As for WPS for crack repair, once the crack is removed from the base metal, the joint is either a partial penetration groove weld if the crack/excavation did not go all the way though, or if it did go all the way through, it would be a CJP groove weld. Some things you might have to address that might not come up during regular fabrication are, root openings for CJP welds can become very large. How do you plan on filling up a large root? How are you planning on inspecting the excavation to make sure the crack is completely gone? Joint restraint is normally very high. Do you need to preheat extra, peen, use extra-low hydrogen procedures, or to stress relieving afterwards because of this? What method are you going to use to remove the crack, and does it need preheated before crack removal?

As far as qualification, a regular butt joint on the appropriate thickness plate is adequate. Nothing special really.