Retrofit of Thin-walled Steel Tanks for Low Liquid Level Operation

The safe operation of steel oil storage tanks is vital for the petroleum industry. Such tanks are typically thin-walled, cylindrical, with large diameters and are not mechanically anchored to the foundation. Until recently, code provisions were oriented towards the design of tanks operating at high levels of the contained liquid and emphasis was given on preventing failure modes associated with yielding of the shell (plastic limit state). Based on this concept, the majority of the existing tanks were constructed with variable shell thickness, because tensile stresses (which result majorly from hydrostatic pressure) reduce towards the top of the tank. To account for structural stability issues of the very thin upper shell courses, stiffening rings were commonly placed on their circumference. However, the stiffening arrangements of existing tanks might not be sufficient to prevent buckling of the shell when these operate at low liquid levels. Therefore, the safety of such structures has to be reevaluated. In this paper, retrofit scenarios for two representative, large-scale oil storage tanks (with diameters of approximately 88m and 47m) operating at low liquid level are proposed in order for the provisions of the Eurocodes to be satisfied. For the larger tank, which is open-top, two additional ring stiffeners were necessary to increase (by reduction of the buckling length) the circumferential buckling resistance of the higher tiers, which was critical in meeting the requirements set by the Eurocodes. The retrofit proposal for the other tank (which supports a conical roof) included the attachment of vertical stiffeners to account for the high meridional stresses developed in the thinner courses. Moreover, the attachment of three stiffening rings was suggested to further reduce the effect of comprehensive hoop stresses on shell buckling. Cost data regarding the proposed retrofit scenarios are presented. Helpful conclusions regarding the strengthening of such structures and the efficiency of the Eurocodes are drawn.