1、 STP-PT-048 CRITERIA FOR RELIABILITY -BASED DESIGN AND ASSESSMENT FOR ASME B31.8 CODE Prepared by: Maher Nessim C-FER Technologies -N)ME STANDARDS TECHNOLOGY, llC Date oflssuance: June 30, 2012 This report was prepared as an account of work sponsored by ASME Pressure Technologies Codes and Standards
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9、y ASME Standards Technology, LLC All Rights Reserved Criteria for Reliability-Based Design and Assessment for ASME B31.8 Code STP-PT-048 TABLE OF CONTENTS FORWORD . vi l. PURPOSE . . l 2. SCOPE . 3 3. DEFINITIONS . 4 4. OVERVIEW OF RBDA METHODOLOGY 7 4.1. Implementation Steps 7 4.2. Reliability and
10、Failure Probability 8 4.3. Failure Probability versus Failure Rate . 9 4.4. Time Dependence and Effect of Maintenance I 0 5. LIMIT STATES . l2 5.1. Limit State Categories . 12 5.2. Applicable Limit States . 13 6. RELIABILITY TARGETS . . 17 6.1. General 17 6.2. Ultimate Limit State Targets . 20 6.3.
11、Leakage Limit States . 38 6.4. Serviceability Limit States 40 6.5. Operational Issues . 41 7. DEVELOPING A LIMlT STATE FUNCTION 42 8. PRO BALIS TIC CHARACTERIZA TlON OF INPUT VARIABLES . 43 9. RELIABILITY ESTIMATION 44 10. IMPLICATIONS OF USING THE APPENDIX . 45 10.1. Design of New Pipelines .45 10.
12、2. Maintenance of Operating Pipelines .47 11. EXAMPLEAPPLICATIONS . . 51 11 .1. New Pipeline Design . 51 11.2. Class Upgrade Deferral . 54 12. REFERENCES . 58 Acknowledgments 60 lll STP-PT-048 Criteria for Reliability-Based Design and Assessment for ASME B31.8 Code LIST OF FIGURES Figure l - Steps I
13、nvolved in lmplemeting RBDA 7 Figure 2- Illustration of Load Effect and Resistance Distributions . 9 Figure 3- Illustration of Time Dependence and Effect of Maintenance on Reliablity II Figure 4 - Illustration of the Evaluation Length . 18 Figure 5 - Reliablity Targets for Ultimate Limit States . 23
14、 Figure 6-Risk as a Function of pPD3 for a Range of Design Cases . 25 Figure 7 - Example Population Density Plot 26 Figure 8-Relative Frequency of Unpopulated Areas Around Pipelines . 27 Figure 9 -A Possible Segmentation Scheme for the Example in Figure 7 28 Figure 10- Calculation of the Population
15、Density at a Point Along the Pipeline 29 Figure 11 - Ulustration of the Method of Calculating pi 30 Figure 12- Example Illustrating the Calculation of a Population Density Graph 31 Figure 13 - Ulustration of Distributed and Location-specific Limit States . 34 Figure 14- Ulustration of Location-speci
16、fic Limit States Around a Given Point 35 Figure 15- Reported Defect Locations and Governing Evaluation Lengths . 36 Figure 16 - Calculated Equivalent Rupture Reliability for the General Reliability Check 37 Figure 17 - Calculated Equivalent Rupture Reliability for the Location-specific Reliability C
17、hecks. 38 Figure 18- Peak Small Leak Rates for the Design Cases as a Function of Wall Thickness 40 Figure 19 - Design Factor Comparison Between for RBDA and ASME B31.8 46 Figure 20- Cost Comparison Between RBDA and ASME B3L8 Designs . 47 Figure 21 -Comparison between Failure Rates for RBDA and Curre
18、nt Practice 49 Figure 22 - Calculated ULS Reliability versus Target for Segment B . 53 Figure 23 -Calculated LLS Reliability versus Target for Segment B . 53 Figure 24- LLS Reliability Compared to Target for Status Quo . 55 Figure 25 - LLS Reliability Compared to Target for Status Quo . 56 Figure 26
19、- ULS Reliability Compared to Target for Various Class Upgrade Options . 57 IV Criteria for Reliability-Based Design and Assessment for ASME B31.8 Code STP-PT-048 LIST OF TABLES Table l -List of Applicable Limjt States 51 Table 2-Population Density and Reliability Targets for Each Pipeline Segment .
20、 51 Table 3 -Equipment Impact Prevention Measures Assumed for Design Example 52 Table 4-Wall Thickness and Equivalent Design Factors 54 Table 5-Basic and Enhanced Failure Prevention Measures for Equipment lmpact. . 55 v STP-PT-048 Criteria for Reliability-Based Design and Assessment for ASME B31.8 C
21、ode FORWORD This Criteria Document provides guidance to potential users of the proposed ASME Appendix B3 I .8R on Reliability Based Design and Assessment (RBDA) by documenting the relevant background information required to fully understand the requirements of the Appendix and to apply them correctl
22、y in decision making. The need for a Criteria Document was identified during the process of voting on ASME B31.8 Ballot No. 08-905 as a requirement for further consideration of the RBDA Appendix. Established in 1880, the American Society of Mechanical Engineers (ASME) is a professional not-for profi
23、t organization with more than 127,000 members promoting the art, science and practice of mechanical and multidisciplinary engineering and allied sciences. ASME develops codes and standards that enhance public safety, and provides lifelong learning and technical exchange opportunities benefiting the
24、engineering and technology community. Visit www.asme.org for more information. The ASME Standards Technology, LLC (ASME ST-LLC) is a not-for-profit Limited Liability Company, with ASME as the sole member, formed in 2004 to carry out work related to newly commercialized technology. The ASME ST-LLC mi
25、ssion includes meeting the needs of industry and government by providing new standards-related products and services, which advance the application of emerging and newly commercialized science and technology and providing the research and technology development needed to establish and maintain the t
26、echnical relevance of codes and standards. Visit www.stllc.asme.org for more information. VI Criteria for Reliability-Based Design and Assessment for ASME B31.8 Code STP-PT-048 1. PURPOSE This Criteria Document provides guidance to potential users of the proposed ASME Appendix B31.8R on Reliability
27、Based Design and Assessment (RBDA) by documenting the relevant background information required to fully understand the requirements of the Appendix and to apply them conectly in decision making. The need for a Criteria Document was identified during the process of voting on ASME B31. 8 Ballot No. 08
28、-905 as a requirement for further consideration of the RBDA Appendix. The Appendix provides requirements for the application of reliability-based methods to the design and assessment of non-sour natural gas transmission pipelines. 1 The Appendix is non-mandatory; however, Section Rl.l in the Appendi
29、x states that “if an operator chooses to use the Appendix for designing and operating a pipeline, he must follow it until a different basis for pipeline operation is established with the regulator.“ The reason for this requirement is that the RBDA approach forming the basis for the Appendix permits
30、certain tradeoffs between initial design and planned maintenance (see Section 4.4). For example, the reliability targets may be met by using a thinner wall than would be required by the conventional design approach, combined with a more stringent integrity maintenance plan. The Appendix requires tha
31、t the maintenance plan used to justify the thinner wall be followed and documented to ensure that the reliability targets are met throughout the operational life. lt is therefore essential to review and establish a new comprehensive basis for continued operation in cases where thi s requirement is e
32、limill1ated by discontinued compliance with the Appendix. The Appendix states that “ reliability-based methods are particularly useful for pipelines involving large uncertainties . application of new materials and technologies, unique loading situations, and severe failure consequences.“ This statem
33、ent is based on two key features of the RBDA methodology: . RBDA is a rigorous methodology. While conventional design methods are mostly empirical, RBDA evaluates various design or operational choices from first principles. For example, the design factor used for wall thickness selection in conventi
34、onal standards is a single safety control parameter that is used to design against a combination of threats and is assigned a single value for a range of pipe properties (i.e., diameter, grade, pressure and class). The design factor has been validated through use over the past few decades and theref
35、ore its effectiveness is established for pipeline parameters that were commonly used during that period. However, it is not necessarily adequate for pipelines made of high strength steels for which little experience exists. By contrast, RBDA addr,esses individual threats based on the actual structur
36、al behaviour of the pipe as derived from basic pipe properties. For example, equipment impact resistance is evaluated from a model that compares the applied pressure to the pressure required to fail a gouged dent caused by an excavator hit. This model uses the actual pipe parameters, such as diamete
37、r, wall thickness and steel grade, and can therefore be applied to the entire range of properties for which it is validated (e.g., higher strength steels) without the need for proof based on prior use. The same logic applies for unique loading conditions such as geotechnical loads. 2. RBDA explicitl
38、y acknowledges uncertainty. Safety of possible design or operational alternatives is measured by reliability (l.O minus the failure probability). This measure explicitly incorporates the impact of uncertainty. A larger degree of uncertainty regarding pipeline behaviour or performance results in a lo
39、wer calculated level of reliability and a requirement to make more conservative decisions in order to ensure adequate reliability. As such, one of the built-in features of the RBDA methodology is the ability to reflect the degree of uncertainty in the decisions made. Other key benefits of the RBDA a
40、pproach include the ability to achieve consistent safety for all pipelines. This eliminates unnecessary conservatism in individual cases, allowing more effective use of resources to 1 The Appendix is not applicable to offshore gas transmission pipelines covered by Chapter VIII, or sour gas service c
41、overed by Chapter IX, of ASME Standard B31.8. 1 STP-PT-048 Criteria for Reliability-Based Design and Assessment for ASME B31.8 Code achieve better overall safety. The methodology also permits integration of design and operational decisions to develop more cost-effective overall solutions. The Append
42、ix in its entirety is explicitly applicable to onshore pipelines transporting non-sour lean natural gas. This statement is not intended to convey that any of the content is inapplicable to other types of pipelines, but rather that there are certain aspects of the document that are specific to non-so
43、ur lean natural gas pipelines. Specifically, “ the reliability targets in Section R1.6 are based on a model that evaluates the consequences of an ignited lean natural gas release at pressures consistent with the assumption of ideal ga.s behaviour.“ These targets should therefore not be used directly
44、 for other gas compositions or ultra-high pressures that may have significantly different release consequences than those of lean natural gas. For rich gas (depending on the particular composition), it may be possible to demonstrate that the underlying release consequence model just mentioned is app
45、licable, and in such cases, the targets can be applied directly. If the model does not apply directly, the Appendix may be used with case-specific reliability targets that meet the risk criteria underlying the Appendix. Such targets can be developed by adjusting the targets in the Appendix based on
46、the relative magnitude of the release consequences associated with the rich gas composition and/or ultra-high pressure (as calculated from a suitable model) and those calculated from the model underlying the Appendix for the same pipeline. Details of this process can be inferred from the original me
47、thodology used in developing the reliability targets in the Appendix (Nessim et al.) 1 , 2. It may also be possible to extend applicability of the Appendix to other fluids, such as sour gas, by making similar adjustments to the reliability targets, as long as the release consequences associated with
48、 these fluids are dominated by human safety considerations. Apart from the reliability targets and the specific procedure used in demonstrating compliance with them, much of the content of the Appendix is applicable to a wide variety of pipelines. This includes all requirements and other information
49、 related to the calculation of reliability with respect to different integrity threats. Users are advised “to consult the Commentary and the reference material that support the provisions of this Appendix to ensure that the parameters to be used in the design are within the range of applicability of the consequence models used for reliability target calibration.“ The targets were developed based on a safety benchmark that was calculated from a set of pipeline designs represented by different combinations of
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