Topsides of offshore platforms contain different equipments, therefore earthquake excitation may impose large forces on them. In designing these topsides, the interaction of the secondary systems and primary structure is not usually considered, which is not realistic. In order to study this subject a 3D model of a typical platform in the Persian Gulf is constructed and analyzed, using OpenSees software, when subjected to different records. These records are scaled for two earthquake levels. In the first level the structure is in elastic condition but for the second level it behaves inelastically. According to the results, in secondary systems with low periods of vibration, structure and secondary system interactions can be neglected. But when the period of secondary system is increased, it is necessary to consider interactions effects.