In the past, basic assumptions used for design calculations of underground steel vessels for storing liquid gases have not taken the real load behaviour, characterized by circumferential bending stresses, into account. Using a general shell model of a partially, elastically bedded circular cylindrical shell with ring stiffeners, stress and deformation behaviour is analysed by variation of the significant influencing parameters such as horizontal bedding, change in the form of the bedding and foundation, and wheel loads from construction vehicles during filling. Finally, recommendations for the sandbed quality are given. Also, conclusions are formulated to find an adequate design model for the simplified design calculations of circumferential stresses.
Apart from lack of space or an aesthetic point of view, safety requirements are the main reason, that storage vessels for explosive and dangerous liquids such as benzine, heating oil or liquid gas are placed underground in trenches or earth covered above ground. Considerations for safety have led to an increase in the use of underground vessels in the last ten years. Even the supplementary earth coverage of existing saddle supported storage tanks has to be considered. Horizontal underground steel vessels consist of a circular cylindrical shell, ring stiffeners with a rectangular or T-cross section and semi-elliptical or torispherical ends as shown in Fig.l. In addition to axi-symmetric loads due to gas pressure from operational filling, vertical loads due to earth pressure, dead weight, liquid filling either during pressure tests or during operation influence the strength of the vessels. To avoid large wall thicknesses for large diameters, ring stiffeners are used.
In the past, for testing and design of underground vessels, in addition to the pressure vessel design, a calculation procedure was used which is based on an in-plane buckling model used by Pfefferkorn (1969).