Mitigation of buckling in subsea pipe-in-pipe systems

A subsea pipeline of pipe in pipe configuration, comprising:

• an inner pipe;
• an outer pipe spaced radially from the inner pipe;
• an annulus defined by the radial spacing between the inner and outer pipes;
• a series of longitudinally spaced outward projections that extend radially outwardly into the annulus from the inner pipe and that are movable longitudinally relative to the outer pipe, wherein the outward projections are equi-spaced along the length of the inner pipe;
• and
• a series of longitudinally spaced inward projections that extend radially inwardly into the annulus from the outer pipe and that are movable longitudinally relative to the inner pipe, wherein the inward projections are equi-spaced along the length of the outer pipe, and wherein the longitudinal spacing between successive inward projections is substantially equal to the longitudinal spacing between successive outward projections;
• wherein, 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;
• wherein the longitudinal spacing between the inward and outward projections defines, by interlocking abutment of the inward projections with the outward projections, a range of said longitudinal movement of the inner pipe relative to the outer pipe, said range of relative longitudinal movement being delimited by a first position in which the inward projections bear against first faces of the outward projections and a second position in which the inward projections bear against second faces of the outward projections opposed longitudinally to the first faces.