Subsea mud pump with reduced pulsation

A positive-displacement pump, comprising:

• a plurality of pumping elements, each of the pumping elements comprising a pressure vessel having a first and a second chamber and a separating member disposed between the first and second chambers, the first chamber and the second chamber being hydraulically connected to receive and discharge a working fluid and a driving fluid, respectively, wherein the separating member moves within the pressure vessel in response to pressure differential between the first and second chambers;
• a valve assembly having actuated suction and discharge valves each in communication with one of the first chambers, the suction and discharge valves being selectively operable to permit fluid to alternately flow into and out of the first chambers;
• a variable-displacement pump for supplying the driving fluid to the second chambers;
• a plurality of flow control valves for alternately communicating the variable-displacement pump with the second chambers;
• and
• a control module for controlling the discharge rate of the variable-displacement pump and the positions of the flow control valves such that the fluid discharged from the first chambers is substantially free of pulsation.