You have put your question wrongfully! You should put it in 'Engineering' field.. To reply your question, and with regard to the fact that you are fully familiar with welding procedures, you must note that the main subject that a pipe-fitter should know is the gravity center of objects for rigging and fit-up. Second, is the material concept, and you should know about material standards (ANSI, ASME, ...), however, according to your former position I believe that you are familiar with them enough. The last thing is being familiar with pipe-fitting tools, such as external/internal clamps, overhead-cranes, special wrenches, tack-welding tools, etc.