Abstract: A pressure compensator for a sub-sea pumping system for pumping an effluent from a deep-sea wellhead comprising a topside module, a sub-sea module, and an umbilical connection between the topside and sub-sea modules. Hydraulic fluid is circulated through the sub-sea module for cooling and lubricating the motor and the pump. The hydraulic cooling and lubricating fluid is preferably a single hydraulic fluid that is compatible with the effluent being pumped. The hydraulic fluid is circulated through a submerged pressure compensator which may include a bellows assembly. The pressure compensator is responsive to the pressure of the effluent being pumped and pressurizes the hydraulic cooling and lubricating fluid to a pressure above the pressure of the effluent. The pressure compensator also responds when the volume of hydraulic cooling and lubricating fluid in the system is reduced or increased, such as by leakage or thermal expansion, in order to maintain the pressure of the hydraulic fluid.