Abstract: In order to improve the utilization efficiency of oil tanks in a tankfarm, two surplus 5×10⁴ m³ external floating roof crude oil tanks are proposed to transform into inner floating roof diesel tanks. In this regard, tank defects which need to be treated in the process of transformation are analyzed. In view of the foundation settlement and uneven settlement, the ovality, perpendicularity and levelness of tank wall and the steel plate thickness of each part of tanks are correspondingly detected. In accordance with relevant specifications, check and verification are conducted, and specific technical requirements for transformation of each part of tanks are determined. Furthermore, the two-way meridian net shell is taken as the net shell roof of inner floating roof. Related experiences provide a reference to the subsequent transformation and repair of storage tanks.