It seems that several fie types that use u3d extension are 3D models that are eligible for conversion to other 3D formats, including IGES format (.igs, .iges), so some kind of u3d to igs conversion or export is certainly possible.
Most tools that deal with 3D graphics usually have support for IGES format or it can be added with some plugin. You can also always use some dedicated 3D converter utility to perform this conversion.