An apparatus that may reduce friction of a tubular structure in a horizontal or deviated well. The apparatus is for mounting on a tubular structure, such as a casing or drill string having a longitudinal axis. The apparatus comprises a tubular segment for mounting over the casing or drill string such that the apparatus is freely rotatable about the longitudinal axis. The apparatus also includes a plurality of ridges on the outer face of the tubular segment, the ridges being at an angle to an axial direction of the tubular segment to cause the apparatus to rotate responsive to movement of the apparatus against a wall of the wellbore as the apparatus traverses the wellbore. The raised ridges have a non-uniform height from the outer face of the tubular segment.