Seismic Design of Oil Storage Tanks

Abstract

One of the important and functional types of structures widely used in Petroleum In- dustries due to the ease of design, including oil depots are cylindrical Oil Storage Tanks. Their use is multi-purpose including oil storage of different types of liquids including oil in Oil Depots. Oil Storage Tanks are commonly adopted due to its ease of design and construction. Analysis and Design of Oil Storage Tanks requires specialized attention due to its multi functionality. If not properly designed, damage caused to Oil Storage Tank leads to hazardous Impact in the form of fire, leakage of toxins and pollution to the environment. Therefore study on analysis and design of Oil Storage Tank is important specially under Seismic Force. There are a variety of Codes and Standards stating minimum requirements and some additional Codes stating requirements from company standards or specifications to be followed in design of Oil Storage Tanks. The present study is an attempt to analyze and design the Oil Storage Tank using variety of standards practiced worldwide. In the present study, four standard codes are considered which are-(a)American Standards API 650 (Welded Steel Tanks for Oil Storage) , (b) Indian Standard 803.1976 (Code of Practice for Design, Fabrication and Erection of Vertical Mild Steel Cylindrical Welded Oil Storage Tanks) (c) The European Standards EN 14015 (Specification for the Design and Manu- facture of Site Built, Vertical, Cylindrical, Flat-bottomed, Above Ground, Welded, Steel Tanks for the Storage of Liquids at Ambient Temperature and Above) (d) The British Standards BS 2654 (Manufacture of Vertical Storage Tanks with Butt-welded Shells for the Petroleum Industry). Special emphasis is given to API 650 due to it's broad use and international acceptance. Various design criteria employed by these standard codes for the design of components of Oil Storage Tank such as Roof, Shell courses, Bottom Plates, Anchorages, Stiffeners, Annular Rings, Wind Girders are compared and utilized for design. Oil Storage Tank of 22 million liter capacity for Naptha storage use is considered for the study and is analyzed and designed using aforementioned four distinct standards.From this study one can compare the provisions and results provided by aforementioned stan- dards. It has been found that IS 803 results into heaviest design amongst all the codes considered while EN 14015 yields lightest design. EN 14015 Design Standard favored provision of intermediate girders over increasing thicknesses of shell courses, thereby pro- viding an economically efficient design. The earthquake design procedures contained in this study are based on response spectra analysis methods and consider two response modes of the tank and its contents|impulsive and convective. The equivalent lateral seismic force and overturning moment applied to the shell as a result of the response of the masses to lateral ground motion are determined. The necessary measures are taken to assure the stability of the tank shell with respect to overturning and to resist buckling of the tank shell as a result of longitudinal compression.