The importance of the evaluations of the response amplitude operators as well as of the accelerations in several points of interest on board of ship is directly linked to the index of operation of a floating structure. The higher the sea state to operate, the better the ship efficiency and operability. The aim of the present paper is to underline the importance of the evaluation of seakeeping performances of an AHTS ship in order to evaluate the upper limits of the affordable sea states from the point of view of roll the motions amplitudes and the comfort on board. The effects of bilge keel on roll motions are also investigated. The paper is dedicated to the memory of our Prof. Dr. Eng. Ion Bidoae, pioneer in ship hydrodynamics research and education in Romania, who passed away 20 years ago.