Cutting may be done by machine, arc, or gas methods. I believe plasma hot cutting methods are preferred in order to limit hydrogen uptake. If hot cutting methods are used, the parent metal shall be preheated before any cutting to the same temperature as intended for welding, and effort shall be made to grind/machine remove the surface scale and coarse grain HAZ associated with the hot cutting method. Removal of the HAZ may not be as critical for subsequent welding by SAW, but for other processes, it can be important. Conduct suface NDE of the grind finished joint surface prior to welding. P91 materials are reputed to stress corrosion crack in air. Chemically clean the joint surfce with a non-residue light solvent immediately prior to welding. Do a google seach on: P91, P9, steel, weld and you will turn up much edifying information.