A hydraulically-operated ripper tool mounted as an attachment on a hydraulic arm of a carrier or excavator is used for demolition of large metal walled structures such as storage tanks in a very short time. The ripper tool has a sharp point for punching entry holes into the tank walls, so that no torch cutting of entry holes needs to be used, particularly where there might be volatile residues left in the tank. The ripper tool has a concave, sharp inner edge and is operated by the hydraulic arm in reciprocation so as to provide a shearing force to cut quickly through metal walls in “can-opener” fashion. The ripper tool facilitates making long vertical cuts into the tank wall from top to bottom, so that a series of vertical cuts can be made to weaken a side of the tank and allow the top of the tank structure to be brought down.