Damage due to corrosion in the form of local thin area (LTA) is a widespread problem in hydrocarbon storage tanks. Fitness for service (FFS) methods are quantitative engineering evaluations that are performed to demonstrate the structural integrity of an in-service tank containing damage like LTAs and make run, repair or replace decisions. The mα-tangent method is a simplified limit load procedure that can be used for FFS evaluations. The method uses a kinematically active reference volume for evaluating the limit load multiplier. This paper reports preliminary results on the use of a modified reference volume approach by considering the reference volume inside the LTA, formed by the overlapping decay lengths from the LTA boundaries. The results from this approach are compared with the existing method which considers a much broader volume outside the damage as the reference volume and with nonlinear FEA. The Remaining Strength Factor (RSF) calculated from the modified reference volume also compares reasonably well with the results from American Petroleum Institute (API)/American Society of Mechanical Engineers (ASME) FFS procedure widely referred as API 579-1/ASME FFS-1. 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 (2.5Rt rather than 6.3Rt).

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