with wave force experiments for surface-piercing cylinders subjected to steep shallow waves. It is found that of the methods considered, the nonlinear diffraction predictions agree most closely with the measured forces. In addition, it is theoretically justifiable and reasonably efficient.