Helically wound tubular structures

A tubular structure comprising:

• a) a first inner region of the tubular structure comprising layers of a first sheet metal helically wound about a longitudinal axis;
• b) a second middle region of the tubular structure comprising layers of a second sheet metal having a plurality of voids disposed therein, said second sheet metal being helically wound about said longitudinal axis and coaxially about said first inner region, wherein a first set of the plurality of voids is located in a first layer of said second sheet metal and a second set of said plurality of voids is located in a second layer of said second sheet metal, said second layer being more radially distant from said longitudinal axis than said first layer of said second sheet metal,
• wherein the voids of said first set are each aligned with a respective one of said voids of said second set such that the second middle region comprises internal pockets each defined by a void of the first set and an aligned void of the second set;
• wherein the plurality of voids further comprises at least one of:
• 1) said voids of said second set of said plurality of voids being more spaced from one another than said voids of said first set of said plurality of voids are from each other;
• and
• 2) a second length dimension of each of said voids of said second set of said plurality of voids being greater than a corresponding first length dimension of each of said voids of said first set of said plurality of voids;
• c) a third outer region of the tubular structure comprising layers of an additional sheet metal helically wound about said longitudinal axis and coaxially about said first sheet metal and said second sheet metal, enclosing the internal pockets of the second middle region;
• and
• d) an insulation disposed within said enclosed internal pockets, said insulation being selected from the group consisting of mineral wool, fiberglass, rigid polyurethane foam, silica, aerogels, a vacuum, and combinations thereof.