In 2015 OCI Nitrogen inspected two ammonia tanks located in Rotterdam, The Netherlands. Both tanks consist of a single inner steel tank with an outer concrete tank around it. An internal inspection was performed since at the same time the tanks also had to undergo various modifications to bring them to more modern standards. During the inspection, cracks were found in the bottom plate welds of the tanks. The cracks originated from original manufacturing defects. This paper will go into detail on the repair of the cracks, and all the other tank modifications that were done to these old tanks to bring these up to more modern standards.