Optimal Design Of Coke Drum Skirt Slots And Analysis Of .

Optimal Design of Coke Drum Skirt Slots and Analysis of Alternative Skirt Support Structuresfor Thermal-Mechanical Cyclic LoadingbyEdward Lee WangA thesis submitted in partial fulfillment of the requirements for the degree ofMaster of ScienceDepartment of Mechanical EngineeringUniversity of Alberta Edward Lee Wang, 2017

ABSTRACTThe skirt-to-shell attachment weld on coke drums is susceptible to low-cycle fatigue failure due tosevere thermal-mechanical cyclic stresses. Therefore, various skirt attachment designs have beenproposed and implemented to reduce stress and thus improve reliability. The most common skirt design isa cylindrical shell attached tangentially by a fillet weld to the coke drum vessel. One inexpensive methodto decrease stress in the junction weld is to add vertical slots near the top of the skirt, thereby reducing thelocal stiffness close to the weld. The conventional skirt slot design is thin relative to its circumferentialspacing. An alternative skirt design where the vessel is supported by a number of welded attachmentplates and allowed to expand and contract freely through the use of lubricated horizontal sliding platesalso exists. In this study, thermal-mechanical elastoplastic 3-D finite element models of coke drums arecreated to analyze the effect of different skirt designs on the stress/strain field near the shell-to-skirtjunction weld, as well as any other critical stress locations in the overall skirt design. The results confirmthat the inclusion of the conventional slot design effectively reduces stress in the junction weld. However,it has also been found that the critical stress location migrates from the shell-to-skirt junction weld to theslot ends. The results from an optimization study of the slot dimensions indicate that wider skirt slotsimprove the stress and strain response and thus increase fatigue life of the weld and slot area compared tothe conventional slot design. An optimal slot design is presented. The sliding plate design is found tofurther improve the stress and strain response at the point of attachment. However, bending of the vesseldue to the rising water level during the quench stage is found to cause severe plastic deformation in thesharp corners which are inherent to the design. Thus, a novel design which includes pinned connections atthe point of attachment in addition to sliding plates is proposed. The pinned-sliding plate design is foundto completely prevent plastic deformation from occurring at the point of attachment and significantlyreduce critical stress. Accordingly, the pinned-sliding plate design is the most promising candidate from sstudy.ii

ACKNOWLEDGEMENTSI would like to express my utmost gratitude to my supervisor Dr. Zihui Xia, who hasprovided endless opportunities, guidance, and support throughout this endeavour.I would like to thank Dr. Feng Ju, Dr. Jie Chen, Dr. Yejian Jiang, and John Aumuller fortheir support and advice.I would also like to acknowledge Suncor Energy Inc. and Mitacs for funding this research.I am very grateful to my parents, my brother, and my girlfriend for their unwaveringsupport and encouragement.iii

Table of ContentsCHAPTER 1 INTRODUCTION. 11.1Overview of Delayed Coking Process and Coke Drums . 11.2Literature Review. 41.2.1Common Coke Drum Issues. 41.2.2Skirt Support Structure Designs and Improvements . 71.3Thesis Objectives . 12CHAPTER 2 STOPLASTIC FINITE ELEMENT ANALYSIS . 152.1Introduction . 152.2Coke Drum Geometry and Materials . 162.2.1Vessel and Skirt Geometry . 162.2.2Skirt Slot Geometry . 172.2.3Materials . 182.3Model Set-Up . 202.3.1Solid Modeling and Meshing . 202.3.2Boundary Conditions . 232.3.3Model Simplifications . 242.4Thermal-Elastoplastic Finite Element Analysis Results . 252.4.1Thermal Analysis . 25iv

2.4.2Skirt Deformation . 282.4.3Comparison of Un-Slotted and Slotted Skirt Junction Stress/Strain Responses. 292.4.4Stress and Strain Response in Slot Area of Original Slot (OS) Model . 342.4.5Comparison of Stress/Strain Response at Critical Locations of NS and OSDesigns. 402.5Summary . 42CHAPTER 3 PARAMETRIC STUDY OF SKIRT SLOT DIMENSIONS USINGTHERMAL-ELASTOPLASTIC FINITE ELEMENT ANALYSIS . 433.1Introduction . 433.2Skirt Slot Design Methodology . 443.3Model Set-Up . 463.4Thermal Analysis Results . 473.5Stress Analysis Results . 493.5.1Effect of Skirt Slot Length L on Junction Stress/Strain Response . 503.5.2Effect of Skirt Slot Length L on Slot Area Stress/Strain Response . 513.5.3Effect of Junction-to-Slot Distance d on Junction Stress/Strain Response . 563.5.4Effect of Junction-to-Slot Distance d on Slot Area Stress/Strain Response . 583.5.5Effect of Skirt Slot Width w on Junction Stress/Strain Response . 643.5.6Effect of Skirt Slot Width w on Slot Area Stress/Strain Response . 66v

3.6Summary and Conclusions . 71CHAPTER 4 ANALYSIS OF ORIGINAL AND OPTIMAL SKIRT SLOT DESIGNSUSING ACCURATE QUENCH MODEL. 744.1Introduction . 744.2Model Set-Up . 754.2.1Validation of the Local Sub-Model . 774.2.2Mesh Dependency of Junction Face (Global Model) and Slot Area (LocalModel) . 794.3Thermal Analysis of Coke Drum Skirt . 834.4Stress Analysis of Coke Drum Skirt . 854.4.1Deformation of Coke Drum Vessel and Skirt . 854.4.2Junction Face Stress Response . 884.4.3Slot Area Stress Response . 894.5Estimation of Fatigue Life . 914.6Summary . 95CHAPTER 5 ANALYSIS OF SLIDING AND PINNED-SLIDING SKIRT SUPPORTSTRUCTURES. 975.1Introduction . 975.2Model Set-Up . 995.3Analysis of Sliding Plate Design . 103vi

5.3.1Transient Thermal Analysis of Sliding Plate Design . 1035.3.2Stress Analysis of Sliding Plate Design . 1045.4Analysis of Pinned Sliding Plate Design . 1105.4.1Transient Thermal Analysis of Pinned Sliding Plate Design . 1105.4.2Stress Analysis of Pinned Sliding Plate Design . 1115.5Summary . 116CHAPTER 6 CONCLUSIONS . 1186.1Summary . 1186.2Recommendations for Future Work. 119BIBLIOGRAPHY . 121vii

List of TablesTable 2-1: Dimensions for Original Slot Design . 18Table 2-2: Material Properties of SA387-12-2 Base Metal . 19Table 2-3: Material Properties of SA240-TP410S Clad Metal . 19Table 2-4: Prescribed Boundary Conditions for Each Process Stage [8] . 24Table 2-5: Summary of stress and strain results at the inner junction face of the No Slot(NS) model . 31Table 2-6: Summary of stress and strain results at the inner junction face of the OriginalSlot (OS) model . 33Table 2-7: Percent difference due to inclusion of skirt slots on maximum equivalent stressand plastic strain at the inner junction face location . 34Table 2-8: Summary of stress and strain results at the top keyhole of the Original Slot(OS) model . 37Table 2-9: Summary of stress and strain results at the bottom keyhole of the Original Slot(OS) model . 38Table 2-10: Summary of stress and strain results at the mid-column location of theOriginal Slot (OS) model . 40Table 3-1: Characteristic dimension values for each of the examined skirt slot designs . 45Table 3-2: Effect of altering slot width and length on critical buckling load of slottedsection . 46Table 3-3: Inner junction stress amplitude results and percent change due to slot length 51Table 3-4: Maximum equivalent stress and plastic strain results at inner junction andpercent change due to slot length . 51viii

Table 3-5: Top keyhole location stress amplitude results and percent change due to slotlength during second cycle. 52Table 3-6: Maximum equivalent stress and plastic strain results at top keyhole locationand percent change due to slot length during second cycle . 52Table 3-7: Bottom keyhole location stress amplitude results and percent change due toslot length during second cycle . 54Table 3-8: Maximum equivalent stress and plastic strain results at bottom keyholelocation and percent change due to slot length during second cycle . 54Table 3-9: Mid-column location stress amplitude results and percent change due to slotlength during second cycle. 55Table 3-10: Maximum equivalent stress and plastic strain results at mid-column locationand percent change due to slot length during second cycle . 55Table 3-11: Inner junction stress amplitude results and percent change due to junction-toslot distance during second cycle . 57Table 3-12: Maximum equivalent stress and plastic strain results at inner junction andpercent change due to junction-to-slot distance during second cycle . 57Table 3-13: Top keyhole location stress amplitude results and percent change due tojunction-to-slot distance during second cycle . 59Table 3-14: Maximum equivalent stress and plastic strain results at top keyhole andpercent change due to junction-to-slot distance during second cycle . 60Table 3-15: Bottom keyhole location stress amplitude results and percent change due tojunction-to-slot distance during second cycle . 61ix

Table 3-16: Maximum equivalent stress and plastic strain results at bottom keyhole andpercent change due to junction-to-slot distance during second cycle . 62Table 3-17: Mid-column location stress amplitude results and percent change due tojunction-to-slot distance during second cycle . 62Table 3-18: Maximum equivalent stress and plastic strain results at mid-column andpercent change due to junction-to-slot distance during second cycle . 63Table 3-19: Inner junction stress amplitude results and percent change due to slot widthduring second cycle. 65Table 3-20: Maximum equivalent stress and plastic strain results at inner junction andpercent change due to slot width during second cycle . 65Table 3-21: Top keyhole location stress amplitude results and percent change due to slotwidth during second cycle . 67Table 3-22: Maximum equivalent stress and plastic strain results at top keyhole andpercent change due to slot width during second cycle . 67Table 3-23: Bottom keyhole location stress amplitude results and percent change due toslot width during second cycle . 68Table 3-24: Maximum equivalent stress and plastic strain results at bottom keyhole andpercent change due to slot width during second cycle . 69Table 3-25: Mid-column location stress amplitude results and percent change due to slotwidth during second cycle . 70Table 3-26: Maximum equivalent stress and plastic strain results at mid-column andpercent change due to slot width during second cycle . 70Table 3-27: Dimensions for optimal slot design . 72x

Table 3-28: Changes in stress amplitudes, equivalent stress and plastic strain due tooptimal slot. 73Table 4-1: Maximum equivalent stress and plastic strain results from the global modelinner junction surface at different mesh densities. 81Table 4-2: Maximum equivalent stress and plastic strain results from the local model topkeyhole location at different mesh densities . 82Table 4-3: Summary of inner junction equivalent stress and plastic strain maximums andranges of each considered design . 89Table 4-4: Summary of top keyhole equivalent stress and plastic strain maximums andranges of each considered design . 91Table 4-5: Estimated fatigue life of junction weld area . 94Table 4-6: Estimated fatigue life of top keyhole location . 94Table 5-1: Summary of sliding plate and slotted skirt second-cycle equivalent stressresults at point of attachment . 106Table 5-2: Summary of sliding plate and slotted skirt equivalent plastic strain results atpoint of attachment . 107Table 5-3: Summary of sliding plate and slotted skirt second-cycle equivalent stressresults at critical stress location . 109Table 5-4: Summary of sliding plate and slotted skirt plastic strain results at critical stresslocation . 109Table 5-5: Summary of pinned-sliding plate and slotted skirt second-cycle equivalentstress results at point of attachment .

proposed and implemented to reduce stress and thus improve reliability. The most common skirt design is a cylindrical shell attached tangentially by a fillet weld to the coke drum vessel. One inexpensive method to decrease stress in the junction weld is to add vertical slots near the top of the skirt, thereby reducing the