I have a plugged 4" corrugated poly deain pipe. It is plugged with washing machine hardened soap residue. It is underground and a straight run to an open outfall. I think there is one plug where there was a slight rise in elevation, but no fittings other than unions. The plug is about 60 to 70 feet from where I can get into the pipe. A 2 or 21/2 inch opening in the plug should be OK. Question: what cutter would do the job and have the least possibility of cutting into the corrugatings of the pipe?