Transmission of power underwater

1. A method of providing electric power to subsea equipment underwater, the method comprising:

temporarily coupling a rotary coupling element of a power unit to the subsea equipment;

effecting electromagnetic coupling between the rotary coupling element of the power unit and a stator of the subsea equipment by bringing together the rotary coupling element of the power unit and the stator of the subsea equipment, the electromagnetic coupling of the rotary coupling element of the power unit and the stator of the subsea equipment forms a generator that generates the electric power on board the subsea equipment upon rotation of the rotary coupling element of the power unit;

turning the rotary coupling element to generate electric power on board the subsea equipment; and

stabilizing the power unit and/or the subsea equipment against torque reaction forces by effecting a secondary attachment between the power unit and the subsea equipment.

2. A subsea system for providing electric power to subsea equipment underwater, wherein the subsea equipment requires intermittent supply of power from an external source, the equipment comprising:

at least one on-board electrical load;

an on-board electric power generator connected to the load to supply electric current to the load when the generator is driven;

a first rotary coupling element arranged to be turned by an external drive to drive the generator wherein the first rotary coupling element is arranged to effect magnetic coupling with the external drive and is housed in an enclosure that is arranged to separate the first rotary coupling element from sea water in use; and

one or more secondary attachment points for stabilizing the subsea equipment against torque reaction forces when the first rotary coupling element is turned by the external drive;

the system further comprising a power unit that is separate from the subsea equipment and that implements the external drive;

wherein the external drive comprises a second rotary coupling element that is arranged to effect magnetic coupling with the first rotary coupling element and that turns the first rotary coupling element to drive the on-board electric power generator; and

wherein the second rotary coupling element is housed in an enclosure that is arranged to separate the second rotary coupling element from sea water in use.

3. The equipment of claim 2 , wherein the electrical load comprises at least one on-board rechargeable battery and the generator is connected to the battery to charge the battery when the generator is driven.

4. The system of claim 2 , wherein the enclosure comprises a window aligned with the second rotary coupling element, the window being magnetically transparent relative to a surrounding wall of the enclosure.

5. The system of claim 2 , comprising one or more alignment formations for aligning the subsea equipment and the power unit.

6. A subsea system for providing electric power to subsea equipment underwater, wherein the subsea equipment requires intermittent supply of power from an external source, the equipment comprising:

at least one on-board electrical load;

a male or female stator that is connected to the load and that is arranged to have an externally-driven rotor temporarily placed in or around the stator to complete an electric power generator for supplying electric current to the load when the rotor is placed in or around the stator and turned relative to the stator; and

one or more secondary attachment points for stabilizing the subsea equipment against torque reaction forces when the rotor is turned relative to the stator;

the system further comprising a power unit that is separate from the subsea equipment and that implements the external drive for the rotor,

wherein the rotor is housed in a magnetically-transparent enclosure to separate the rotor from sea water in use.

7. The system of claim 6 , wherein the electrical load comprises at least one on-board rechargeable battery and the stator is connected to the battery to charge the battery when the externally-driven rotor is placed in or around the stator and turned relative to the stator.

8. The system of claim 6 , comprising one or more alignment formations for aligning the subsea equipment and the power unit.

9. The system of claim 6 , comprising one or more secondary attachment points for stabilising the subsea equipment and/or the power unit against torque reaction forces in use.

10. A subsea power unit for providing power to discrete subsea equipment underwater, the power unit comprising:

a rotary coupling element being a rotor for a generator including magnets or windings;

a motor for turning the rotor; and

one or more secondary attachment points for stabilizing the power unit and/or the subsea equipment against torque reaction forces in use; and

comprising an enclosure that houses the generator rotor and is arranged to separate the generator rotor from sea water in use.

11. The power unit of claim 10 , wherein the rotor comprises bearing formations spaced axially along an axis of rotation of the rotor.

12. The power unit of claim 11 , wherein the bearing formations comprise a circumferential flange extending around the rotor.

13. The power unit of claim 11 , wherein the bearing formations comprise a pivot formation disposed on the axis of rotation of the rotor.

14. A subsea system for providing electric power to subsea equipment underwater, wherein the subsea equipment comprises:

at least one on-board electrical load;

a male or female stator that is connected to the load and that is arranged to have an externally-driven rotor temporarily placed in or around the stator to complete an electric power generator for supplying electric current to the load when the rotor is placed in or around the stator and turned relative to the stator; and

comprising a subsea power unit that is separate from the subsea equipment and is as defined in claim 10 .