Determining flow rates of multiphase fluids

1. A method for determining flow rate of a multiphase fluid comprising:

measuring a first pressure differential of the multiphase fluid flowing through a rotodynamic pump operating at a rotational speed thereby driving the multiphase fluid;

measuring a second pressure differential of the multiphase fluid flowing through a portion of a fluid conduit positioned in series with said rotodynamic pump; and

determining, based on the first and second pressure differentials and the rotational speed of said rotodynamic pump, a flow rate of the multiphase fluid.

2. The method of claim 1 further comprising altering a pump speed based on said determined flow rate of the multiphase fluid so as to maintain the multiphase flow rate within a predetermined range.

3. The method of claim 1 wherein the multiphase fluid comprises an oil liquid phase and a water liquid phase, and said pump and portion of the fluid conduit are located in a subsea location.

4. The method of claim 1 wherein the multiphase fluid comprises an oil liquid phase and a water liquid phase, and said pump and portion of the fluid conduit are located on land.

5. The method of claim 1 wherein the multiphase fluid comprises a gas phase, an oil liquid phase and a water liquid phase.

6. The method of claim 1 wherein said rotodynamic pump is a single-stage pump, and said first pressure differential is a pressure difference between an inlet pressure and an outlet pressure of said single-stage pump.

7. The method of claim 1 wherein said rotodynamic pump is a multi-stage pump, and said first pressure differential is a pressure difference between an inlet pressure and an outlet pressure of said multi-stage pump.

8. The method of claim 1 wherein said rotodynamic pump is a multi-stage pump, and said first pressure differential is a pressure difference between an inlet pressure and an outlet pressure of said a single-stage of said multi-stage pump.

9. The method of claim 1 wherein said portion of the fluid conduit is a pressure-based flowmeter configured to measure flow rate of a single-phase fluid.

10. The method of claim 1 wherein said portion of the fluid conduit is a venturi having an inlet and a throat, and said second pressure differential is a pressure difference between said inlet and said throat of the venturi.

11. The method of claim 1 wherein said portion of the fluid conduit comprises a constriction of a type selected from a group consisting of: venturi; V-cone; orifice plate; dall tube; Pitot tube; and multi-hole pressure probe.

12. The method of claim 1 further comprising comparing said determined flow rate for the multiphase fluid with a flow rate for the multiphase fluid determined using a multiphase flow meter.

13. The method of claim 1 further comprising using said determined flow rate for the multiphase fluid to supplement a multiphase flow meter during times of unavailability of the multiphase flow meter.

14. A system for determining flow rate of a multiphase fluid comprising:

a rotodynamic pump configured to drive a multiphase fluid;

a fluid conduit positioned in series with said rotodynamic pump and between a wellhead and a topside receiving facility; and

a processing system configured to:

receive a first pressure differential of the multiphase fluid flowing through said pump;

receive a second pressure differential of the multiphase fluid flowing through a portion of said fluid conduit;

receive a rotational pump speed at which the pump is operating; and

determine, based on the first and second pressure differentials and the rotational pump speed, a flow rate of the multiphase fluid.

15. The system of claim 14 wherein the processing system is further configured to alter a pump speed based on said determined flow rate of the multiphase fluid so as to maintain the multiphase flow rate within a predetermined range.

16. The system of claim 14 wherein the multiphase fluid comprises an oil liquid phase and a water liquid phase, and said pump and portion of the fluid conduit are located in a subsea location.

17. The system of claim 14 wherein the multiphase fluid comprises a gas phase, an oil liquid phase and a water liquid phase.

18. The system of claim 14 wherein said first pressure differential is a pressure difference between an inlet pressure and an outlet pressure of said rotodynamic pump.

19. The system of claim 14 wherein said portion of the fluid conduit is a venturi having an inlet and a throat, and said second pressure differential is a pressure difference between a said inlet and said throat of the venturi.

20. The system of claim 14 wherein said portion of the fluid conduit comprises a constriction of a type selected from a group consisting of: venturi; V-cone; orifice plate; dall tube; Pitot tube; and multi-hole pressure probe.

21. A method for determining flow rate of a multiphase fluid comprising:

measuring a pressure differential of the multiphase fluid flowing through a rotodynamic pump operating at a first rotational speed thereby driving the multiphase fluid;

determining a mixture density for the multiphase fluid flowing across a portion of a fluid conduit in series with the rotodynamic pump; and

combining the differential flow rate, mixture density, and the rotational speed of the rotodynamic pump thereby determining a flow rate of the multiphase fluid.

22. The method of claim 21 wherein the portion of the fluid conduit comprises a volume or velocity flowmeter that measures a volumetric flowrate, and said mixture density is based on a known relationship between differential pressure across said pump and flow rate for a plurality of mixture densities.

23. The method of claim 22 wherein the flowmeter is of type selected from a group consisting of: turbine meter; positive displacement meter; drag force meter; and ultrasonic flow meter.

24. The method of claim 23 wherein the flowmeter is a mass flow meter, and said mixture density is determined based on a known relationship between flow rate and mixture density for a mass flow rate measured by the mass flow meter.