Conclusions
A review of design factors in American and major pipeline standards and codes from other countries was conducted. In addition, recent on going and planned changes in design margins in codes covering pressure vessels, boilers and piping have been examined and assessed.
Based on this review it has been concluded that it may be possible to improve the design factors used in B31.8 without reducing the historical safety built into the pipeline design. Design factors have been used historically to address uncertainties such as in the design and operating loads, material manufacture, fabrication of components, examination, testing, analytical techniques, modes of failure, failure causes and other quality related factors.
The major design factors in the present B31.8 Code, which are applied against the Specified Minimum Yield Strength for the design of internal pressure, first appear in the 1935 American Standards Association Code for Pressure Piping, ASA B31.1, in the cross-country pipeline rules.
In the last 65 years major quality improvements have been made in all areas which have significantly reduced the uncertainties and the need for conservative design factors.
Consequently, changes in the design factors are appropriate at this time. This will lead to more economical operation of pipelines, better optimization of resources, and will address international competition for the American pipeline industry, while preserving and improving upon the same historical margins of safety and risk to the industry and the public.
Recommendations
It is recommended that the B31.8 Code Committee begin an in-depth study of the current design practices used for pipelines in relation to the improvements in materials, design and fabrication techniques that have been made over the past several decades. Such a study could provide the technical justification to revise the design factors as presented in Table 9.1. The ASME Boiler and Pressure Vessel Code Committee has undertaken such a task in the past few years resulting in an improvement in the design margins for their respective Codes (Upitis and Mokhtarian, 1996 and 1997).
Table 9.1 summarizes the design factors that are recommended for consideration and adoption in the B31.8 and U.S. Department of Transportation design rules. Appropriate changes in the design factors in other areas of the code can be made consistent with the above recommendations.
A comparison of existing and recommended design factors is presented in Figure 9.1. The resulting ratio increases in the design factors are illustrated in Figure 9.2. The increases in the design pressures for B31.8 range from 0% to 15% depending on the class location. For DOT rules the increases range from 6% to 15% depending on the class location.
In addition, it is recommended that the B31.8 Code Committee undertake a major effort to fully incorporate risk-based principles in the code so that pipeline companies, which are now using risk management for their pipeline operations, can optimize the pipeline designs and improve safety margins as well. A number of pipeline standards from other countries have incorporated some aspects of reliability or risk-based principles. These are referenced in Chapter 4 of this report. In particular the Canadian, Australian, British and Dutch standards have incorporated risk based principles which B31.8 should consider as a minimum. Presently, various ASME Code Committees are assessing development of risk-based design codes under the names of partial safety factors, limit state design etc. However, presently all on-going efforts are in reality reliability based using concepts introduced in Chapters 7 and 8. They are similar to the AISC LRDF approach, which address only half of the risk term, namely the probability of failure or its complement, reliability. Some codes try to address consequences using various categories or classes to differentiate some requirements
It is also recommended that B31.8 take a leadership role towards developing a fully risk-based design approach where both the probability of failure and the consequence are treated with the same level of importance and mathematical rigor. Such an approach will lead to improved and consistent safety in pipelines, increased maximum allowable operating pressures, provide more economical designs and operations and overcome the limits imposed by the present single design factor approach where all uncertainties are combined into a conservative single design factor. In addition, it will bring back to B31.8 its recognized leadership in its international use by having the most advanced, sophisticated and economical design rules. The historical leadership of B31.8 is clearly evident in other foreign standards, which are based on past B31.8 design philosophy and rules. The incorporation of different design factors as a function of location class by B31.8 (a forerunner to reliability concepts) has influenced foreign codes to incorporate reliability or risk-based concepts.
In order to have the safest, best pipeline operations in the world, the B31.8 Code must make the best technical methods and the best design codes available to pipeline operators.
Table 9.1 Recommended Design Factors
CLASS LOCATION EXISTING B31.8 DESIGN
FACTOR
RECOMMENDED DESIGN FACTOR
Class 1, Division 1 0.80 0.80
Class 1, Division 2 0.72 0.76
Class 2 0.60 0.68
Class 3 0.50 0.57
Class 4 0.40 0.46
Figure 9.1 Comparison of Design Factors
DESIGN FACT OR F
0.8
0.72
0.6
0.5
0.4
0.72 0.72
0.6
0.5
0.4 0.8
0.76
0.68
0.57
0.46
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Class 1, Div ision 1 Class 1, Div ision 2 Class 2 Class 3 Class 4
Location Class
Design Factor F
B31.8 DO T
RECO M M ENDED
Figure 9.2 Recommended Design Pressure Increases
Ratio of Recommended Design Factor to Existing Design Factor
1.00
1.06
1.13 1.14 1.15
1.11
1.06
1.13 1.14 1.15
0.9 0.95 1 1.05 1.1 1.15 1.2
Class 1, Division 1 Class 1, Division 2 Class 2 Class 3 Class 4
Location Class ( R
FROM B31.8 FROM DOT