Load-sharing in parallel fluid pumps

1. A method of sharing load between a plurality of parallel fluid pumps in a subsea fluid pumping system, said subsea fluid pumping system comprising:

a first fluid pumping unit comprising a first pump and a first motor which is drivingly connected to the first pump, the first pump comprising a suction conduit which is in fluid communication with a fluid inlet conduit and a discharge conduit which is in fluid communication with a fluid outlet conduit;

a second fluid pumping unit comprising a second pump and a second motor which is drivingly connected to the second pump, the second pump comprising a suction conduit which is in fluid communication with the fluid inlet conduit and a discharge conduit which is in fluid communication with the fluid outlet conduit, thus rendering the second pumping unit parallel to the first pumping unit;

a return conduit which is connected between the fluid outlet conduit and the fluid inlet conduit, thus providing a feed-back path for a fluid from the fluid outlet conduit to the fluid inlet conduit; and

a control valve which controls a flow of the fluid through the return conduit;

the method comprising the steps of:

establishing a pump limit characteristics diagram for the first pumping unit by mapping a minimal allowable torque of the first pumping unit as a function of a differential pressure across the first pump, and identifying a permissible operating region of the first pumping unit defined by a set of minimum allowable torque values for the first pumping unit;

establishing a pump limit characteristics diagram for the second pumping unit by mapping a minimal allowable torque of the second pumping unit as a function of a differential pressure across the second pump, and identifying a permissible operating region of the second pumping unit defined by a set of minimum allowable torque values for the second pumping unit;

monitoring the torque of the first pumping unit and the differential pressure across the first pump, acquiring a monitored torque value (T m a) and a monitored differential pressure value (DP m a) of the first pumping unit, and identifying the minimum allowable torque value (T 0 a) of the first pumping unit corresponding to the monitored differential pressure value (DP m a) of the first pumping unit;

monitoring the torque of the second pumping unit and the differential pressure across the second pump, acquiring a monitored torque value (T m b) and a monitored differential pressure value (DP m b) of the second pumping unit, and identifying the minimum allowable torque value (T 0 b) of the second pumping unit corresponding to the monitored differential pressure value (DP m b) of the second pumping unit; and

regulating the rotational speed of the first and the second pumps such that T m a/T 0 a equals T m b/T 0 b.

2. The method according to claim 1 , further comprising the step of:

regulating the control valve such that the monitored torque value (T m a) of the first pumping unit does not fall below the minimum allowable torque value (T 0 a) of the first pumping unit, and such that the monitored torque value (T m b) of the second pumping unit does not fall below the minimum allowable torque value (T 0 b) of the second pumping unit.

3. The method according to claim 1 , wherein said steps of establishing the pump limit characteristics diagrams for the first and the second pumping units are performed prior to operating the pumping system, and wherein said steps of monitoring the torque of the first pumping unit and the differential pressure across the first pump, monitoring the torque of the second pumping unit and the differential pressure across the second pump, and regulating the rotational speed of the first and the second pumps are performed at a predetermined interval during operation of the pumping system.

4. The method according to claim 1 , wherein said steps of establishing the pump limit characteristics diagrams for the first and the second pumping units comprises storing the pump limit characteristics diagrams in a control system of the pumping system.

5. The method according to claim 4 , wherein the step of storing the pump limit characteristics diagrams in the control system comprises storing said set of minimum allowable torque values for said first and second pumping units in look-up tables.

6. The method according to any one of claims 4 and 5 , wherein the step of storing the pump limit characteristics diagrams in the control system comprises storing the pump limit characteristics diagrams in a data storage unit in the control system.

7. The method according to claim 1 , wherein said fluid is a multiphase or a variable density fluid.