A heated subsea pipeline 10 comprises a direct electrical heating (DEH) system that heats a central major portion 40 of the pipeline. Supplementary heating systems 42 extend along respective end portions 38 of the pipeline, longitudinally outboard of the central portion 40 heated by the DEH system. A flow of heating fluid may be circulated along the end portions 44, and may be circulated through an underwater vehicle that pumps and heats the flow and connects to the supplementary heating system 42 by connectors such as hot stab connectors 46 when required.
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