An aluminum riser assembly has a plurality of riser joints connectable together to form a riser string. When upper and lower joints are assembled, clamps support auxiliary pipes for carrying hydraulic lines. A drive rotates a sleeve rotatably supported on the upper joint, and the sleeve rotates a union nut rotatably disposed on the upper joint. As the union nut rotates, it moves axially along the riser joint and threads onto the lower joint. The union nut is tightened until it engages an external collar to complete the coupling between the upper and lower riser joints. The entire process can then be repeated for additional upper riser joints to make up a riser string.