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

A transition section disposed between successively spoolable, electrically trace-heated PiP pipelines comprises an inner pipe, an outer pipe, and an annulus between the inner and outer pipes. The annulus contains heating cables that extend longitudinally between annuli of the pipelines and longitudinally spaced seals that, when deactivated, allow fluid communication between the annuli of the pipelines and, when activated, isolate the annuli of the pipelines from each other. Longitudinally spaced blocking plates close the lumen of the inner pipe and define an inner chamber between them. Longitudinally spaced openings penetrate a wall of the inner pipe at locations longitudinally inboard of the blocking plates and the seals. The openings effect fluid communication between the annulus and the inner chamber and also define a diversion path for the heating cables that extends from the annulus to the inner chamber and back to the annulus.