Tube stiffening

A method of forming a tubular structure, the method comprising:

• positioning a first edge region of a first material and a second edge region of the first material in an edge-to-edge abutting relationship to one another to form a first spiral extending along an outer surface of the first material;
• moving an elongate edge of a second material onto the outer surface of the first material to form a second spiral as the first spiral of the edge-to-edge abutting relationship of the first material is being formed, wherein the first spiral and the second spiral are longitudinally spaced relative to one another along the outer surface of the first material;
• joining the first edge region of the first material to the second edge region of the first material in the edge-to-edge abutting relationship along the first spiral;
• and
• welding the elongate edge of the second material to the first material along the second spiral,
• wherein
• the first material and the second material are each formed of metal,
• the elongate edge of the second material has a first area in contact with the first material along the second spiral,
• a surface of the second material has a second area in a direction extending radially from the elongate edge of the second material, and
• the second area of the surface of the second material is greater than the first area of the elongate edge of second material in contact with the outer surface of the first material.