Hi Pierre, if you do a "Save As" and have both curves and surfaces, the curves are not written to SKP format in that case, that's because the curves can be kind of strange in a certain way to export since they get embedded in objects because SketchUp merges things together.

But if you select both surfaces and curves and do a File > Export instead of just a "Save As" then they should export.

Also the curve SKP export can occasionally crash inside the SketchUp library in some cases when it does end up merging some curve vertices together with other neighboring ones so make sure to save your work before doing that kind of export.

- Michael