This article presents a comparative study of two practical methods for the estimation of the hydrodynamic lends and motion responses of a semisubmersible to demonstrate some important shortcomings inherent with each method. These methods are: Morison's equation with constant hydrodynamic coefficients, and panel method based on two-dimensional (2-D) Green's function integral equation technique. The article includes a brief formulation of each method dedicated to a twin-hull semisubmersible and presents the results of the comparative calculations with each method performed on a twin-circular hull of eight-column semisubmersible model. The results of the calculations are discussed by emphasizing relative importance of these shortcomings and recommendations are made to the designers to be aware of them in the rise ef both methods. [S0892-7219(00)00401-5].