this is a pipe construction set which supports 3 LOD-resolutions: 6, 12 and 24

it contains a version of the pipe with thickness (inner faces)

the preview image with flat shading shows the LOD-effect very good.

for unwrapping i used project as cube, that works good for these models

the metal texture is mine

you can join multiple objects of the same LOD-resolution, but don't forget to remove the doubles then and to unwrap again.