Abstract: A thermally insulated pipeline assembly comprises an inner pipe surrounded by and spaced from a jacket, which is in turn surrounded by and spaced from an outer casing. The annular space between the inner pipe and the jacket is substantially filled with thermal insulating material such as open cell polyurethane foam, which is preferably injected into the jacket and cured in situ. The space between the jacket and the outer casing may be filled with pressurized gas. The jacket may have apertures to allow the gas access to the annular insulated space. The arrangement provides improved thermal insulation, particularly when the casing is pressurized, by reducing thermal transfer by convection between the inner pipe and the outer casing. The arrangement is also applicable to pipeline bundles, with a plurality of inner pipes each having an associated jacket and insulation, all surrounded by a common casing.