Operations such as lofting of surfaces, offset of surfaces and offset of structures of surface will sometimes produce self-intersecting surfaces, as will offset self-intersecting shells. Often the self-intersections will be large and easily detected by visual inspection. Sometimes minor editing of the input data can solve the problem. In other cases, expensive redesign or working around them is required. However, when the self-intersections are small they will easily remain in the CAD-model, later inducing problems when the CAD-model is used in applications such as simulation or production planning.