Self-intersections prevent the T-Spline from converting to a surface or solid. The problem is not neccesarily the pipe command itself, there are many ways this can happen. In the image you provided those red areas indicate the self-intersections. The difficulty for the software is to determine when there is a self-intersection which side of the intersection should remain and which should be removed. It is usually helpful to put the T-Spline in "box mode" with the Modify - Dispaly Mode or with the body or any of its elements selected use Alt-1 for box, Alt-2 for Control Frame and Alt-3 for Smooth. Editing of the self-intersections will be easier when you are looking at the box model.
