Fatigue Evaluation of API 12F Shop-Welded Tanks With a New Roof-to-Shell Junction Detail

The American Petroleum Institute (API) provides a series of standards and specifications on storage tanks, in which the API 12F specification provides 12 tank designs that can be fabricated in the shop and transported to the field. The nominal capacity of the 12 API 12F tank designs ranges from 90 bbl (14.3 m³) to 1000 bbl (158.99 m³). The minimum required thickness and operational pressure levels that each tank case can sustain are given in Table 1 of the current 13th edition of API 12F (API, 2019, “API 12F Specification for Shop-Welded Tanks for Storage of Production Liquids,” 13th ed., API Publishing Services, Washington, DC, Standard No. API 12F). The objective of this study is to estimate the fatigue life of API 12F tanks under normal operation pressure cycles following the procedure presented in ASME VIII-2. The stored liquid product specific gravity is assumed to be 1.2 when the liquid height is half of the tank height, while the specific gravity is assumed to be 0.7 when the stored liquid height is 18 in. (460 mm). Meanwhile, a new roof–shell attachment detail is proposed in this study, the new rectangular cleanout junction detail presented in the 13th edition of API 12F is modeled, and various component thickness combinations are considered to investigate the effect of component thickness on fatigue life. The roof–shell joint (top junction) and shell–bottom plate (bottom junction) are studied by axisymmetric models under axisymmetric idealization as they are away from the cleanout junction, while the cleanout junction is studied by applying a submodeling technique. Stress classification is performed at each location of interest to obtain the stress components to calculate the stress range within each loading cycle that is needed to perform fatigue evaluation. The results and discussion about fatigue evaluation of API 12F tanks are presented in this report.