AN OVERVIEW AND VALIDATION OF THE FITNESS-FOR-SERVICE ASSESSMENT PROCEDURES FOR LOCAL THIN AREAS A Thesis Presented to The Graduate Faculty of the University of Akron In Partial Fulfillment of the Requirements for the Degree Masters of Science Mechanical Engineering J.L. Janelle December, 2005 ii AN OVERVIEW AND VALIDATION OF THE FITNESS-FOR-SERVICE ASSESSMENT PROCEDURES FOR LOCAL THIN AREAS J.L. Janelle Thesis Approved: Accepted: Advisor Department Chair Dr. Paul Lam Dr. Celal Batur Committee Member Dean of College Dr. Jiang Zhe Dr. George K. Haritos Committee Member Dean of the Graduate School Dr. Xiaosheng Gao Dr. George R. Newkome Date iii ABSTRACT In todays petroleum refining industry, aging infrastructure is a primary concern when considering replacement costs and safe operation. As vessels, piping, and tankage age in service, they are subjected to various forms of degradation or damage that may eventually comprise structural integrity. An engineering or Fitness-For-Service (FFS) assessment is required to evaluate structural integrity and safely extend the life of damaged equipment. Guidelines for performing a FFS assessment have been documented in API RP 579. The goal of API 579 is to ensure the safety of plant personnel and the public while aging equipment continues to operate, provide technically sound Fitness-For-Service assessment procedures for various forms of damage, and help optimize maintenance and operation of existing facilities while enhancing long-term economic viability. The procedures in API 579 (2000 release) provide computational methods to assess flaws that are found in in-service equipment caused by various damage mechanisms. The focus of this study is to review the technical basis for the Fitness-For-Service assessment procedures for general and local metal loss. Extensive validation of these procedures along with additional development is presented. The conclusions of the study are recommended as the best practices to be included in future versions of API 579. The specific objectives for the study are as follows: Objective 1: Validate the API 579 Section 5 LTA rules in addition to the validation in WRC 465. The validation includes comparison of the API 579 methodology to other industry method and to a database of full scale tests. Objective 2: Develop new or improve upon the existing methodology to increase the accuracy of the assessment procedures and eliminate some of the limitations. iv Objective 3: Standardize the safety margin between MAWP and failure pressure for industry analysis methods and different Design Code margins on allowable stress. Objective 4: Improve the existing rules for LTAs subject to supplemental loading (circumferential extent of the LTA). This study is part of a series of WRC Bulletins that contain the technical background to the assessment procedures in API 579: WRC 430 Review of Existing Fitness-For-Service Criteria for Crack-Like Flaws WRC 465 Technologies for the Evaluation of Erosion/Corrosion, Pitting, Blisters, Shell Out-of-Roundness, Weld Misalignment, Bulges, and Dents in Pressurized Components WRC CCC An Overview and Validation of The Fitness-For-Service Assessment Procedures for Crack-Like Flaws in API 579 (not complete as of this printing) WRC 471 Development of Stress Intensity Factor Solutions for Surface and Embedded Cracks in API 579 WRC 478 Stress Intensity and Crack Growth Opening Area Solutions for Through-Wall Cracks in Cylinders and Spheres WRC MMM An Overview of the Fitness-For-Service Assessment Procedures for Weld Misalignment and Shell Distortions in API 579 (not complete as of this printing) WRC PPP An Overview of the Fitness-For-Service Assessment Procedures for Pitting Damage in API 579 (not complete as of this printing). This study represents a significant improvement to the current techniques available in the public domain for the analysis of Local Thin Areas. Information is also included that can be used to standardize the different LTA analysis techniques available in industry. However, further research, development and testing is required to further increase the accuracy of LTA analysis methods. The shortcomings of the assessment procedures are discussed as well as areas for future research. v TABLE OF CONTENTS Page LIST OF TABLES................................................................................................................. xii LIST OF FIGURES............................................................................................................... xiv CHAPTER I. INTRODUCTION.......................................................................................................... 1 1.1 Industry Needs................................................................................................. 1 1.2 Flaw Types and Damage Mechanisms in API 579.......................................... 2 1.3 General Corrosion and Local Thin Areas (LTAs) ............................................ 3 1.4 Need for Standardized Assessment ................................................................ 3 II. LTA ASSESSMENT AND VALIDATION OVERVIEW................................................. 5 2.1 Introduction ...................................................................................................... 5 2.2 Acceptance Criteria.......................................................................................... 6 2.2.1 Overview .......................................................................................... 6 2.2.2 Linear Elastic Allowable Stress Classification ................................. 6 2.2.3 Non-linear Elastic-Plastic Stress Criteria......................................... 7 2.2.4 Remaining Strength Factor .............................................................. 8 2.3 Original LTA Assessment Methodology........................................................... 9 2.4 LTA Development and Validation Work........................................................... 10 2.4.1 Introduction....................................................................................... 10 2.4.2 Kiefner, et al ..................................................................................... 10 2.4.3 Stephens, Bubenik, Leis, et al.......................................................... 11 2.4.4 Coulson, Worthington....................................................................... 12 vi 2.4.5 Mok, Pick, Glover, Hoff .................................................................... 13 2.4.6 Chell ................................................................................................. 13 2.4.7 Hopkins, Jones, Turner, Ritchie, Last .............................................. 14 2.4.8 Kanninen, et al ................................................................................. 15 2.4.9 Chouchaoui, Pick ............................................................................. 15 2.4.10 Valenta, et al.................................................................................... 15 2.4.11 Zarrabi, et al .................................................................................... 16 2.4.12 Sims, et al ........................................................................................ 16 2.4.13 Batte, Fu, Vu, Kirkwood................................................................... 16 2.4.14 Fu, Stephens, Ritchie, Jones .......................................................... 17 2.5 ASME Section XI Class 2 and 3 Piping ........................................................... 17 2.6 Current In-Service Inspection Codes............................................................... 17 III. API 579 METAL LOSS ASSESSMENT PROCEDURES............................................. 19 3.1 Introduction...................................................................................................... 19 3.2 Multi-Level Assessment Procedure ................................................................ 20 3.3 Inspection Data Requirements........................................................................ 21 3.3.1 Point Thickness Readings ............................................................... 21 3.3.2 Critical Thickness Profiles................................................................ 22 3.4 Assessment of General Metal Loss ................................................................ 23 3.4.1 Overview .......................................................................................... 23 3.4.2 Applicability and Limitations............................................................. 24 3.4.3 Metal Loss Away from Structural Discontinuities............................. 25 3.4.3.1 Assessment with Point Thickness Readings................... 25 3.4.3.2 Assessment with Critical Thickness Profiles ................... 26 3.4.4 Metal Loss at Major Structural Discontinuities................................. 29 3.5 Assessment of Local Metal Loss .................................................................... 31 3.5.1 Overview .......................................................................................... 31 3.5.2 Applicability and Limitations............................................................. 31 vii 3.5.3 Assessment Procedure Circumferential Stress Direction............. 33 3.5.3.1 Overview.......................................................................... 33 3.5.3.2 API 579 Section 5, Level 1 Assessment ......................... 33 3.5.3.3 API 579 Section 5, Level 2 Assessment ......................... 34 3.5.4 Assessment Procedure Longitudinal Stress Direction.................. 36 3.5.4.1 Overview.......................................................................... 36 3.5.4.2 API 579 Section 5, Level 1 Assessment.......................... 37 3.5.4.3 API 579 Section 5, Level 2 Assessment.......................... 38 3.5.5 Non-Cylindrical Shells...................................................................... 41 3.5.5.1 Overview.......................................................................... 41 3.5.5.2 Spherical Shells and Formed Heads............................... 41 3.5.5.3 Conical Shells .................................................................. 43 3.5.5.3 Elbows ............................................................................. 43 3.6 API 579 Advanced Assessment of Metal Loss ............................................... 44 3.6.1 Overview .......................................................................................... 44 3.6.2 Assessment with Numerical Analysis .............................................. 45 3.6.3 API 579, Level 3 Assessment (Lower Bound Limit Load)................ 46 3.6.4 Plastic Collapse Load....................................................................... 48 3.7 Comparison of General and Local Metal Loss ................................................ 49 3.8 Remaining Life Evaluation ............................................................................... 50 3.8.1 Overview .......................................................................................... 50 3.8.2 Thickness Approach......................................................................... 50 3.8.3 MAWP Approach.............................................................................. 51 IV. LTA ASSESSMENT PROCEDURES FOR CIRCUMFERENTIAL STRESS............... 52 4.1 Introduction...................................................................................................... 52 4.2 Calculation of Undamaged MAWP ................................................................. 52 4.3 Calculation of Undamaged Failure Pressure.................................................. 53 viii 4.4 Calculation of Damaged MAWP and Damaged Failure Pressure................... 55 4.5 Thickness Averaging Assessment................................................................... 57 4.5.1 Overview .......................................................................................... 57 4.5.2 API 510 Assessment (Method 8) ..................................................... 57 4.5.3 API 653 Assessment (Method 9) ..................................................... 58 4.5.4 API 579 Section 4, Level 1 and Level 2 Assessment (Methods 25 and 26) ........................................................................................ 58 4.6 ASME B31.G Assessment ............................................................................... 59 4.6.1 Overview .......................................................................................... 59 4.6.2 Original ASME B31.G Assessment (Method 7) ............................... 59 4.6.3 Modified B31.G Assessment, 0.85dl Area (Method 4)..................... 62 4.6.4 Modified B31.G Assessment, Exact Area (Method 6) ..................... 64 4.7 RSTRENG Method (Method 5)........................................................................ 65 4.8 PCORR Assessment (Method 20)................................................................... 66 4.9 API 579 Assessment........................................................................................ 68 4.9.1 Overview .......................................................................................... 68 4.9.2 API 579, Level 1 Assessment (Method 1)........................................ 69 4.9.3 API 579, Level 2 Assessment, Effective Area (Method 2) ............... 69 4.9.4 API 579, Level 2 Assessment, Exact Area (Method 3).................... 70 4.9.5 API 579 Hybrid 1, Level 1 Assessment (Method 14) ....................... 70 4.9.6 API 579 Hybrid 1, Level 2 Assessment (Method 15) ....................... 71 4.9.7 API 579 Hybrid 2, Level 1 Assessment (Method 16) ....................... 72 4.9.8 API 579 Hybrid 2, Level 2 Assessment (Method 17) ....................... 72 4.9.9 API 579 Hybrid 3, Level 1 Assessment (Method 18) ....................... 73 4.9.10 API 579 Hybrid 3, Level 2 Assessment (Method 19) ...................... 74 4.9.11 API 579 Modified, Level 1 Assessment (Method 27) ...................... 74 4.9.12 API 579 Modified, Level 2 Assessment (Method 28) ...................... 75 4.10 Chell Assessment .......................................................................................... 76 ix 4.10.1 Overview.......................................................................................... 76 4.10.2 Chell Assessment (Method 12) ....................................................... 78 4.10.3 Modified Chell Assessment (Method 13) ........................................ 79 4.11 British Gas Assessment................................................................................. 79 4.11.1 Overview.......................................................................................... 79 4.11.2 British Gas Single Defect Analysis (Method 10) ............................. 81 4.11.3 British Gas Complex Defect Analysis (Method 11) ......................... 83 4.12 BS 7910 Assessment..................................................................................... 86 4.12.1 BS 7910, Appendix G Assessment, Isolated Defect (Method 21) .. 86 4.12.2 BS 7910, Appendix G Assessment, Interacting Flaws (Method 22)................................................................................................... 87 4.13 Kanninen Assessment (Method 23)............................................................... 87 4.14 Shell Theory Assessment (Method 24).......................................................... 89 4.15 Janelle Method............................................................................................... 90 4.15.1 Janelle Level 1 Assessment (Method 29) ....................................... 90 4.15.2 Janelle Level 2 Assessment (Method 30) ....................................... 91 V. VALIDATION OF LTA ASSESSMENT PROCEDURES FOR CIRCUMFERENTIAL STRESS....................................................................................................................... 93 5.1 Introduction...................................................................................................... 93 5.2 Validation Databases ...................................................................................... 93 5.3 New LTA Analysis Methods ............................................................................ 94 5.3.1 API 579 Hybrid Assessment Procedures......................................... 95 5.3.2 New Folias Factor Development for Hybrid Methods ...................... 96 5.3.3 Modified API 579, Level 2 Folias Factor for Long Flaws ................. 97 5.3.4 Janelle Method................................................................................. 99 5.4 Statistical Validation of LTA Methodology Using a Failure Ratio.................... 100 5.5 Summary of Validation Results....................................................................... 101 VI. ALLOWABLE RSF FOR DIFFERENT DESIGN CODES ............................................ 102 6.1 Introduction...................................................................................................... 102 x 6.2 Design Codes for Pressurized Equipment ...................................................... 102 6.3 Margin of MAWP to Failure Pressure per Design Code ................................. 105 6.4 Allowable RSF Results.................................................................................... 105 VII. LTA ASSESSMENT PROCEDURES FOR LONGITUDINAL STRESS ...................... 106 7.1 Introduction...................................................................................................... 106 7.2 Kanninen Assessment Method ....................................................................... 106 7.3 Thickness Averaging....................................................................................... 106 7.3.1 API 510............................................................................................. 107 7.3.2 API 653............................................................................................. 107 7.4 API 579 Assessment Methods......................................................................... 107 7.4.1 API 579 Section 5, Level 1 Analysis ................................................ 107 7.4.2 API 579 Section 5, Level 2 Analysis ................................................ 107 7.4.3 Modified API 579 Section 5, Level 2 Analysis.................................. 107 7.4.4 Janelle, Level 1 Analysis.................................................................. 108 7.4.5 Janelle. Level 2 Analysis.................................................................. 112 VIII. VALIDATION OF LTA ASSESSMENT PROCEDURES FOR LONGITUDINAL STRESS....................................................................................................................... 116 8.1 Introduction...................................................................................................... 116 8.2 Validation Databases ...................................................................................... 116 8.3 Summary of Validation Results....................................................................... 117 IX. LTA PROCEDURES FOR HIC DAMAGE.................................................................... 118 9.1 Introduction...................................................................................................... 118 9.2 Subsurface HIC Damage ................................................................................ 118 9.3 Surface Breaking HIC Damage....................................................................... 120 X. LTA PROCEDURES FOR EXTERNAL PRESSURE .................................................. 123 XI. CONCLUSIONS AND RECOMMENDATIONS............................................................ 125 11.1 Introduction .................................................................................................... 125 11.2 LTA Assessment Procedures for Circumferential Stress .............................. 125 xi 11.2.1 Recommended Methods for Circumferential Stress ....................... 125 11.2.2 Allowable Remaining Strength Factors ........................................... 126 11.3 Recommended Methods for Longitudinal Stress........................................... 126 11.4 Further LTA Assessment Development......................................................... 127 11.4.1 Material Toughness Effects............................................................. 127 11.4.2 Stress Triaxiality from LTAs ............................................................ 128 11.4.3 Rules for LTAs Near Structural Discontinuities............................... 128 XII. NOMENCLATURE....................................................................................................... 129 XIII. TABLES........................................................................................................................ 134 XIV. FIGURES ..................................................................................................................... 224 REFERENCES..................................................................................................................... 258 xii LIST OF TABLES Table Page 1 Stress Classification ................................................................................................ 134 2 Examples of Stress Classification ........................................................................... 135 3 Thickness Averaging for In-Service Inspection Codes............................................ 138 4 Section Properties for Computation of Longitudinal Stress in a Cylinder with a LTA .......................................................................................................................... 139 5 LTA Assessment Methods....................................................................................... 141 6 Validation Cases for the Undamaged Failure Pressure Calculation Method.......... 143 7 Parameters for a Through-Wall Longitudinal Crack in a Cylinder Subject to a Through-Wall Membrane and Bending Stress ........................................................ 144 8 LTA Database 1 Case Descriptions ........................................................................ 145 9 LTA Database 2 Case Descriptions ........................................................................ 147 10 LTA Database 3 Case Descriptions ........................................................................ 147 11 LTA Database 4 Case Descriptions ........................................................................ 148 12 FEA Results for a Cylindrical Shell with a LTA........................................................ 149 13 FEA Results for a Spherical Shell with a LTA......................................................... 150 14 API 579 Folias Factor Values for a Cylinder and a Sphere..................................... 151 15 Cases Omitted from Statistics ................................................................................. 153 16 Stress Limits Based on Design Codes.................................................................... 154 17 Stress Limits Based on Design Codes.................................................................... 157 18 MAWP Ratio vs. Allowable Stress for ASME Section VIII, Division 1 (Pre 1999) and ASME B31.1 (Pre 1999) ................................................................................... 158 19 MAWP Ratio vs. Allowable Stress for ASME Section VIII, Division 1 (Post 1999) and ASME B31.1 (Post 1999) ................................................................................. 163 x iii20 MAWP Ratio vs. Allowable Stress for ASME Section VIII, Division 2 and ASME B31.3........................................................................................................................ 168 21 MAWP Ratio vs. Allowable Stress for the New Proposed ASME Section VIII, Division 2 ................................................................................................................. 173 22 MAWP Ratio vs. Allowable Stress for CODAP........................................................ 178 23 MAWP Ratio vs. Allowable Stress for AS 1210 and BS 5500................................. 183 24 MAWP Ratio vs. Allowable Stress for ASME B31.4 and ASME B31.8, Class 1, Division 2 ................................................................................................................. 188 25 MAWP Ratio vs. Allowable Stress for ASME B31.8, Class 1, Division 1................ 193 26 MAWP Ratio vs. Allowable Stress for ASME B31.8, Class 2.................................. 198 27 MAWP Ratio vs. Allowable Stress for ASME B31.8, Class 3.................................. 203 28 MAWP Ratio vs. Allowable Stress for ASME B31.8, Class 4.................................. 208 29 MAWP Ratio vs. Allowable Stress for API 620........................................................ 213 30 MAWP Ratio vs. Allowable Stress for API 650........................................................ 218 31 Geometry Parameters for the Circumferential Extent Validation Cases ................. 223 32 Circumferential Extent Validation Results ............................................................... 223 x iv LIST OF FIGURES Figure Page 1 Logic Diagram for the Assessment of General or Local Metal Loss in API 579 ..... 224 2 Logic Diagram for the Assessment of Local Thin Areas in API 579 ....................... 225 3 Coefficient of Variation for Thickness Reading Data (a) Small Variability in Thickness Profiles and the COV (b) Large Variability in Thickness Profiles and the COV.............................................................................. 226 4 Examples of an Inspection Grid to Define the Extent of Metal Loss Damage ........ 227 5 Establishing Longitudinal and Circumferential Critical Thickness Profiles from an Inspection Grid (a) Inspection Planes and Critical Thickness Profile (b) Critical Thickness Profile (CTP) Longitudinal Plane (Projection of Line M) (c) Critical Thickness Profile (CTP) Circumferential Plane (Projection of Line C) ............................................. 228 6 Critical Thickness Profiles for Isolated and Multiple Flaws (a) Isolated Flaw (b) Network of Flaws................................................................... 229 7 Zone for Thickness Averaging in a Nozzle.............................................................. 230 8 LTA to Major Structural Discontinuity Spacing Requirements in API 579............... 231 9 Example of a Zone for Thickness Averaging at a Major Structural Discontinuity ... 232 10 Level 1 Assessment Procedure for Local Metal Loss I Cylindrical Shells (Circumferential Stress)........................................................................................... 233 11 Determination of the RSF for the Effective Area Procedure (a) Subsection for the Effective Area Procedure (b) Minimum RSF Determination .......................................................................................................... 234 12 Exact Area Integration Bounds................................................................................ 235 13 Supplemental Loads for a Longitudinal Stress Assessment ................................... 236 14 Assessment Locations and Parameters for a Longitudinal Stress Assessment (a) Region of Local Metal Loss Located on the Inside Surface (b) Region of Local Metal Loss Located on the Outside Surface.................................................. 237 15 Longitudinal Stress, Level 1 Screening Curve ........................................................ 238 16 BG Depth Increment Approach ............................................................................... 238 x v17 Table Curve 3D Fit of the Shell Theory Folias Factor ............................................. 239 18 Comparison Between Analysis Methods and FEA Trends for a Cylinder with a LTA .......................................................................................................................... 239 19 3D Solid FEA Model Geometry of a Cylinder for = 5............................................ 240 20 Axisymmetric FEA Model Geometry of a Cylinder for = 5.................................... 240 21 Table Curve 2D Fit of the Modified API 579 Folias Factor ...................................... 241 22 Comparison of the Old API 579 Folias Factor to the Modified Folias Factor and the Original Folias Factor ........................................................................................ 241 23 Screening Curve for the Circumferential Extent of an LTA ..................................... 242 24 Comparison of the Old API 579 Level 1 Screening Curve to the Modified API 579 Folias Factor Level 1 Screening Curve ................................................................... 243 25 Axisymmetric FEA Model Geometry of a Sphere for = 5 ..................................... 244 26 Comparison Between Analysis Methods and FEA Trends for a Sphere with a LTA .......................................................................................................................... 244 27 Table Curve 3D Plot of the Janelle Method............................................................. 245 28 RSFA vs. MAWP Ratio for ASME Section VIII, Division 1 (Pre 1999) and ASME B31.1 (Pre 1999) for the Modified API 579 Assessment (Method 28) .................... 246 29 RSFA vs. MAWP Ratio for ASME Section VIII, Division 1 (Post 1999) and ASME B31.1 (Post 1999) for the Modified API 579 Assessment (Method 28) .................. 246 30 RSFA vs. MAWP Ratio for ASME Section VIII, Division 2 and ASME B31.3 for the Modified API 579 Assessment (Method 28) ...................................................... 247 31 RSFA vs. MAWP Ratio for the New Proposed ASME Section VIII, Division 2 for the Modified API 579 Assessment (Method 28) ...................................................... 247 32 RSFA vs. MAWP Ratio for CODAP for the Modified API 579 Assessment (Method 28) ............................................................................................................. 248 33 RSFA vs. MAWP Ratio for AS 1210 and BS 5500 for the Modified API 579 Assessment (Method 28)......................................................................................... 248 34 RSFA vs. MAWP Ratio for ASME B31.4 and ASME B31.8, Class 1, Division 2 for the Modified API 579 Assessment (Method 28)................................................. 249 35 RSFA vs. MAWP Ratio for ASME B31.8, Class 1, Division 1 for the Modified API 579 Assessment (Method 28).................................................................................. 249 36 RSFA vs. MAWP Ratio for ASME B31.8, Class 2 for the Modified API 579 Assessment (Method 28)......................................................................................... 250 37 RSFA vs. MAWP Ratio for ASME B31.8, Class 3 for the Modified API 579 Assessment (Method 28)......................................................................................... 250 x vi38 RSFA vs. MAWP Ratio for ASME B31.8, Class 4 for the Modified API 579 Assessment (Method 28)......................................................................................... 251 39 RSFA vs. MAWP Ratio for API 620 for the Modified API 579 Assessment (Method 28) ............................................................................................................. 251 40 RSFA vs. MAWP Ratio for API 650 for the Modified API 579 Assessment (Method 28) ............................................................................................................. 252 41 Maximum Bending Factor as a Function of the Radius to Thickness Ratio............ 252 42 Screening Curve for the Circumferential Extent of a LTA ....................................... 253 43 Longitudinal Stress Folias Factor ............................................................................ 254 44 Subsurface HIC Damage (a) Subsurface HIC Damage Actual Area (b) Subsurface HIC Damage Area Modeled as an Equivalent Rectangle...................................................................... 255 45 Surface Breaking HIC Damage (a) Surface Breaking HIC Damage Actual Area (b) Surface Breaking HIC Damage Area Modeled as an Equivalent Rectangle ........................................... 256 46 Idealized Geometry for a LTA Subject to External Pressure................................... 257 1 CHAPTER I INTRODUCTION 1.1 INDUSTRY NEEDS Most US design codes and standards for pressure containing equipment do not adequately address degradation and damage during operation. In the pressure vessel and pipeline industries, surface flaws are major limiting factors of vessel or pipe life, and this type of degradation due to age and aggressive environment eventually threatens the structural integrity of equipment. Replacing vessel and piping equipment is expensive, making it cost effective and desirable to operate slightly damaged equipment. For corrosion beyond a specified limit or other damage mechanism like cracking, a Fitness-For-Service (FFS) assessment is required. A FFS assessment is a quantitative engineering evaluation to determine the structural integrity of equipment containing a flaw or damage. The American Petroleum Institute (API) Recommended Practice (RP) 579 [1] is a comprehensive document for evaluating common flaws and damage in pressure vessels, piping, and tankage. The guidelines presented in API 579 may also be used in other industries as long as the applicability and limitations for an assessment are satisfied. API 579 is intended to supplement and expand upon the requirements in the inspection codes NBIC [2], API 510 [3], API 570 [4], and API 653 [5]. The goals are to ensure an acceptable margin of safety, provide accurate remaining life predictions, and help optimize maintenance and inspection for damaged equipment still in operation. The focus of this study is to further develop and validate the rules for assessing metal loss or corrosion damage in API 579. 2 1.2 FLAW TYPES AND DAMAGE MECHANISMS IN API 579 Various types of flaws can occur in piping systems and pressure vessels due to environmental and in-service factors. API 579 addresses the following geometric flaws and damage mechanisms: Brittle Fracture: Brittle fracture is the susceptibility of a material to form crack-like flaws or experience a catastrophic failure typically at lower temperatures. General Metal Loss: General metal loss is a uniform reduction in wall thickness caused by corrosion and is one of the simplest defects to assess. Local Metal Loss: Local metal loss or Local Thin Areas (LTAs) are similar to general metal loss. The geometry of these defects is more complex than general metal loss and includes most types of isolated metal loss that can occur in pipe and vessel walls. Pitting: Pitting corrosion is closely related to local metal loss and is characterized by large numbers of small pits in a given area of pipe or vessel wall. The damage can be assessed with the same rules that are provided for LTAs with a few additional requirements. Blisters and Laminations: Blisters most often appear in equipment that is in some form of hydrogen service. Hydrogen molecules impregnate the steel, forming high-pressure bubbles of hydrogen gas or blisters in the vessel wall. Laminations occur during the steel plate manufacturing process and are a plane of non-fusion in the interior of the steel plate. Blisters may also be evaluated with the analysis methodology provided for LTAs with additional requirements. Weld Misalignment and Shell Distortion: Weld misalignment is an offset of plate centerlines that occurs in the longitudinal or circumferential weld joints of vessels during the vessel fabrication process. Shell distortion usually occurs during fabrication and is the result of improperly rolled shell plates. 3 Crack-Like Flaws: Crack-like flaws can have widely varying geometry and are caused by multiple mechanisms. Rules are provided for analyzing crack-like flaws as they are, or grinding them out and treating them like a LTA. Creep Damage: Creep damage occurs mostly in high temperature service and is a relation between time, temperature, stress, and excessive strain. This damage can also lead to cracks and crack growth. Dents and Gouges: Dents and gouges are forms of damage usually resulting from mechanically cold working a material. These defects are similar to shell distortions and LTAs respectively, but additional requirements must be met to prevent brittle fracture. 1.3 GENERAL CORROSION AND LOCAL THIN AREAS (LTAS) Local thin areas appear in several different geometries. The first is isolated areas of general corrosion. These "patches" of corrosion are areas of isolated uniform corrosion in a pipe or vessel wall and are characterized by a non-varying flaw thickness profile. Areas of local metal loss are similar to general metal loss but may have extreme variations in the flaw thickness profile. Isolated pits are another classification of local thin area that have a circular shape and are usually smaller than areas of general corrosion. Combinations of general metal loss, local metal loss, and pitting can give rise to an infinite number of local thin area geometries. General pitting, blisters, and gouges can also be thought of as local thin areas and assessed using similar analysis methods. Likewise, a crack-like flaw may be ground out and the resulting groove evaluated like a LTA. With many types of common defects being classified as local thin areas, the importance of finding a reliable analysis method is evident. 1.4 NEED FOR STANDARDIZED ASSESSMENT Currently there are twenty-five different methods compiled in this study for analyzing local thin areas in pipes and vessels. These analysis methods all have roots in various industries, codes, and standards. In industry, at least five of these methods are actively used in Fitness-For- 4Service assessments today. This can make communication difficult between parties using different assessment procedures, and some parties may be using methods with low accuracy or reliability. Depending on the assessment code that is used, assessment results may vary drastically. One standardized set of analysis guidelines is needed to eliminate confusion regarding which method should be used. The focus of this study is to find the most statistically accurate and reliable method currently available and to validate the guidelines in API 579. 5 CHAPTER II LTA ASSESSMENT AND VALIDATION OVERVIEW 2.1 INTRODUCTION Determining the Fitness-For-Service or safe operating pressure of corroded equipment is not yet an exact science. As such, assessment accuracy is extremely important. In an attempt to improve reliability, researchers have implemented test programs involving full-scale burst tests and finite element analysis of corroded pipes and vessels. With the data collected from test programs, many different methods and acceptance criteria for analyzing LTAs have evolved. The questions are: which of these methods are the most accurate and can the accuracy be further improved? In an attempt to answer these questions, large databases of burst tests and finite element analysis have been compiled in this study from various sources. The cases in each database are analyzed with each of analysis methods available in the public domain and some newly developed methods. Statistical analysis of the various Fitness-For-Service assessment methods will provide the best gage for measuring the accuracy of each method. Alterations to the current API 579 Fitness-For-Service guidelines will be recommended based on the findings of this study. The current procedures for inspection and analysis of an LTA from the document are presented in later sections. The assessment methods in API 579 will be validated and compared to all other closed formed methods presented in this study. The validated assessment methods will be used with various construction codes, and code based assessment guidelines will be developed and included in API 579. This will allow standardized assessment of components designed to different construction codes. 6 2.2 ACCEPTANCE CRITERIA 2.2.1 Overview Depending of the type of mechanical analysis being performed, different acceptance criteria have been developed for various failure modes to insure safety in a given design. For example, a primary concern in the design of a vacuum tower is buckling of the shell wall due to external pressure. To prevent this type of failure, structural stability criteria have been developed for use with buckling analysis for equipment with large compressive stresses. There are other types of acceptance criteria such as fatigue initiation used to evaluate components subject to cyclical loading, and similarly, creep-fatigue initiation criteria used for components exposed to cyclical loading in the creep regime. One of the most widely used acceptance criterion is stress criteria. Stress criteria are limits placed on stresses generated in a given component due to geometry, loading, damage (such as an LTA), or other conditions and is based on material properties of the component at a given temperature. The two types of stress criteria that are relevant to a LTA assessment are linear elastic stress classification and non-linear elastic-plastic stress evaluation. A separate approach for evaluating a LTA is the Remaining Strength Factor (RSF) criteria. With the RSF approach, the load carrying capacity of a damaged component is compared to the load carrying capacity of the undamaged component to calculate a reduction in strength. Either linear elastic stress or RSF criteria are used for the closed form assessment procedures presented in this report. Non-linear elastic-plastic stress criteria is most commonly used for advanced (numeric) analysis of a LTA, but other criteria for fatigue, buckling, creep, or any other failure mode may also be used. 2.2.2 Linear Elastic Allowable Stress Classification For LTAs a quantity known as stress intensity can be computed and compared to an allowable value of stress intensity. Stress intensity is a measure of stress derived from a yield criterion. Two yield criteria to establish stress intensity are recommended by API 579. Maximum 7yield stress intensity is equal to twice the maximum shear stress which is equal to the difference between the largest and smallest principle stress as follows: max 1 2 2 3 3 12 max , , S = = ( (1) The other yield criterion is maximum distortion energy. This is the preferred criteria and is also known as the Von Mises equivalent stress. ( ) ( ) ( ) 0.52 2 21 2 2 3 3 112von MisesS (= = + + (2) Determination of structural integrity is based on a comparison between calculated stress intensity and the allowable stress intensity of the material. There are five stress intensity categories based on location and origin of the stress field. The five categories and their associated limits along with the tri-axial stress limits are shown in Table 1. Examples of stress classification based on component, location, and loading is provided in Table 2. Establishment of the allowable stress intensity for structural integrity comparison is based on the design code used to construct the component. A detailed description of the design codes and associated allowable stress intensities can be found in Paragraph 6.2. 2.2.3 Non-linear Elastic-Plastic Stress Criteria Non-linear elastic-plastic stress criteria typically provide a better prediction of safe load carrying capacity for a component. Traditional linear elastic stress classification and allowable stress criteria make only a rough estimate of failure loads because they ignore non-linear phenomenon that may occur in component failure. Non-linear elastic plastic analysis takes into account geometric, material, and combined non-linearity directly, to develop plastic collapse loads. Plastic collapse loads are defined as the maximum load where material response is elastic-plastic including strain hardening and large displacement effects. Closed form solutions for plastic collapse loads are not readily available, so numerical techniques such as Finite Element Analysis (FEA) may be used to obtain a solution. The calculated stress intensity for limit 8or plastic collapse loads can be compared to allowable stress intensities to determine a components structural integrity. The concept of plastic collapse load can be used to develop a simplified strength factor for LTAs called the Remaining Strength Factor. 2.2.4 Remaining Strength Factor The Remaining Strength Factor (RSF) has been introduced to define the acceptability for continued service of components containing a flaw in terms non-linear elastic plastic stress criteria. For a LTA analysis, plastic collapse loads can be calculated using FEA or full scale burst tests. The RSF was originally proposed by Sims [6] to evaluate LTAs and is defined as: { }{ }Collapse Load of Damaged ComponentRSFCollapse Load of Undamaged Component= (3) Acceptance criteria can be established using the RSF in combination with traditional code formulas, elastic stress analysis, limit load theory, or elastic-plastic analysis, depending on complexity of the assessment. The RSF is the value calculated by many of the assessment procedures presented in API 579. Each of the LTA assessment methods presented in this study has been reworked in terms of the RSF where possible for ease of comparison. Detailed procedures for calculating the RSF for each analysis method are found in Paragraphs 4.6 through 4.14. The RSF can be used to calculate either the failure pressure or the Maximum Allowable Working Pressure (MAWP) of damaged components. The calculation for determining the failure pressure of damaged equipment is: 0 fP P RSF = (4) The MAWP is slightly different and can be calculated using the RSF and an allowable RSF as follows: 0 aaRSFMAWP MAWP for RSF RSFRSF| |= < |\ . (5) 9 0 aMAWP MAWP for RSF RSF = (6) In a Fitness-For-Service assessment, the calculated RSF is compared to an allowable value. If the calculated RSF is greater than the allowable, the component may be returned to service. If the calculated RSF is less than the allowable, the component may be derated using Equation (5). The recommended value for the allowable remaining strength factor that is currently in API 579 is 0.9 for equipment in process services. This value can be overly conservative or un-conservative based on the design code used in construction, type of loading, or consequence of failure. One of the objectives of this study is to standardize the amount of conservatism in the determination of a damaged MAWP for different design codes and assessment methods. This will be achieved by tuning the allowable RSF so that a fixed margin on MAWP to failure pressure is maintained regardless of design code. 2.3 ORIGINAL LTA ASSESSMENT METHODOLOGY Before specific LTA assessment procedures were developed, regions of metal loss in were assessed using thickness averaging techniques. The origins of this method are unclear, although some guidelines still use these procedures which have been shown to be greatly conservative. To improve the assessment techniques for corroded pipelines, additional criteria was developed in the late 1960s and early 1970s through research sponsored by Texas Eastern Transmission Corporation and the AGA pipeline research committee. The criterion was incorporated into ASME B31.4 and B31.8 piping design codes and is commonly referred to as the B31.G [7] assessment criteria. The B31.G criteria are based on a fracture mechanics relationship developed by the AGA NG-18 Line Pipe Research Committee. The relationship was introduced by Maxey [8] and is based on a Dugdale plastic zone model, a Folias [9] bulging factor for a through wall crack in a cylindrical shell, and a flaw depth to thickness relationship. A series of corroded pipe burst tests were performed by Kiefner [10] to demonstrate the relationship between the remaining strength of pipes with and without LTAs. The B31.G method is the 10foundation for most of the local thin area assessments that are currently in use. Details of the original B31.G calculation procedure are presented in Paragraph 4.6.2. 2.4 LTA DEVELOPMENT AND VALIDATION WORK 2.4.1 Introduction Since initial development of local thin area assessment in the late 1960s, many other groups and individuals have conducted research related to this topic. Twenty-five analysis methods developed by various authors are contained in this study for general LTAs, and many more methods exist for analyzing specific cases. In addition to new development work, much effort has gone into validating the existing methods and comparing the methods to determine which is the most accurate. The following paragraphs have a brief summary of the validation and development work that is available in the public domain. 2.4.2 Kiefner, et al Kiefner [11], [12], [13], [14], [15], [16], [17] has published multiple papers with other authors on the subject of local thin area assessments for pipes. Contained in the papers from the late 1960s and early 1970s is the basis for most of todays assessment procedures, in addition to a large number of corroded pipe burst test cases that were used to validate the developed methodology. Kiefner also contributed to the development of techniques that improved upon the basic procedure, including the RSTRENG [18] (see Paragraph 4.7) method and software analysis tool. 112.4.3 Stephens, Bubenik, Leis, et al Bubenik [19] showed that finite element analysis can be used to predict the load carrying capacity of corroded pipes. Comparisons between FEA and over 80 burst tests showed that failure stresses were well over yield. It was also concluded that load redistribution is dependent on geometry and strain hardening and is more significant for small deep corroded regions than for large corrosion regions. Stephens [20] conducted research with full scale testing and FEA on the failure of corroded pipe subjected to internal pressure and axial loading. For pipe defects subjected only to internal pressure, defect width was of secondary importance to defect length and depth. For pipe defects subject to combined axial and pressure loads, defect width is significant, and results indicated that axial loads increased the combined von Mises stress in the pipe, resulting in lower failure pressure. Interaction of separated defects was also examined. The interaction of separated defects is dependant on the defect size. Small defects have small interaction length and large defects have large interaction lengths. Axial spaced defects increase the stresses when compared to an isolated defect, which may decrease failure pressure. Circumferentially spaced defects decrease the stresses when compared to an isolated defect, which may increase failure pressure. This study was also used in the development of PCORR. The PCORR analytic model uses traditional finite element analysis applied to local thin areas in pipelines. Stephens [21] compared some of the prominent LTA assessment methods to determine the most accurate method. Methods used in the comparison were B31.G, modified B31.G, RSTRENG, Chell, Kanninen, Ritchie, Sims, and API 579. Conclusions showed the API 579 method to have the least variability. The modified B31.G, RSTRENG, and Chell methods also had small variability. Stephens [22], [23], [24], [25], [26] has investigated the fundamental mechanisms driving failure of pipeline corrosion defects. The research involved three phases: development of an analytic model known as PCORR, comparative evaluation of material and defect geometry variables controlling failure, and development of a simple closed form failure assessment 12method. A parametric study with PCORR was used to identify variables that influence failure in moderate to high toughness pipe. The variables are ranked according to the magnitude of their influence as follows: 1. Internal pressure 2. Vessel or pipe diameter 3. Flaw depth and wall thickness 4. Ultimate material strength 5. Defect Length 6. Defect shape and characteristics 7. Yield strength and strain hardening characteristics 8. Defect Width 9. Fracture toughness The authors observed that pipes with low material toughness may fail at stresses below ultimate stress. This could be caused by crack initiation at the base of corrosion defects, resulting in failure pressures below the fully ductile prediction. PCORR was also used to develop a closed form solution for analyzing corrosion defects. The method is fully described in Paragraph 4.8 and is called the PCORR Assessment Method. 2.4.4 Coulson, Worthington Coulson and Worthington [27], [28] examined spirally oriented local thin areas and the interaction spacing between adjacent local thin areas. A full-scale burst test program was used in the study. Axial oriented flaws were compared to spiral flaws of equal length, and it was found that the spirally oriented flaws were less severe. A factor was developed that scaled the severity of spiral flaws to axial flaws of equal length. Failure pressure for spiral flaws is determined by calculating the failure pressure of an equivalent axial flaw and multiplying the result by the spiral factor. Additionally, general rules for the interaction of adjacent defects were developed as follows: 13 Flaws may interact in the axial direction if the separation between them is less than or equal to the length of the shortest flaw. Flaws may interact in the circumferential direction if the separation between them is less than or equal to the width of the narrowest flaw. Spiral flaws may interact if the separation between them along the spiral direction is less than or equal to the length. Spiral flaws separated by at least 12 inches normal to the spiral direction are not expected to interact. For the assessment of interacting flaws, assessment of the individual components is also necessary. The burst tests to verify these rules consisted of four spiral flaw tests, two axial flaw tests, three axial spaced flaw tests, one spirally spaced flaw test, and two circumferentially spaced flaw tests. Further validation of this method was performed by British Gas. 2.4.5 Mok, Pick, Glover, Hoff Mok, Pick, Glover, and Hoff [29], [30] examined the effects of long external corrosion by expanding on the work by Coulson and Worthington. Their objective was to develop a less conservative approach for evaluating long and long spiral flaws. Using previous tests and FEA analysis, the authors developed a burst pressure criterion for those types of flaws based on an orientation angle with respect to the circumferential plane of a cylindrical shell. 2.4.6 Chell In the original B31.G assessment methodology, a Folias factor is calculated based on a non-dimensional length parameter for the LTA. The Folias factor is used with the flaw profile to calculate a surface correction factor and subsequent acceptance criterion. Chell [31] developed 14an alternate form for the surface correction factor for LTA assessments. Details of the Chell surface correction factor are presented in Paragraph 4.10.1. 2.4.7 Hopkins, Jones, Turner, Ritchie, Last Hopkins and Jones [32] performed experimental tests to examine long flaws, interactions of slots, interaction of small and moderate size flaws, and short deep flaws contained in a larger shallow flaw. The experiments were performed in 24 inch pipe and included the following tests. Long slots: 4 cases Ring slots: 4 cases Short flaws and pits: 9 cases Interaction of medium flaws: 9 cases Short, deep flaws in a larger shallow flaw: 6 cases Jones, Turner, and Rithcie [33] performed FEA tests to examine plane stress failures (infinite length flaw) in 36 inch pipe. The authors were able to show that the failure sequence for the flaws were as follows. Yielding of the thinned section Full plastic behavior of the thinned section. Bending stresses exceeding yield develop in the undamaged section adjacent to the thinned section. Ductile failure occurs in the thinned section Ritchie and Last [34] developed a calculation procedure to calculated the failure pressure of a corroded shell based on the original B31.G equations. The authors modified the procedure to remove some of the conservatism and take into account ultimate strength and strain hardening for the damaged component. 15 2.4.8 Kanninen, et al Kanninen [35], [36], [37], [38] and others developed methodology to analyze the failure of LTAs subject to supplemental loading. As part of the research, full scale failure tests were performed to study the behavior of a LTA defect in a cylindrical shell that fails due to an applied net section bending moment. The assessment methodology developed by Kanninen is the bases for the evaluation of the circumferential (longitudinal stress) profile of a LTA. The details of his assessment method are presented in Paragraphs 4.13 and 7.2. 2.4.9 Chouchaoui, Pick Chouchaoui and Pick [39], [40], [41], [42], [43], [44] investigated the behavior of isolated or closely spaced corrosion flaws oriented circumferentially or longitudinally in pipe. The study included full scale burst tests and FEA of the test cases. For isolated flaws, it was shown that the B31.G and RSTRENG methods result in reasonable characterization of the damage. It was also concluded that longitudinally aligned pits within a certain spacing decreases the failure pressure of the pipe. 2.4.10 Valenta, et al Valenta [45], [46] developed a Finite Element Analysis model and a theoretical model for evaluating corrosion defects in gas transmission pipelines. The models were compared to the B31.G assessment and experimental verification. It was concluded that the FEA model would more accurately predict failure in corroded gas transmission pipelines than the ASME B31.G assessment method. 16 2.4.11 Zarrabi, et al Zarrabi [47] has presented methodology for assessing the integrity of cracked, eroded, or corroded vessels, tubes, or pipe. The methodology involves Finite Element models of cylindrical shells with part through rectangular slots. Plastic collapse pressures from the FEA are reported for a wide range of shells and slots through the use of non-dimensional parameters. Zarrabi [48] has developed methodology for assessing locally thin boiler tubes. By using elastic-plastic Finite Element Analysis models of boiler tubes with local thinning, a procedure is presented to calculate primary stress in the thinned section. The primary stress combined, material properties of the boiler tube, and operating conditions are used to calculate the creep and plastic lives of the boiler tube. 2.4.12 Sims, et al Sims [49], [50] was responsible for developing the RSF acceptability criterion for LTAs as described in Paragraph 2.2.4. In addition the authors reviewed existing methodology and developed modified rules for evaluating LTAs and groove-like flaws. 2.4.13 Batte, Fu, Vu, Kirkwood Batte, Fu, Vu, and Kirkwood [51], [52] undertook a British Gas group sponsored project to improve the assessment of corroded pipelines, resulting in the BG assessment methods. Included in that study are numerous full-scale pipe burst tests and FEA models. The burst tests were performed on high strength steel pipes with machined single or adjacent local thin areas. The full scale burst tests were reproduced with FEA models and the numeric results were compared to the actual results. The BG methods are presented in Paragraph 4.11 and the databases are presented in Paragraph 5.2. 17 2.4.14 Fu, Stephens, Ritchie, Jones Fu, Stephens, Ritchie, Jones [53] are the authors of the most current publication from the Pipeline Research Council. In the document, the original B31.G, modified B31.G, RSTRENG, and British Gas (BG) closed form methods for assessing local thin areas are compared. The study did not include the methodology currently in API 579. The cases are validated with full scale tests which are included in Database 1 and Database 3 of this report. The study recommends using the B31.G method for analyzing low toughness pipes and the RSTRENG and BG methods for high toughness pipes based on statistical analysis of the burst pressures predicted by the different methods. The BG methods (10 and 11) presented in this report have been expanded on to include methodology for analyzing groups of closely spaced local thin areas. Some spacing criteria is presented, but the method is still largely empirical. 2.5 ASME SECTION XI CLASS 2 AND 3 PIPING The ASME Section XI [54], [55], [56], group on pipe flaw evaluation is currently developing requirements for analytical evaluation of pipe wall thinning. The evaluation involves two separate assessments for a LTA in a pipe, elbow, or reducer. The first assessment is a thickness evaluation to determine if the minimum wall thickness is acceptable for internal pressure loads. The second is a stress evaluation to determine if primary and secondary loads cause stress that exceeds the material allowable limits specified by the code of construction. 2.6 CURRENT IN-SERVICE INSPECTION CODES Current in-service inspection codes for pressure vessels, piping, and tankage in the refinery and petrochemical industries contain assessment guidelines to evaluate LTAs. Although these rules have been in existence for many years, they are empirically based and do not have a sound technical background that is required to extend current limitations. A summary of the existing rules for the API 510, API 653, API 570, and NBIC inspection codes is shown in Table 3. These 18rules are based on average measured thickness data over a prescribed length. The advantages and limitations of thickness averaging are discussed in Chapter 3. As an alternative, the in-service inspection codes provide an option for evaluation by stress analysis. In this option, assessment results are evaluated using the ASME Boiler and Pressure Vessel Code, Section VIII, Division 2, Appendix 4 (Hopper diagram). This option provides flexibility in the analysis but becomes difficult to apply because the categorization procedure in Appendix 4. However, results may be arbitrary due to stress classification with the Hopper diagram. 19 CHAPTER III API 579 METAL LOSS ASSESSMENT PROCEDURES 3.1 INTRODUCTION The Fitness-For-Service (FFS) assessment procedures proposed in API 579 were developed to provide a standardized assessment methodology for inspectors, plant engineers, and engineering specialists. The rules include classification, limitations, and acceptance criteria for different types of metal loss. The option to calculate a derated MAWP based on the extent of damage is also provided. The procedures are valuable for extending the life of damaged equipment, setting inspection intervals, or determining the remaining life of damaged equipment. Most in-service inspection codes and standards use a thickness averaging procedure to evaluate areas of metal loss. API 579 includes modified thickness averaging rules as well as specific LTA analysis methodology to be consistent with the inspection standards. Therefore, metal loss is divided into two categories in API 579. General metal loss includes regions of corrosion or erosion that have uniform or non-uniform remaining thickness. The rules for evaluating general metal loss are presented in Section 4 of API 579. Local Metal Loss includes regions of metal loss that have a non-uniform thickness and more detailed assessment rules are used to provide an accurate result. The rules for evaluating local metal loss are presented in Section 5 of API 579. The difference between general and local metal loss assessments has to do with the amount and type of data that is required for the assessment. For general metal loss, point thickness readings or detailed thickness profiles are required. For local metal loss, detailed thickness profile information, which involves thickness readings and their spacing, is required. 20The assessment procedures for general metal loss in API 579 are based on a thickness averaging approach similar to other existing codes and provide a suitable result when applied to uniform metal loss. For local areas of metal loss, the thickness averaging approach may still be used; however, the results will be overly conservative. For these cases, the API 579 assessment procedures for local metal loss can be used to reduce the conservatism in the analysis. The local metal loss rules may also be used to evaluate general metal loss, but the amount of inspection data and complexity of the analysis is greater. The distinction between general and local metal loss is difficult to make without detailed knowledge of the metal loss profile, so the rules in API 579 have been structured to provide consistent results between the two methods. It is recommended that a simpler general metal loss assessment be initially performed for either type of metal loss. If the results are not satisfactory, an assessment using the local metal loss rules can be used for a less conservative estimate. 3.2 MULTI-LEVEL ASSESSMENT PROCEDURE Three levels of assessment are provided in API 579 for each flaw and damage type. In general, each assessment level has a balance between degree of conservatism, the amount of information required to perform the assessment, the skill of the personnel performing the assessment and the complexity of the analysis. A logic diagram is included in each section to illustrate how these assessment levels are interrelated. The overall logic diagram for assessing general or local metal loss is shown in Figure 1, and the logic diagram for evaluating local metal loss specifically is shown in Figure 2. Level 1 is the most conservative, but is easiest to use. Practitioners usually proceed sequentially from a Level 1 to a Level 3 assessment (unless otherwise directed by the assessment techniques) if the current assessment level does not provide an acceptable result or a clear course of action cannot be determined. A general overview of each assessment level and its intended use are described below: Level 1: The assessment procedures included in this level provide conservative screening criteria that require a minimum amount of inspection or component information. 21The Level 1 assessment procedures are intended for use by either plant inspection or engineering personnel. Level 2: The assessment procedures included in this level provide a more detailed evaluation that is less conservative than those from a Level 1 assessment. In a Level 2 assessment, inspection information similar to that required for a Level 1 assessment is required; however, more detailed calculations are used in the evaluation. Level 2 assessments are intended for use by plant engineers or engineering specialists experienced and knowledgeable in performing FFS assessments. Level 3: The assessment procedures included in this level provide the most detailed evaluation that produces results that are less conservative than those from a Level 2 assessment. In a Level 3 assessment additional inspection and component information is typically required, and the recommended analysis is based on numerical techniques such as finite element analysis. The Level 3 assessment procedures are intended for use by engineering specialists experienced and knowledgeable in performing FFS evaluations. 3.3 INSPECTION DATA REQUIREMENTS 3.3.1 Point Thickness Readings (PTR) There are two inspection techniques that may be used when characterizing a region of metal loss. Point Thickness Readings (PTR) are a random sampling of thickness measurements in a corroded region. PTR are only suitable for assessments where the variation in thickness readings is statistically small. The test for significance in the variability is based on the Coefficient of Variation (COV) of the thickness reading population. The COV is defined as the standard deviation of a sample divided by the mean of a sample. As shown in Figure 3, if the COV of the thickness reading population is small, then the variability in thickness readings is small. Alternatively, if the variability in thickness readings is large, so is the COV. If the COV of the thickness reading population minus the Future Corrosion Allowance (FCA) is less than 10%, 22then the general metal loss is defined to be uniform and the average thickness can be computed directly from the population of thickness readings. If the COV is greater than 10%, then the use of thickness profiles is required to determine the average thickness. PTR data may only be used for an API 579 Section 4 general metal loss assessment. As recommended in API 579, if point thickness readings are used in an assessment, the assumption of general metal loss should be confirmed considering the following: A minimum of 15 thickness readings is recommended unless the level of NDE utilized can be used to confirm that the metal loss is general. In some cases, additional readings may be required based on the size of the component, the construction details utilized, and the nature of the environment resulting in the metal loss. Additional inspection may be required such as visual examination, radiography or other NDE methods. 3.3.2 Critical Thickness Profiles (CTP) The other technique for characterizing metal loss is by using a Critical Thickness Profile (CTP). If possible, it is recommended that CTPs are always used for the assessment of metal loss. They are required for a detailed API 579 Section 5 local metal loss assessment and may also be used for an API 579 Section 4 general metal loss assessment. In addition the CTPs are better for inspections records if continued damage is expected. If the COV test for point thickness readings is greater than 10%, then the general metal loss is defined to be non-uniform and the use of thickness profiles is required. An inspection grid covering the region of metal loss is typically required to determine the extent of the damage. Examples of inspection grids used to map the metal loss damage on a cylinder, cone, and elbow are shown in Figure 4. Once the inspection grids have been established and the thickness readings are taken, the Critical Thickness Profiles (CTPs) can be determined. The CTPs in the longitudinal and circumferential directions are required for the assessment. The process to establish the CTP is shown in Figure 5. The longitudinal and circumferential CTPs are found by taking the lowest readings along the 23lines designated by Mi and Ci, respectively, as noted in the figure. This establishes the maximum metal loss or minimum thickness readings in the region of damage by using a "river bottom" approach. Once the minimum thicknesses along all of the lines identified with Mi and Ci lines are taken, these values are projected onto longitudinal and circumferential planes, respectively, to form the CTP in these directions as shown in Figure 5. In the figure, the dimension s is the length of the longitudinal CTP and the dimension c is the length of the circumferential CTP. The spacing of the CTPs is the spacing of the thickness grid in the longitudinal and circumferential directions. This process can be used for both isolated and multiple flaws as shown in Figure 6. 3.4 ASSESSMENT OF GENERAL METAL LOSS 3.4.1 Overview The API 579 Section 4 assessment procedures can be used to evaluate uniform and non-uniform metal loss on the outside diameter or inside diameter of a component. The results obtained for general metal loss may be overly conservative for flaws with significant thickness variations. To account for this, an initial screening can be performed using general metal loss guidelines, and an additional assessment may be performed using local metal loss guidelines if the component does not meet the general metal loss criteria. Two procedures for evaluating general metal loss away from structural discontinuities are provided based on the type of inspection data available. One procedure uses Point Thickness Readings (PTR) and the other uses Critical Thickness Profiles (CTP). Point thickness readings should be used in assessments where variance in thickness readings is small. Critical thickness profiles are suited to handle all types of assessment. It is recommended that CTPs be used whenever possible. Acceptability for both methods is determined from a strength criterion dictated by the original construction code, and each has criteria to ensure against leakage. If the strength criterion is not satisfied, rules are provided to determine the MAWP of pressurized components or the maximum fill height for atmospheric storage tanks. Procedures are also 24provided to establish an inspection interval based on a remaining life assessment, or to specify a future corrosion allowance for continued operation. A different procedure is required for metal loss at structural discontinuities. Structural discontinuities include nozzles and branch connections, axisymmetric discontinuities such as stiffening rings, piping systems which have thickness interdependency, or any other structural component that affects the shell stiffness in the region of metal loss. The current assessment methodology defines a zone of interaction between the shell and discontinuity. Acceptance for the region of metal loss is established by determining an average thickness for each component in the interaction zone and using the average thickness with the original design code equations for each component and the interdependency of the two. 3.4.2 Applicability and Limitations The following are the limitations and applicability for the Level 1 and Level 2 assessment procedures specified in API 579. The component must be designed and constructed in accordance with a recognized code or standard. This insures construction to a standard quality level and requires normal scheduled inspections. The component must not be operating in the creep range. The assessment guidelines presented here have not been validated for these conditions, although they may be applicable. Accumulated creep strains usually become concentrated in reduced stiffness regions. Stiffness reduction is a function of wall thickness, flaw geometry, material properties, and load conditions. These effects have not yet been addressed, so this type of assessment may not be conservative for these conditions. The region of metal loss must have relatively smooth contours without notches, crack-like flaws, or other locations of stress concentration. Notches and other areas of stress concentration may lead to cracking or brittle fracture, which is not considered in this type of assessment. Similarly, the material of the component must have sufficient material 25toughness. The local metal loss rules do not apply to materials that may be embrittled due to temperature or operating environment The component is not subject to cyclic service. Fatigue screening guidelines in API 579 are separate from a general LTA assessment. The cut-off for cyclic service in API 579 is 150 cycles. These limitations result in an acceptable level of conservatism when performing this type of assessment. Limitations based on loading conditions are also included. Internal pressure, maximum fill height, or supplemental loads must be governed by equations that relate the load to a required wall thickness. A summary of load limitations in API 579 for each assessment level are given as follows. Level 1 assessments are applicable to internal or external pressure only Level 2 assessments may have internal or external pressure and/or supplemental loading from weight and occasional loads Level 3 assessment can be performed when any of the above limitations are not satisfied or for any load conditions. 3.4.3 Metal Loss Away from Structural Discontinuities 3.4.3.1 Assessment with Point Thickness Readings The acceptance criteria for metal loss can be determined once the average and minimum thicknesses have been established. The Level 1 Assessment criteria are shown below. min amt FCA t (7) lim mmt FCA t (8) Where the minimum permissible thickness for pressure vessels and piping is | |lim minmax 0.5 , 0.10 t t inches = (9) 26 and the minimum permissible thickness for tanks is | |lim minmax 0.6 , 0.10 t t inches = (10) The Level 2 Assessment criteria are shown below. min am at FCA RSF t (11) lim mmt FCA t (12) The minimum permissible thickness, tlim, is evaluated using Equations (9) and (10). If the component fails the above criteria, a damaged MAWP can be determined by substituting the average thickness back into the original design equations as long as the minimum thickness requirement is satisfied. For example, for a cylindrical shell subjected to internal pressure, the MAWP could be determined as follows using a typical design equation. ( )( )10.6a ama amt FCAMAWPRSF R t FCA = + (13) The Level 1 calculation does not include the allowable RSF. The MAWP with inclusion of the allowable RSF may not be higher than the original calculated MAWP. 3.4.3.2 Assessment with Critical Thickness Profiles To perform a thickness averaging assessment with CTPs, the length for thickness averaging, L, is computed using the following equations. minL Q Dt = (14) 0.5211.123 11 tt aRQR RSF (| | ( = | (\ . (15) minamt t FCARt= (16) 27The Q factor is actually derived from the API 579, section 5 assessment rules for regions of local metal loss and can be thought of as a conservative screening method for local metal loss. A remaining strength factor based on the remaining thickness ratio and the flaw length is calculated as follows: ( )11 1tttRRSFRM= (17) 21 0.48tM = + (18) 1.285 lDt = (19) minmmt t FCARt= (20) In the above equations, l is the length of the local thin area based on the CTP. By setting the RSF equal to the allowable RSF and solving for l, conservative screening criteria can be derived which relates the length for thickness averaging to the remaining thickness ratio as follows: 1.285 lDt = (21) 21 0.48 1.285t lMDt| |= + |\ . (22) 2111 0.7926tatRRSFRlDt= + (23) Solving for l or the length for thickness averaging yields: 28 211.262 11 ttaRl DtRRSF (| | ( | ( | = ( | ( |\ . ( (24) Setting l equal to L and factoring out Q yields the following: L Q Dt = (25) 0.5211.123 11 ttaRQRRSF (| | ( | ( | = ( | ( |\ . ( (26) When the thickness averaging rules are applied to an area of metal loss that is an actual LTA, the length for thickness averaging will be small because a small Rt ratio produces a small Q value. This small length for thickness averaging when centered on the minimum thickness reading will produce a small average thickness that subsequently results in a small or conservative MAWP. The rules of API 579 have been structured to direct the user to the LTA assessment procedures for these cases. Alternatively, when the LTA has a high remaining thickness ratio, the value of Q becomes larger thus increasing the length for thickness averaging. When this longer length is centered on the minimum thickness reading value, a large average thickness and corresponding MAWP will result. This MAWP will approach the value that would be obtained using the LTA assessment procedures. The consistency in the rules is guaranteed because the length for thickness averaging given by Equation (14) is derived by substituting RSFa for RSF in equation (35) and solving for l; the resulting value of l is then set to the length for thickness averaging, L. After the length for thickness averaging, L, is determined, the assessment is completed based on the relative values of s and L: s > L the local metal loss assessment rules can be used for the evaluation s < L the general metal loss rules are used for the evaluation 29When using the general metal loss rules, the average thickness for both the meridional and circumferential planes must be considered. The average thickness in the meridional direction, tsam, is determined by averaging the thickness readings within the dimension s over the length L, and the average thickness is in the circumferential direction, tcam, is determined by averaging the thickness readings within the dimension c over the length L. The minimum thickness is based on the minimum thickness reading in the grid. In a Level 1 assessment, tam = tsam for cylindrical shells because the only loading permitted is internal pressure. For spheres and formed heads, the average thickness is taken as tam=max[tsam, tcam]. In a Level 2 assessment, tsam and tcam are used directly in the analysis to account for supplemental loads. For cylindrical shells, the acceptance criterion for the average thickness is the same as specified in Paragraph 3.4.3.1 except Equation (11) is replaced with the following equations. mins Cam at FCA RSF t (27) minc Lam at FCA RSF t (28) For spherical shells and formed heads the assessment criterion is identical to the cylindrical shell methodology. The only difference is how tmin is calculated. If the component fails the specified criteria, a damaged MAWP can be determined as described in Paragraph 3.4.3.1. 3.4.4 Metal Loss at Major Structural Discontinuities One advantage the general metal loss rules have over the local metal loss rules is that they allow the assessment of metal loss at structural discontinuities. Examples of structural discontinuities include local erosion and/or corrosion at vessel nozzle and piping branch connections, internal tray support rings, stiffening rings, conical shell transitions, and flanges. In the current edition of API 579, general and local areas of metal loss at structural discontinuities are evaluated by determining an average thickness within a thickness averaging zone, and using 30the thickness with the original construction code design rules to determine acceptability for continued service. Design rules for components at a major structural discontinuity typically involve satisfying a local reinforcement requirement (e.g. nozzle reinforcement area), stress requirement based upon a given load condition, geometry, and thickness configuration (e.g. flange design). These rules typically have a component with thickness that is dependent upon the thickness of another component. To evaluate components with thickness interdependency, the MAWP should be computed based upon the average measured thickness minus the future corrosion allowance including the thickness required for supplemental loads for each component using the equations in the original construction code. The calculated MAWP should be equal to or exceed the design MAWP. The average thickness of the region can be obtained as follows for components with thickness interdependency as described in API 579. Nozzles and branch connections: The average measured thickness is determined as the average of the thickness readings taken within the nozzle reinforcement zone as shown in Figure 7. Axisymmetric Structural Discontinuities: Determine L using Equation (14) and Lv based on the type of structural discontinuity as shown in Figures 8 and 9. The average thickness is computed based on the smaller of these two distances. If L < Lv, the midpoint of L should be located where the wall thickness is equal to tmm to establish a length for thickness averaging unless the location of tmm is within L/2 of the zone for thickness averaging. In this case, L should be positioned so that it is entirely within Lv to compute the average thickness. Piping Systems: Piping systems have thickness interdependency because of the relationship between the component thickness, piping flexibility, and the resulting stress. For straight sections of piping, determine L using the procedure described above and compute the average thickness to represent the section of pipe with metal loss in the piping analysis. For elbows or bends, the thickness readings should be averaged within the bend and a single thickness used in the piping analysis (i.e. to compute the flexibility 31factor, system stiffness and stress intensification factor). For branch connections, the thickness should be averaged within the reinforcement zones for the branch and header, and these thicknesses should be used in the piping model (to compute the stress intensification factor). An alternative assumption is to use the minimum measured thickness to represent the component thickness in the piping model. This approach may be warranted if the metal loss is localized; however, this may result in an overly conservative evaluation. 3.5 ASSESSMENT OF LOCAL METAL LOSS 3.5.1 Overview The local metal loss assessment rules are used to evaluate regions of metal loss resulting from erosion/corrosion, mechanical damage such as grooves and gouges, blend ground areas used to remove crack-like flaws, and the damage associated with pitting and blisters. The local metal loss assessment rules may only be used with CTP data. These procedures use the concept of an RSF for acceptance criteria, and contain separate rules for evaluating the longitudinal and circumferential stress direction of a flaw in cylindrical shells. The local metal loss rules are divided into rules for evaluating the circumferential stress direction or longitudinal profile of an LTA and the longitudinal stress direction or circumferential profile of an LTA. The circumferential stress assessment is used to evaluate LTAs in equipment subject to internal pressure only where circumferential stresses dominate. The longitudinal stress assessment is used to evaluate LTAs in equipment subject to internal pressure and supplemental loads that may cause the longitudinal stresses to effect the flaw behavior. As in the rules for general metal loss, two levels of assessment are provided. 323.5.2 Applicability and Limitations The applicability and limitations of Level 1 and Level 2 local metal loss assessment procedures have the same limitations as those described for general metal loss in Paragraph 3.4.2. In ad