Estimating moonpool piston mode response at resonance is important for operation safety. This is a difficult task, in particular, due to nonlinear nature of the moonpool response connected to the damping imposed by the flow separation at the moonpool’s inlet. In the present work, the applicability of a simplified model, based on decomposing the problem into potential and viscous components, is investigated. The moonpool piston mode response is modeled as an additional degree of freedom. The coupling terms between this new degree of freedom and other vessel’s modes of motions are calculated based on potential flow calculations. Radiation and diffraction problems are considered separately. A finite volume solver with linearized boundary conditions is used to obtain the moonpool response under forced vertical motions. A quadratic damping model is fitted to the obtained responses and added to the free-surface condition of the potential flow formulation. The problem is solved both in frequency and time-domain. The validity of the obtained model is investigated by model test comparison for a dummy vessel with moonpool undergoing regular and irregular forced oscillations, as well as an offshore operation vessel with moonpool exposed to irregular waves. The benefits and shortcomings of the model are discussed. It is suggested that this method can be used as a practical tool to address moonpool piston mode response in irregular waves.