Largest pipe I ever welded was 96" in diameter and that seemed to take forever. bead brush the end of each electrode change every pass had to be back ground to a contour and depth gage. Any porosity found during each pass meant grind out re weld to depth then move on to the next pass.
Pipelining on the 96incher was nearly as tedious as welding hull plates on a work barge. I can just imagine what was involved in welding up a submarine that had to withstand the extreme pressures they encounter.