STRUCTURAL SYSTEMS: MISCELLANEOUS - Lecture 15C.1:
The lecture describes the basic principles used in the design of tanks for the storage of oil or water. It covers the design of vertical cylindrical tanks, and reference is made to the British Standard BS 2654  and to the American Petroleum Industry Standard API650 .
Lecture 8.6: Introduction to Shell Structures
Lecture 8.8: Design of Unstiffened Cylinders
Welded cylindrical tanks are commonly used to store oil products or water.
The principal structural element of these tanks is a vertical steel cylinder, or shell, which is made by welding together a series of rectangular plates and which restrains the hydrostatic pressures by hooptension forces. The tank is normally provided with a flat steel plated bottom which sits on a prepared foundation, and with a fixed roof attached to the top of the shell wall.
This lecture explains the design basis for the structural elements of cylindrical tanks and illustrates the arrangements and the key details involved.
1. DESIGN OF WELDED CYLINDRICAL TANKS
Oil and oil productsare most commonly stored in cylindrical steel tanks at atmospheric pressure or at low pressure. The tanks are flat bottomed and are provided with a roof which is of conical or domed shape.
Water is also sometimes stored in cylindrical steel tanks. When used to store potable water they are of a size suitable to act as a service reservoir for a local community; they have a roof to preventcontamination of the water. Cylindrical tanks are also used in sewage treatment works for settlement and holding tanks; they are usually without a roof.
The sizes of cylindrical tanks range from a modest 3m diameter up to about 100m diameter, and up to 25m in height. They consist of three principal structural elements - bottom, shell and roof.
For petroleum storage, the bottom is formed of steel sheets,laid on a prepared base. Some tanks for water storage use a reinforced concrete slab as the base of the tank, instead of steel sheets.
The shell, or cylindrical wall, is made up of steel sheets and is largely unstiffened.
The roof of the tank is usually fixed to the top of the shell, though floating roofs are provided in some circumstances. A fixed roof may be self supporting or partiallysupported through membrane action, though generally the roof plate is supported on radial beams or trusses.
1.2 Design Standards
Clearly, common standards are generally applicable whether a tank holds oil or water, though it is the petroleum industry which has been responsible for the development of many of the design procedures and standards.
The two standards applied most widely are BritishStandard BS 2654  and the American Petroleum Institute Standard API 650 . These two Standards have much in common; although there are some significant differences (see Appendix A). Other standards, American and European, are not applied much outside their respective countries.
This lecture will generally follow the requirements of BS 2654 . This standard is both a design code and aconstruction specification. The design code is based on allowable stress principles, not on a limit state basis.
1.3 Design Pressure and Temperature
Tanks designed for storage at nominally atmospheric pressure must be suitable for modest internal vacuum (negative pressure). Tanks may also be designed to work at relatively small positive internal pressures (up to 56 mbar (5,6 kN/m2), according to BS2654.Non-refrigerated tanks are designed for a minimum metal temperature which is based on the lowest ambient air temperature (typically, ambient plus 10oC) or the lowest temperature of the contents, whichever is the lower. No maximum service temperature is normally specified.
Tanks are usually manufactured from plain carbon steel plate (traditionally referred to as mild steel) of...