Analysis of the mechanical behavior of a delayed coker drum with a circumferentially cracked skirt

The skirts of four coke drums of a delayed coker plant in an oil refinery became severely cracked as result of the service and a poor design. The cracks were located in the circumference just below the welded joint of the skirt with the pressure vessel of the coke drums. Since the cracks grew up to one hundred percent of the circumference, the drums were free to move both laterally and vertically. The present paper describes the results of the measurement of these displacements, as well as other non destructive test, done in order to analyze by finite element the mechanical behavior of the skirt and the drum-skirt system. The results showed that the greatest risk of failure was the plastic collapse of the skirt due to an uneven distribution of the vertical loads resulting from the lateral and vertical displacements of the drum. The analysis is used then to propose a unique reinforcement of the skirt that allows to fully rehabilitate the coker drums without re-welding the fractured skirts.