Abstract: A double rack and pinion device (17) is used to maintain substantially constant upward tension on a marine riser (16) that extends from stationary equipment (11) on the ocean floor to a floating structure (10) on the ocean surface. A preferred embodiment of the double rack and pinion device (17) comprises a sleeve (21) fitted over an upper segment of the riser (16), with a generally cylindrical weighted structure (40) surrounding the sleeve (21). Racks (33, 34, 35) are secured to or formed integrally on the exterior surface of the sleeve (21), and corresponding racks (53, 54, 55) are secured to the interior surface of the weighted structure (40). Pinions (63, 64, 65) are supported by the floating structure (10), and extend into the spacing between corresponding pairs of racks on the sleeve (21) and on the weighted structure (40).