I would say, just shootin from the hip that you better run some mechanical tests before you commit. I get nervous when welding ferritics/bainitics over austenitics. The 8018 is intended to produce a bainitic microstructure. With a little more alloy you may end up with martensite. Since this is a linepipe application I am assuming you probably do not wish to heat treat. Which means you might have unrelieved martensite hard as a rock. Which makes the microstructure crack prone.