Improvements in design and fitness evaluation of above ground steel storage tanks

Above ground steel storage tanks are widely used to store liquids in a variety of industries. The design and fitness for service procedures for such tanks are a concern for international standards and need to be continually improved upon to ensure better safety and serviceability. Several important aspects about tank design and assessment are studied in this thesis. The bottom plate material near the shell to bottom joint in the tank is generally in plastic range. It is a critical failure point in many modes of tank failure. The effect of increasing the bottom plate projection length at this joint for tanks with rigid ring wall foundations is studied both theoretically and numerically. A theoretical beam model is validated using finite element analysis (FEA) and extended to determine the length of bottom plate projection needed for maximum effect. The formation of plastic hinges in the bottom plate on the inside and outside of this joint is discussed in detail using FEA. Tanks operating at elevated temperatures (200ﾟF to 500ﾟF) need to consider additional stresses due to thermal expansions and restraints from the tank shell and bottom plate interactions. The frictional forces from the foundation cause significant stresses at the tank bottom. The design guidelines by API 650 standard address this issue using a factor named ‘C’ that defines the ratio of actual expansion against free expansion of the tank bottom. At present, an empirical range of ‘C’ values (0.25 – 1.0) is allowed without clear guidelines for selecting a suitable value. This thesis evaluates the current procedure and suggests an alternate method by incorporating the friction coefficient directly in the stress equations, instead of the C-factor. The fill/draw down cycle of the stored liquid could lead to low cycle fatigue near shell to bottom joint. The peak alternating stress (strain) at this location determines the fatigue life of the tank. The widely used API 650 procedure employs beam on elastic foundation theory to determine the fatigue life for all tanks. The thesis shows that this is incorrect for tanks on concrete ring wall. The appropriateness of using this theory is studied and an alternative beam model is proposed. It is verified using FEA. Damage due to corrosion in the form of local thin area (LTA) is a widespread problem in storage tanks. Fitness for service (FFS) methods are quantitative engineering evaluations used to demonstrate the structural integrity of an in-service tank containing damage like LTA and make run, repair or replace decisions. The mα-tangent method is a simplified limit load procedure that can be used for such FFS evaluations. This thesis uses a modified reference volume for mα-tangent method applied to tanks and reports initial results for FFS evaluations. The study also finds that for large cylinders like tanks with very high R/t ratio, the circumferential decay lengths will be smaller than those previously reported ( 2.5 Rt rather than 6.3 Rt ).