How to obtain the continuous surfaces?

Hi, I want to get the contour of a shell with certain thickness (like what is shown in the right picture). Archived image: external imageAnd I have tried the method  ShapeAnalysis_FreeBounds::GetClosedWires() but it can only get the contour of a given face, what I can think is to select the faces manually and sewing them together, then call the GetClosedWires() method.

How can I get all the continuous faces from the shell automatically or is there any other way I can do this except GetClosedWires?