PSI - Issue 3

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

39 7

To verify the effectiveness of the proposed rehabilitation method, a finite element simulation was done, with the same load conditions and geometrical characteristics as the simulation of the unrepaired circumferentially cracked skirt. The result is shown in Figure 9, where it can be seen that the maximum effective stress is reduced from 36.7 ksi, to 26.9 ksi, but it si located in the stringer’s edges, not in the skirt. More important is the fact that the stresses are more evenly distributed in the skirt, with a maximum effective stress in the skirt of 11.9 ksi, which is well below the maximum allowable stress of 21.4 ksi. The simulation also shows that the stresses induced in the shell are well below the maximum allowable stresses, so the risk of plastic instability or high local stresses is completely mitigated.

Fig. 9. Von Mises stress distribution of the reinforced coke drum skirt, calculated by finite element analysis.

4. Summary and conclusions 1.

The mechanical behavior of the skirt and the drum-skirt system of a coke drum skirt fully circumferentially cracked at the skirt junction was done in order assess the mechanical failure risks that arise from such condition. 2. The immediate consequence of the skirt junction cracking was the uneven load distribution in the skirt due to the lateral and tilting displacements of the drum. This uneven load distribution causes local high stress zones in the skirt which are prone to plastic instabilities that may lead to the local plastic collapse of the skirt, thus making the drum to fall vertically or tilting to cause the interruption of the service of even a catastrophic failure. 3. The results of the finite element stress analysis of the coke drum with a cracked and misaligned skirt showed the formation of two local zones of high stresses in diameter opposed locations in the skirt, but with a maximum effective stress is 36.7 ksi, which is lower than the yield strength of the skirt material, but higher than the maximum allowable stress making necessary to take a rehabilitation action. 4. A reinforcement of the skirt, without welding the circumferential crack, designed in such way that the effects of the lateral and vertical movements of the drum are mitigated is proposed. This reinforcement is an orthogonal stiffened ring in the skirt and a stiffening ring with tripping brackets in the shell side. A finite element stress analysis of the drum with the proposed reinforcement showed that this solution provides high vertical stability to the skirt, an even distribution of the vertical load, and greatly reduces the effective stress in the skirt, thus mitigating the risk of plastic instability.