Heave compensating system

1. A heave compensating system for a marine vessel, the system comprising:

a hydraulic actuator of the vessel configured to couple to a load suspended from the vessel and vary the distance between the load and the vessel in response to heaving motion of the vessel;

wherein the hydraulic actuator is connected to a first hydraulic machine for actuation by the first hydraulic machine;

a second hydraulic machine connected to a hydraulic accumulator, wherein both the first and second hydraulic machines are coupled to one another and to a shared electric motor;

a controller configured to control hydraulic movement of the first and second hydraulic machines and to control power to the electric motor in response to one or more signals representative of at least one of a wave-induced heave movement of the vessel and a wave induced force applied to the load;

wherein the first and second hydraulic machines and the electric motor are all mounted via a common drive shaft.

2. The system of claim 1 , wherein the first and second hydraulic machines and the electric motor are coupled via a direct 1:1 ratio.

3. The system of claim 1 , wherein the electric motor is disposed between the first hydraulic machine and the second hydraulic machine.

4. The system of claim 1 , wherein the electric motor is an asynchronous motor.

5. The system of claim 1 , further comprising a valve coupled to the accumulator and the actuator, wherein the valve is configured to move between a first position in which the accumulator and the actuator are fluidly isolated from one another, and a second position in which the accumulator and the actuator are connected.

6. The system according to claim 5 , wherein the controller is configured to control operation of the valve in response to a signal representative of the hydraulic pressure in the accumulator, the controller being configured to move the valve from the first position to the second position in response to the pressure falling to a predetermined threshold value.

7. The system of claim 1 , wherein the controller is configured to receive a signal representative of the hydraulic pressure in the accumulator, and to control power to the electric motor in response thereto.

8. The system of claim 1 , wherein the controller is configured to receive a signal representative of the position of the load relative to the vessel and to control movement of the first and second hydraulic machines in response thereto.

9. The system of claim 1 , wherein the controller is configured to maintain a substantially constant support force on the load suspended from the vessel during heaving movement of the vessel.

10. The system of claim 1 , wherein the controller is configured to maintain the load suspended from the vessel in a substantially constant position during heaving movement of the vessel.

11. A method of operating a heave compensating system, comprising:

providing a first hydraulic machine, a second hydraulic machine, and an electric motor, wherein the first and second hydraulic machines and the electric motor are all mounted via a common drive shaft;

detecting a wave-induced heave movement of a vessel;

operating the first hydraulic machine in response to the detected wave-induced heave movement using a controller;

actuating a hydraulic actuator to vary the distance between the vessel and a load suspended from the vessel using the first hydraulic machine; and

operating the second hydraulic machine using a controller to drive the first hydraulic machine.

12. The method according to claim 11 , further comprising applying torque to a shaft coupled to the first and second hydraulic machines using an electric motor.

13. The method of claim 12 , further comprising actively controlling energization of the electric motor using the controller.

14. The method of claim 11 , further comprising connecting a hydraulic accumulator to the hydraulic actuator using a valve, in response to the pressure within the accumulator falling below a predetermined threshold value.

15. A heave compensating system for a marine vessel, the system comprising:

a hydraulic actuator of the vessel configured to couple to a load suspended from the vessel and vary the distance between the load and the vessel in response to heaving motion of the vessel;

wherein the hydraulic actuator is connected to a first hydraulic machine for actuation by the first hydraulic machine;

a second hydraulic machine connected to a hydraulic accumulator, wherein both the first and second hydraulic machines are coupled to one another;

a valve connected to the actuator and the accumulator, the valve configured to move between a first position in which the accumulator and the actuator are fluidly isolated from one another, and a second position in which the accumulator and the actuator are connected; and

a controller configured to control hydraulic movement of the first and second hydraulic machines in response to one or more signals representative of at least one of a wave-induced heave movement of the vessel and a wave induced force applied to the load;

wherein the first and second hydraulic machines and an electric motor are all mounted via a common drive shaft.

16. The system of claim 15 , wherein the electric motor is mechanically coupled to the first and second hydraulic machines, and wherein the electric motor is configured to provide a torque to the first and second hydraulic machines.

17. The system of claim 16 , wherein the controller is configured to control power to the electric motor in response to one or more signals representative of at least one of a wave-induced heave movement of the vessel and a wave induced force applied to the load.