Abstract: A subsea pipeline (10) of pipe-in-pipe configuration comprises an inner pipe (14), an outer pipe (16) spaced radially from the inner pipe and an annulus (20) defined by the radial spacing between the inner and outer pipes. A series of longitudinally-spaced outward projections (22) extend radially outwardly into the annulus from the inner pipe and are movable longitudinally relative to the outer pipe. A corresponding series of longitudinally-spaced inward projections (24) extend radially inwardly into the annulus from the outer pipe and are movable longitudinally relative to the inner pipe. When the inner pipe is subject to thermal elongation or contraction in use of the pipeline, the inner pipe is movable longitudinally relative to the outer pipe, hence moving the outward projections between and relative to the inward projections. The pipeline may be buried to restrain the outer pipe. The annulus may be flooded, in which case the inner pipe is covered with wet insulation.