Skirt to bottom head attachments of coke drums experience severe thermo-mechanical cyclic stresses, causing failures due to low cycle fatigue. Accordingly, many skirt attachment designs have evolved over a period of time starting with simple conventional weld build up design, improved weld build up design, integral forged attachment design and others. The objective of this paper is to compare thermo-mechanical fatigue life of three different skirt attachment designs using elastic-plastic fatigue assessment methods of ASME Section VIII, Division 2. A transient thermal analysis model is first developed incorporating appropriate boundary conditions. The time-dependent variable heat transfer coefficients at the inner surface of the coke drum, which change with the operation stages and the levels of oil filling and water quenching, are determined based on the field measured thermocouple temperature data on the outer surface of the coke drum. Sequentially coupled elastic-plastic transient thermo-mechanical stress analyses of coke drum skirt attachments are carried out using both Twice Yield and cycle-by-cycle methods. The effective strain ranges and the fatigue life of three different skirt attachment designs are calculated and compared.

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