Modified Fatigue Stress at Shell-to-Bottom Joint of Steel Tanks on Ring Walls

Abstract

Shell-to-bottom joint of hydrocarbon storage tanks is a critical failure location which needs careful evaluation especially in the case of elevated temperature tanks. The fill/draw down cycle of the stored liquid causes low cycle fatigue near this joint and hence a fatigue evaluation is recommended. The peak alternating stress at this location, used to enter the fatigue curves is currently determined using a pseudo elastic stress that represents strain range due to inelastic deformations. For this, API 650 employs beam on elastic foundation theory. This theory is being used for tanks resting fully on earthen foundation as well as those on concrete ring wall. This paper studies the validity of using this theory for tanks with concrete ring wall foundation which are much more rigid compared to earthen foundations. Some of the difficulties in the current practice are highlighted. Alternatives to using the current model are derived to determine the stresses in such tanks. The results are validated using finite element analysis.