Abstract: In the regular inspection of in-service spherical storage tanks, when the buried defect of the weld exceeds the corresponding standard, it is necessary to carry out the safety condition rating as required, but TSG 21—2016 does not specify the relevant requirements for the safety condition rating of the buried defect in the nature of the crack. In this paper, after the safety assessment of the spherical storage tank with cracked buried defects is carried out by TOFD, the buried crack defects in the safe area of the assessment map are selected for online monitoring of 2 regular inspection cycles, and finally this type of inactive defects during the use of the spherical storage tank is found. No obvious expansion was found. After anatomical verification, the reasons for the failure of the crack to expand were analyzed. The innovation of this paper lies in the application of TOFD detection technology to the online monitoring of buried defects in spherical tanks, which verifies that the detection method has certain feasibility in monitoring buried cracks in spherical tanks, and provides a practical exploration for the inspection of spherical tanks in the same industry.Highlights: (1) The application of TOFD technology to the on-line monitoring of spherical tank’s embedded defects has proved the feasibility of this method in the monitoring of spherical tank’s embedded cracks, which provides an experience reference for the same industry.(2) On the premise of meeting the production process, the spherical tank with some inactive or closed buried crack defects can not be repaired, which saves the cost for the enterprise and improves the economic benefit.