Subsea pipelines equipped with direct electrical heating systems

1. A heated subsea pipeline, comprising:

a direct electrical heating (DEH) system that is arranged to heat a central elongate portion of the pipeline;

at least one supplementary heating system that extends along an end portion of the pipeline, longitudinally outboard of the central portion heated by the DEH system, wherein the supplementary heating system comprises at least one heating element in thermal communication with an inner flowline of the pipeline; and

external connectors provided at ends of the at least one heating element for the supply of energy from an underwater vehicle to the supplementary heating system, wherein the connectors are supported by a bulkhead, connector plate or end structure of the pipeline.

2. The pipeline of claim 1 , wherein the central portion extends along the pipeline between current transfer zones that are mutually spaced along the pipeline.

3. The pipeline of claim 2 , wherein the end portions include longitudinally outboard portions of the current transfer zones.

4. The pipeline of claim 1 , wherein the central portion extends along the pipeline between current injection points that are mutually spaced along the pipeline.

5. The pipeline of claim 4 , wherein the end portions are longitudinally outboard of the current injection points.

6. The pipeline of claim 1 , wherein the heating element is in contact with the flowline.

7. The pipeline of claim 1 , wherein the heating element is surrounded by thermal insulation that encircles the flowline.

8. The pipeline of claim 7 , wherein the heating element is disposed within an annulus between the inner flowline and the thermal insulation.

9. The pipeline of claim 7 , wherein the heating element is embedded in the thermal insulation.

10. The pipeline of claim 1 , wherein the heating element is a hot fluid conduit.

11. The pipeline of claim 10 , wherein the conduit has a diameter of less than two inches (5.1 cm).

12. The pipeline of claim 1 , in combination with a feeder cable that is connected electrically to the flowline.

13. The pipeline of claim 1 , in combination with a return cable that is connected electrically to the flowline.

14. The pipeline of claim 1 , wherein the connectors are hot stab couplings.

15. In combination, the pipeline of claim 1 with an underwater vehicle that is configured to supply energy to the supplementary heating system.

16. The combination of claim 15 , wherein the underwater vehicle carries connectors that are cooperable with the connectors of the pipeline to complete a heating circuit.

17. The combination of claim 16 , wherein the underwater vehicle carries a pump that is arranged to drive a flow of heating fluid around the heating circuit.

18. The combination of claim 17 , wherein the underwater vehicle carries a heater that is arranged to heat the flow of heating fluid.

19. A method of heating a subsea pipeline that is heated primarily by a direct electrical heating (DEH) system, the method comprising:

connecting an underwater vehicle to external connectors of at least one supplementary heating system that extends along an end portion of the pipeline, longitudinally outboard of a central portion of the pipeline heated by the DEH system, the external connectors being supported by a bulkhead, connector plate or end structure of the pipeline; and

activating the supplementary heating system by circulating a flow of heating fluid between the underwater vehicle and the supplementary heating system, along the end portion of the pipeline.

20. The method of claim 19 , comprising pumping the flow of the heating fluid from the underwater vehicle into the supplementary heating system.

21. The method of claim 19 , comprising heating the flow of the heating fluid aboard the underwater vehicle.

22. The method of claim 19 , comprising activating the supplementary heating system when the pipeline is shut down.