PSI - Issue 3

J.L. González et al. / Procedia Structural Integrity 3 (2017) 33–40 Author name / Structural Integrity Procedia 00 (2017) 000–000

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As seen in the table above, the lateral displacements of the magnitude observed in the coker drums analyzed here cause a 2.3 times reduction of the contact area, so by simple definition of stress, as load divided by area, the stresses in the skirt may increase in the same proportion as the contact area reduces. In the scenario described above, it becomes clear that the skirt may reach a plastic instability condition as the lateral displacement increases or the vertical buckling increases, while the drum is on full charge and in the heat-up stage. The failure mode then will be the plastic collapse of two diameter opposed sections of the skirt, causing the tilt and fall of the drum. This failure condition will cause not only the discontinuation of the coker drum operation, but very likely a fire of enormous proportions due to the spill of the hydrocarbons contained in the drum. To assess the potential risk of plastic instability of the cracked and misaligned skirt, a finite element calculation of the stress distribution in the skirt was made. The geometrical model is shown in figure 6. The dimensions are the ones given in the section 1 of this paper. The lateral displacement is 25.4 mm and the vertical load is 2,763 metric tons, which corresponds to the maximum possible loads from dead weight, seismic and wind loads. The temperature of the skirt was 140°C. And the mechanical properties of the fabrication material are the specified for the ASTM A387 Gr. 11 CL2 steel plates at the simulation temperature (45 ksi). The results of the finite element stress analysis of the cracked and misaligned skirt are shown in Figure 7. As expected, two local zones of high stresses appear in diameter opposed location in the skirt. These high stress zones are orthogonal to the direction of maximum lateral displacement and correspond to the contact surface areas. The stresses depicted are effective or von Mises stresses, so they can be directly compared to the yield strength of the skirt fabrication steel to predict if there is plastic deformation. The maximum effective stress is 36.7 ksi, which is lower than the yield strength (45 ksi), therefore plastic deformation is not expected, at least for the simulation conditions. This leads to the conclusion that the observed bulging in the skirt may have been caused by higher temperatures, since higher loads are less likely to occur. Another interesting result of the finite element simulation is that the most stressed zone coincides with the zone where the maximum bulging height was observed, confirming the idea that the zones of the fractured skirt edge that are in contact with the edge in the shell side are more prone to suffer plastic instability.

Fig. 6. Geometrical model for finite element determination of the stress distribution of the cracked and misaligned skirt junction.

Finally, even though for the finite element simulation conditions, there is not risk of plastic instability of the fractured skirt, the average effective stresses in the high stress zone are above the maximum allowable stress of 21.4 ksi, determined from Table 1A of the ASME Secc. II Part D [Code ASME (2013)], which is indeed a highly risky condition, making necessary to take a rehabilitation action.