Abstract: A riser tensioner system for tensioning a riser at an offshore vessel includes a frame adapted to affix to the vessel. An elongate riser joint is provided for coupling into the riser. The riser joint has an axial flange portion projecting laterally outward therefrom from and extending axially along a length of the riser joint. Ram pistons are arranged about the frame to couple to the riser and to support the riser in tension. Centralizer arms are arranged on the frame to extend into proximity to the flange portion of the riser joint when the riser joint is received through the frame.