to maintain a riser under tension , the riser can be connected to a gripper which is connected to vertical toothed racks that are in constant engagement with gear wheels which are driven with constant torque , preferably by hydraulic motors at constant pressure . an apparatus is also described for putting the riser under tension . the apparatus includes vertical guides in a derrick , toothed racks in the vertical guides , a gripper designed for interaction with a riser connected to the racks , and drive units arranged in the derrick for drive - actuation of the racks in the vertical guides .