Abstract: A centralizer for centering a marine riser in an opening in a working deck. The assembly comprises a plurality of cam arm units pivotally coupled to a portion of the working deck for pivotal movement about pivot axes parallel to the top planar surface of the deck and a plurality of hydraulic power devices for pivoting the cam arm units from a retracted position spaced away from the riser to an active position engaging the riser. Each cam arm unit includes a camming surface which increases in radius from the pivot axis thereof from a first end towards a second end, this second end engaging the riser when each unit is in the active position. Each camming surface is defined by surfaces of a plurality of rollers, two of which are located at the largest radii second end and have the same radii from the pivot axis.