In Russia, there's a special term for a pipe that is not circular in its section: "a profiled pipe" (профильная труба). There's even a Wikipedia page for it: профильная труба. Such pipes come in a wide variety of sizes and usually have an oval, square or rectangular section ("profile" in Russian), but other kinds of sections are possible.
I wonder what is the common cover-all technical term describing "any kind of non-circular pipe".
Example of usage:
Pervouralsk New Pipe Plant produces a range of non-circular pipes, including pipes of variable cross-section.