The Model SL subsea hydraulic rotary stepping actuator is designed to be lightweight, compact, and powerful with a high degree of reliability, precision, and durability. It uses hydraulic spring-return cylinders linked to ratchet-pawl mechanisms to step the valve-stem nut in 1/10-of-a-turn increments. Separate cylinders provide valve-open and valve-close operation.

The operating cylinders are designed to balance the back-pressure in the supply lines; this ensures unhindered piston retraction between stepping cycles and no change in actuator-housing volume throughout the stepping cycle. As well, the actuator housing is pressure compensated to balance the internal pressure with that of the external water pressure.

The actuator includes an override that can be configured to allow the choke to be operated by diver or by remotely operated vehicle (ROV). As well, because the actuator features a modular design, the override is available in either top drive (parallel to centerline) or side drive (perpendicular to centerline). The override unit may also be positioned in 90° increments about the actuator centerline.

A high-visibility local position indicator is standard on this actuator. As well, an optional linear variable differential transformer (LVDT) is available to directly measure the position of the valve stem for accurate electrical feedback of the choke position. The electrical leads for the LVDT are terminated in a bulkhead-mounted wet-mateable connector supplied by third parties.

Additionally, various hydraulic couplings to suit customer requirements can be supplied mounted and tubed to the actuator.

Due to the modular nature of this actuator, it may also be supplied as simply a manual operator without the actuator mechanism. All of the options regarding drive location and electrical feedback still remain.

Advantages:
-simple, robust design.
-high torque capability.
-highly reliable function.
-qualified to extreme water depths.