Power supply system for marine drilling vessel

1. A power supply system for a marine drilling vessel, comprising:

a first power grid section coupled to a generator configured to generate electric power and further coupled to an electrically powered active heave compensator configured to raise and lower a drill string relative to the marine drilling vessel, wherein the active heave compensator is configured to generate electric power when lowering the drill string, and

a second power grid section coupled to a generator configured to generate electric power and further coupled to an electrically powered thruster drive of the marine drilling vessel,

wherein the first power grid section is electrically coupled to the second power grid section, and

wherein the power supply system configured to supply electric power generated by the active heave compensator during heave compensation to the thruster drive for operating the thruster drive.

2. The power supply system of claim 1 , wherein the active heave compensator comprises a heave compensating drawworks comprising one or more electric motors.

3. The power supply system of claim 2 ,

wherein the first power grid section comprises an AC bus operating at a predetermined AC frequency, and

wherein the electric motor of the heave compensating drawworks is an AC electric motor and is coupled to the AC bus via a variable speed drive, the variable speed drive comprising an active rectifier and configured to convert electric power generated by the AC electric motor to electric power having a frequency that is substantially equal to the frequency at which the AC bus is operated.

4. The power supply system of claim 1 , further comprising a control unit configured to control the flow of electric power in the power supply system in such a way that electric power generated by the active heave compensator is supplied to the thruster drive.

5. The power supply system of claim 1 ,

wherein the active heave compensator is configured to compensate for a heave motion of the marine drilling vessel caused by sea waves,

wherein a heave compensation cycle comprises a first phase in which the drill string is raised and the active heave compensator consumes electric power and a second phase in which the drill string is lowered and the active heave compensator generates electric power, and

wherein the power supply system is configured to supply the electric power generated during the second phase to the thruster drive.

6. The power supply system of claim 5 , further comprising a control unit configured to operate the thruster drive at a lower power during the first phase of the heave compensation cycle and at a higher power during the second phase of the heave compensation cycle.

7. The power supply system of claim 1 ,

wherein the thruster drive is part of a propulsion system of the marine drilling vessel, the propulsion system comprising a dynamic positioning system, and

wherein the power supply system is configured so as to supply an average electric power to the thruster drive that is above a threshold power required by the dynamic positioning system for position keeping.

8. The power supply system of claim 1 , wherein the power supply system is configured to control the supply of electric power to the thruster drive in such a way that the sum of the electric power consumed by the thruster drive and of the electric power consumed and generated by the active heave compensator is substantially constant over a heave compensation cycle.

9. A method of controlling a power supply system of a marine drilling vessel,

providing a supply system comprising a first power grid section coupled to a generator configured to generate electric power and further coupled to an electrically powered active heave compensator configured to raise and lower a drill string relative to the marine drilling vessel, wherein the active heave compensator is configured to generate electric power when lowering the drill string and a second power grid section coupled to a generator configured to generate electric power and further coupled to an electrically powered thruster drive of the marine drilling vessel, wherein the first power grid section is electrically coupled to the second power grid section, the method comprising:

generating electric power by means of the active heave compensator when the active heave compensator lowers the drill string during heave compensation,

supplying the generated electric power to the thruster drive, and

operating the thruster drive with the supplied electric power.

10. The method of claim 9 ,

wherein the active heave compensator is configured to compensate for a heave motion of the marine drilling vessel caused by sea waves, wherein a heave compensation cycle comprises a first phase in which the drill string is raised and the active heave compensator consumes electric power and a second phase in which the drill string is lowered and the active heave compensator generates electric power,

wherein the step of supplying the generated electric power to the thruster drive is performed during the second phase of the heave compensation cycle.

11. The method of claim 10 , further comprising controlling the operation of the thruster drive such that the thruster drive is operated at a lower power during the first phase of the heave compensation cycle and at a higher power during the second phase of the heave compensation cycle.

12. The method of claim 9 ,

wherein the thruster drive is part of a propulsion system of the marine drilling vessel, the propulsion system comprising a dynamic positioning system, and

wherein the method further comprises controlling the supply of electric power to the thruster drive in such a way that an average electric power is supplied to the thruster drive that is above a threshold electric power required by the dynamic positioning system for position keeping.

13. The method of claim 9 , further comprising controlling the supply of electric power to the thruster drive in such a way that the sum of the electric power consumed by the thruster drive and of the electric power consumed and generated by the active heave compensator is substantially constant over a heave compensation cycle.

14. The method of claim 9 , further comprising: controlling the supply of electric power to the thruster drive in such a way that the load on the one or more generators coupled to the first and second power grid sections is substantially constant over a heave compensation cycle.