The present invention concerns a robot system (1) for automated performance of working operations in open structures (17) in the heavy industries, comprising a portal structure (2) having at least two parallel longitudinal girders (3) provided with travelling paths (4) (X direction) to support engagement parts (10) at each end of at least one transverse robot gantry (9), which comprises a carrier girder (12) having travelling paths (13) (Y direction) to support engagement parts (14) on a robot arm (15), said robot arm (15) having an upper end which may be advanced at a maximum height (L) above the carrier girder (12), as well as a lower part which may move in an upward and downward direction (Z direction), and which has a lower working head (16) to perform said operations. The system is characterized in that the carrier girder (12) of the gantry (9) is provided at a distance below the upper side of the longitudinal girders (3), said distance corresponding at least to the maximum height (L) of the robot arm above the carrier girder (12).