The hydrodynamic forces generated by the water moving inside a U-shaped tank are predicted by using the open-source Smoothed Particle Hydrodynamic (SPH) solver DualSPHysics. In particular the study focuses on the roll moment prediction of the U-tank undergoing forced roll motion at different requencies. The sensitivity of the hydrodynamic prediction with respect to variations of particle resolution density is investigated by a systematic set of numerical simulations. Results of the SPH simulations are validated by comparison against available experimental data on a particular U-tank shape and discussed both in terms of roll moment amplitude and phase lag with respect to the imposed motion.