Spooling and installing trace-heated pipelines of pipe-in-pipe configuration

Electrically trace-heated pipe-in-pipe pipelines joined end-to-end via a transition section, wherein the transition section comprises:

• an inner pipe and an outer pipe spaced radially from the inner pipe;
• an annulus defined between the inner and outer pipes, the annulus of the transition section containing heating cables that extend longitudinally between annuli of the pipelines and also containing longitudinally spaced seals that are switchable between a deactivated state to effect fluid communication between the annuli of the pipelines and an activated state to isolate the annuli of the pipelines from each other;
• longitudinally spaced blocking plates that close a lumen of the inner pipe and define an inner chamber between them;
• and
• longitudinally spaced openings that penetrate a tubular wall of the inner pipe at locations longitudinally inboard of the blocking plates and the seals to effect fluid communication between the annulus of the transition section and the inner chamber and to define a diversion path for the heating cables that extends from the annulus of the transition section to the inner chamber and back to the annulus of the transition section.