A Comparative Study of the Thermal-Mechanical Behavior and Fatigue Life Predictions of Different Coke Drum Support Skirt Designs

Abstract

It is well known that coke drums are subjected to severe operating conditions that can lead to progressive plastic strain accumulation and related fatigue damage and cracking. The difficulties of performing an insightful, useful coke drum analysis are numerous. There are a variety of uncertainties and randomness in temperature and stress during the heating and quench portions of the cycle that limits the value of conventional analysis for remaining life prediction. Further complicating the issue, the progression of damage in coke drums often involves gross plasticity, permanent dilation, and variable bulging in the pressure boundary. Since the accumulation of damage in the drum is a history (or path) dependent process, a snapshot of strain or even distortion data has limited value in quantifying damage or remaining life. Additionally, ASME Code elastic-based fatigue methods, with plasticity correction factors, that have been used for coke drum assessments in the past are technically invalid when ratcheting (incremental plastic strain accumulation) is occurring. More modern fatigue methods and damage models may be required to address the permanent irreversible damage and material degradation that is produced during cycling. To date, there are no standard methodologies developed in the refining industry for coke drum life prediction, as is evidenced by the first edition of API Technical Report 934-G [1] (published in April 2016) as well as the limited analytical work typically performed to validate coke drums during the design phase. Simplified approaches have generally been used to attempt to define trends rather than developing more fundamental analytical insight into structural response. API 934-G [1] generally attributes cracking in the bottom cones and skirts of coke drums to cyclic thermal stresses introduced during the fill and water quench portions of the operating cycle, but indicates that weld misalignment can also be a contributor to crack initiation and propagation. In this study, the thermal-mechanical behavior of multiple coke drum skirt designs, including forged, welded, and sliding configurations is investigated using elastic and elastic-plastic finite element analysis (FEA), and corresponding fatigue life predictions are discussed for each type of design. Additionally, the effect of skirt slots is quantified. Finally, a cyclic plasticity material model is employed to establish shake-down, and a modern strain-based fatigue method is implemented, including a critical-plane approach as documented in Part 14 of API 579-1/ASME FFS-1, Fitness-For-Service, (API 579) [2], Welding Research Council (WRC) Bulletin 550 [3], and Reference [4]. These results are compared to more conventional base metal and welded fatigue predictions.