Abstract
The bottom corner joint of welded steel tank for oil storage is the most important in the periodical opening inspection because the annular plate in the vicinity is particularly high stress region. Therefore, the strict inspection with water fill test is required when it is defective. So its zone was previously regulated conservatively as 1 foot of radial direction width which was defined as critical zone in API (American Petroleum Institute) Standard 653. And in recent years, it has been revised as 3 inches of short width in consideration of reduction of the inspection cost. But the critical zone in Japanese Fire Service Act still remains as 600mm width. In order to clarify the grounds for the values of critical zone and the rational values in engineering, the survey for three fiscal years has carried out in Center for Risk Management and Safety Sciences of Yokohama National University. The results are as follows:
1) Clarification of the grounds for conservative critical zone
2) Establishment of the rational critical zone based on stress analysis for several kinds of loads including earthquake
3) Offering a proposal for the rational critical zone according to defective degrees
