Abstract: The present invention relates to a unit element of undersea pipe, in particular for providing a bottom-to-surface connection, constituted by an assembly of two coaxial pipe elements of the PiP type (1) presenting an inner pipe element (2) and an outer pipe element (3) coaxial thereabout with an annular space (41) between the pipes, preferably including a thermally insulating material (42) in said annular space, and including at each end a junction piece (6, 6a-6b) in the form of a steel forging for connecting together two of said unit elements of axial pipe (1), said junction forgings (6) being welded to the ends of both of said inner and outer pipes, said unit elements being characterized in that said inner pipe element (2) comprises in its main portion an inner tube (21) of metal presenting thickness that is small relative to the thickness of the ends of said inner pipes that are welded to said junction forgings (6), the thickness of said inner tube being less than 50% the thickness of said ends of said inn