Load Factors for Use in Wind Buckling Analysis of API650 Tanks

Abstract Design by analysis techniques are often used for the Fitness-For-Service assessment of aboveground storage tanks with shell deformations. A significant mode of failure that should be considered during such an assessment is buckling of the tank wall due to wind loading, vacuum condition, or a combination of the two. This is especially true because the buckling load capacity of any thin-walled cylindrical shell is highly sensitive to geometric imperfections. API 650 tanks, when designed and constructed correctly, have adequate protection against collapse from buckling, as demonstrated by the large worldwide population of tanks in long-time successful service. However, cases arise where tanks are constructed outside code tolerance, or where shell deformation occurred during service or due to repair work. In such cases, a more advanced technique to evaluate the buckling strength of the tank with imperfections is needed. The design by analysis rules used for the assessment of protection against collapse from buckling were updated in the 2023 edition of the ASME BPVC. Among the changes is a significant reduction in the load factor that is required to demonstrate adequate buckling strength. In this paper, work is presented to derive a required load factor specifically for the buckling assessment of API650 storage tanks, when performing the assessment according to the updated ASME BPVC method. A parametric study was performed where a number of different tank geometries, designed in accordance with API 650 requirements, were evaluated by means of advanced buckling analysis. With the understanding gained from the parametric study, and with reference to the underlying philosophy in the wind design of API 650 tanks, a well-founded conclusion could be reached on an appropriate load factor that can be used to demonstrate adequate buckling strength of a tank. Application of the buckling assessment rules and the derived load factor is demonstrated using two example cases.