Assembly of coaxial pipes including junction parts with sealed inner cavities, and method for making same

An assembly of at least two coaxial pipes constituted in a main portion by an outer pipe and an inner pipe defining an annular space preferably containing lagging material, the assembly being constituted by assembling together unit elements of at least two coaxial pipes, comprising at least two coaxial pipe assembly unit elements, one of which has, at at least one end, a first junction part joined to a second junction part at the end of another said coaxial pipe assembly unit element, each of said first and second junction parts being a body of revolution defined as follows:

• in a radial direction relative to a longitudinal axis XX′ of symmetry of said part, it is defined by a cylindrical inner wall of substantially the same diameter as the intermediate portion of said inner pipe, and by a cylindrical outer wall of diameter substantially equal to the outside diameter of the intermediate portion of said outer pipe;
• and
• in said direction of the longitudinal axis XX′
• on the end of said junction part that is joined by welding to the ends of said outer and inner pipes of a said unit element of an assembly of at least two coaxial pipes, said outer and inner walls of said junction part forming respective outer and inner first branches in longitudinal section, said outer and inner first branches defining a first annular cavity;
• and
• at the opposite end of said junction part where it is joined to another said junction part, itself connected by welding to the end of said another unit element of an assembly of two coaxial pipes, said outer and inner walls forming in longitudinal section respective outer and inner second branches defining an internal second annular cavity;
• the two said first and second junction parts being welded to each other solely via the ends of their said outer second branches;
• the bottoms of said first and second cavities being spaced apart in said longitudinal direction XX′ so as to define a massive solid zone of said junction part in which said outer and inner walls form the outer and inner faces of a single cylindrical wall;
• and
• the ends of said two non-welded-together inner second branches of said first and second junction parts are in metal-on-metal contact with each other under elastic deformation stress via their contact surface, which is a surface of revolution, between an outer first surface of revolution on the outer face of the inner second branch of the first junction part and an inner second surface of revolution on the inner face of the inner second branch of the second junction part, said inner second branch of the first junction part forming a male inner second branch situated inside the female inner second branch of the second junction part,
• wherein said inner second surface of revolution is of a shape that is different, but of the same axis of revolution as the outer first surface of revolution, such that said outer first surface of revolution and inner second surface of revolution present, when they are arranged coaxially and engaged in contact one against the other, an initial line of contact that is circular and that transforms into said contact surface after said elastic deformation stress resulting from the elastic deformation due to the shrinkage of the weld between the ends of said outer second branches of said two junction parts, so that said internal second cavities are leaktight relative to the inside of the inner pipe.