Fig. 2. Misalignment of the fractured edges of the skirt junction.

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1 Available online at ScienceDirect Procedia Structural Integrity 3 (2017) XXIV Italian Group of Fracture Conference, 1-3 March 2017, Urbino, Italy Analysis of the mechanical behavior of a delayed coker drum with a circumferentially cracked skirt J.L. González a*, S. Gómez b, G. Gómez c a Professor of the Metallurgy and Materials Department, ESIQIE IPN, México, D.F. b Head of NDT at Grupo de Analisis de Integridad, ESIQIE IPN, México, D.F. c Independent consultant and pressure vessel expert, México, D.F. Abstract 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. Copyright 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the Scientific Committee of IGF Ex-Co. Keywords: Cracked skirt; coke drum; mechanical behavior. 1. Introduction A delayed coker unit in an oil refinery is composed of four drums of 1766 m 3 of volume each. The operating temperature is 449 ºC and the design pressure is 1.05 kg/cm 2 at the top and 3.9 kg/cm 2 at the bottom. The shell is made of SA387-Gr11 CL.2 steel with a 2.8 mm clad of 410S stainless steel. The main dimensions of the drums are: 8534 mm internal diameter, mm shell height and 6160 mm cone height and the nominal thickness is 25.4 mm. The * Corresponding author. Tel.: ext ; fax: address: drjorgeluis@hotmail.com Copyright 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of the Scientific Committee of IGF Ex-Co /j.prostr

2 34 J.L. González et al. / Procedia Structural Integrity 3 (2017) skirt is a cylinder made of SA CL.2 steel, 25.4 mm thickness. The vessel is designed under the ASME Secc. VIII Division I, After eleven years of service, during a routine inspection, the thermal insulation was removed in the area of the skirt junction in all four drums in order to make thickness measurements, after that a cracking along the circumferential skirt junction was detected; the cracks ran aside of the welded joint in the skirt side. An example of the cracked area and the cracking is shown in Figure 1. At the time of inspection, in the four drums, the entire circumference was cracked and the fractured edge of the skirt showed different levels of misalignment between the fractured edges, as shown in Figure 2. There was also observed that there was an opening and closure movement of the mating crack surfaces In addition to the skirt junction cracking; bulging of the skirt, anchorage bolt pull out and corrosion under insulation were observed. Fig. 1. Left: Coker drum with the thermal insulation removed in the skirt junction. Right: Cracking of the skirt junction running along the weld line in the skirt side. Fig. 2. Misalignment of the fractured edges of the skirt junction. By visual observation of the drums in service, it was deducted that the misalignment was caused by the lateral displacement of the drums. It is worth to mention that the movement was fairly cyclic at a frequency of 1 Hz in the heating up stage of the drum operation. A literature survey pointed out that cracking of the skirt junction is a very common form of damage of the coker drums [Boswell (1997), Jani (2012), Schimidt (2012), API RP 571 (2013)], and it was known that these cracks are caused by the cyclic stresses induced by the differential thermal expansions of the pressure vessel and the skirt during the operational stages of the coker drum (load-heat up-quench-discharge), a mechanism referred as thermal fatigue [Boswell (1997), Jani (2012), Schimidt (2012), API RP 571 (2013)]. The Figure 3 shows a sketch of the skirt junction indicating the location of the cracking. The misalignment of the fractured edges of the skirt was measured directly on each drum, taking readings every 30 grads in the circumference (one technical hour). A positive value was assumed if the shell edge was displaced outwards and negative when the

3 J.L. González et al. / Procedia Structural Integrity 3 (2017) shell s edge was displaced towards the center of the drum. The results are shown in Table 1. The height of the bulging with respect to the vertical cylinder line of the skirt was also measured; the maximum bulging height was 25.8 mm which correspond to an internal diameter expansion of 0.6 %. The inspection of the skirts was completed with thickness measurement, metallographic examination and hardness measurements. The minimum measured thickness was 25.4 mm and no local metal loss was observed. The minimum measured hardness value was 140 Brinell, with an average of 156 HBN in the level close to the skirt junction. The surface microstructure of the skirt, shown in Figure 4, was normal and no alterations due to high temperature exposure or micro cracks were observed. Fig. 3. Sketch of the skirt junction showing the crack location. Fig. 4. Surface microstructure of the skirt consisting of a matrix of fine ferrite grains with a dispersion of carbides. Direct metallography, bright field. Image at 200X. Etch Nital 5. Table 1. In service measured displacements at the cracked edges of the skirt junctions. Displacement Drum Maximum positive (mm) Location (technical hour) 04:30 10:00 09:00 02:30 Maximum negative (mm) Location (technical hour) 5:30 11:00 08:30 06:00 Crack opening (mm) Location (technical hour) 11:30 11:00 11:

4 36 J.L. González et al. / Procedia Structural Integrity 3 (2017) The data in table 1 shows that the displacements are result of combined vertical tilt and horizontal movements (referred here as lateral); the worst case in lateral displacement was drum 3, but the drum 2 has the greatest tilting, since it is where there is the most crack opening height. In terms of the directions of the displacement, the distribution is fairly random. 2. Analysis of the mechanical behavior of the cracked skirt junction The results of the hardness measurements indicate that the ASTM A 387 Gr. 11 CL2 steel plates of the skirt still met the tensile strength requirement, since the average measured hardness values of 156 HBN correspond to a tensile strength of 78 ksi, according to the table F.1 of the API579/ASME FFS standard [API570/ASME FFS-1 (2007)], which is greater than the minimum specified tensile strength of 75 ksi. Therefore there is no loss of strength of the skirt fabrication steel. Regarding the bulging, its maximum measured value was equivalent to a 0.6% Out-of-Roundness; this is less than the maximum allowed of 1.0 %, as required by the Subsection A, Part UG-80 of the ASME Section VIII Division 1 code [Code ASME (2001)], therefore it is acceptable, as for a Level 1 fitness for service assessment of Part 8 of the API579/ASME FFS standard. Based on the previous results, an analysis of the mechanical failure risks that could arise from the skirt junction cracking indicated that the most likely scenario of failure is the plastic collapse of some sections of the skirt caused by local high stress arising from the uneven vertical load distribution of the drum on the fractured edge of the skirt. The uneven vertical load distribution is schematically illustrated in the Figure 5 and is caused by the lateral and vertical displacements of the drum, which is free to move because of the fracture of the skirt junction, thus reducing the contact area of the mating surfaces of the cracked edges. Fig. 5. Schematic representation of the formation of high stresses zone in the coker drum skirt due to the misalignment of the skirt junction. Because of the very large diameter to thickness ratio of the skirt (ID/t = 334), even relatively small displacements will cause a dramatic reduction of the contact surface area, as shown in Table 2. Table 2. Contact surface area of cracked edges of the skirt junction as a function of the lateral displacement. Lateral displacement (mm) Contact surface area of cracked edges (mm 2 ) , , , , ,675

5 J.L. González et al. / Procedia Structural Integrity 3 (2017) 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.

6 38 J.L. González et al. / Procedia Structural Integrity 3 (2017) Fig. 7. Von Mises stress distribution of the coke drum skirt cracked and misaligned at the skirt junction, calculated by finite element analysis. 3. Rehabilitation The common sense would suggest welding the cracked line as the first rehabilitation method. Independently of the technical difficulty to realign the skirt junction, to machine the bevel butt joint and to stress relieve the weld, this certainly is not a permanent remedial action, since the drum movements and thermal expansions that caused the skirt junction cracking will be still present and active, thus causing the same cracking again, and perhaps in a shorter time. Other remediation methods that involve reestablishing the continuity between the skirt and the drum shell may suffer similar damages, i.e. cracking and bulging. The authors of this paper propose that a better solution is 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, as well as the misalignment are mitigated. This reinforcement can be in the form of an orthogonal stiffened ring in the skirt and a stiffening ring with tripping brackets in the shell side, as depicted in figure 8. This solution will give a very high vertical stability to the skirt, provide a wider contact surface for the shell support, make an even distribution of the vertical load, even if there is a large lateral displacement, and will restrict the tilting movements of the drum. Fig. 8. Conceptual design of a reinforcement to rehabilitate the coke drum skirt without welding the junction crack.

7 J.L. González et al. / Procedia Structural Integrity 3 (2017) 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.

8 40 J.L. González et al. / Procedia Structural Integrity 3 (2017) Acknowledgements The authors acknowledge Pemex Refinación for the opportunity to perform the work described in this paper. References Boswell, R., Remaining Life Evaluation of Coke Drum Energy Engineering Conference. Plant Engineering, Operations, Design & Realiability Symposium. Code ASME Section VIII Division 1, Rules for Construction of Pressure Vessels. The American Society of Mechanical Engineers, New York, NY Code ASME Section II Materials Part D Properties. The American Society of Mechanical Engineers, New York, NY , pp. 32 Jani, A. B., Coke Drum Monotoring and Inspection for Fatigue Life and Safety Improvement Coking.com Meeting, Fort McMurray, Alberta, Canada. Recommended Practice API RP 571, Damage Mechanisms Affecting Fixed Equipment in the Refining Industry. American Petroleum Institute, Washington, D.C Pp.5-65 Schimidt, M., Samman, M., Repair and Retrofit of a Coke Drum Skirt Attachemet Weld Coking.com Meeting, Fort McMurray, Alberta, Canada. Standard API570/ASME FFS-1, Fitness for Service. American Petroleum Institute, Washington, D.C