This primer presents an overview of Codes that are relevant to pipeline design, and the resulting margins of safety (MoS). As the document descriptions in the adjacent image indicate, the term Code is used here in a generic context to represent not only the usual Codes, like those associated with the ASME or the IGEM, but also National Standards, Country and/or Regional Norms, and Federal Regulatory requirements.
The ASME has recently defined the term Code in reference to the term Standards. The ASME states that a Code is a standard that has been adopted by one or more governmental bodies and has the force of law. In contrast, a Standard is a set of technical definitions and guidelines written by experts – a “how to” set of instructions for designers and manufacturers. The ASME adds that Standards are voluntary as they are considered guidelines, lacking the force of law. In this context any standard incorporated by reference in a National or Regional mandate becomes a code specific to that application.
Pipelines like most other structures fail due to issues in one of three categories: 1) inadequate resistance to the design loadings and/or other conditions, 2) the inability to function and/or remain serviceable, and 3) instability issues. This segment is specific to issues with the resistance to the design loadings and other conditions, as failures in this category tend to be dominate among reportable pipeline incidents.
Working Stress Design
Pipelines in the US and much of the world are designed subject to the guidance of ASME B31 which makes use of Working Stress Design (WSD). The US Code of Federal Regulations adopts these same principals as do many other National Regulations and design Codes. However, not all structures or transport systems rely on WSD, which establishes minimal requirements for the resistance to loadings and other design circumstances. Several other approaches to design exist with many aspects well beyond what can be considered here regarding the choice of a design basis.
Some important factors affecting the choice of a design basis include:
- the consequences of a failure,
- the likely exposure in the event of a failure,
- the certainty of the loadings,
- the nature of the materials and fabrication/construction processes and
- the quality assurance/quality control available through inspection and measurement technologies.
Alternative Design Basis for Pipelines.
Other popular design basis for pipelines include:
- Limit states design (LSD);
- Ultimate stress design (USD);
- Strain-based design;
- Load and resistance factor design; and
- Reliability-based design.
For those not familiar with these concepts the Internet generally provides reliable guidance. Given that other approaches exist for design aside from WSD, it is not surprising that some countries or jurisdictions adopt the WSD approach used in the US Regulations – while others do not. For example, countries such as Canada, Russia, and Australia allow what are Reliability and Risk-based schemes, and forms of Limit-States approaches, and will tend to embrace specialized practices as needed or appropriate. Within a country, an alternative design basis might be adopted to address regional or jurisdictional differences for onshore pipelines and facilities, due to for example to climate or geology or state borders – or to account for the differences in the circumstances that exist for offshore versus onshore structures, and their construction.
Such alternatives are evident in the US and elsewhere. It is apparent for example in the guidance that is trending toward more stringent requirements for some of the States, or in the differences that exist for onshore pipelines versus those operating offshore. In this context there are a host of Codes, Regulations, and Standards involved in the pipeline industry worldwide.
The various design basis for pipelines can be discriminated by the material property that is used to quantify the required resistance, by the allowable level of that property that is accepted by that guidance, and for some the loadings that are involved.
Consider WSD (equally Elastic or Allowable Stress Design) as a benchmark against which to illustrate other design concepts. WSD makes use of a working or allowable stress, whose value is reduced relative to the specified minimum yield stress (SMYS) by a factor of safety (FoS) < 1, which in many pipeline Codes and Regulations is referred to as a design factor (DF):
- Thus, the allowable stress = FoS * yield stress,
where: FoS denotes a factor of safety whose value is < 1, which as noted above is termed the design factor in pipeline Codes with the yield stress usually being set at SMYS in Codes or Regulations.
In contrast, Ultimate Stress Design (USD), which is a form of LSD, the allowable resistance is defined similarly in regard to a specified minimum ultimate state:
- Thus, the allowable resistance = RF * ultimate state,
where RF denotes a resistance factor; whose function is analogous to the FoS such that its value is < 1,with the ultimate state often set at SMTS by Codes or Regulations.
The same format applies across the other design basis, with adaptations that are case specific. Values of the FoS and/or the RF are chosen to offset concern for uncertainty in the resistance property for the material, or by analogy for other approaches such factors reflect uncertainty in the loadings. That said, for some applications their values also are established in regard to the public and environmental consequences, which brings concern for the exposure, and the transported product into consideration.
Margin of Safety and Embedded Conservatism
The adjacent image illustrates the Margin of Safety (MoS) that develops for WSD for cases where the DF = 0.72. This DF is the least conservative of the factors mandated in the US Regulations – with the same being true for ASME B31.4 and B31.8, and many other Codes and Regulations. The MoS shown develops by way of such Pipeline Design requirements, which herein is defined relative to the design allowable stress and the stress at failure, as in a burst test.
So defined, the MoS is an aggregated metric that reflects the combined effects of all FoS relative to the state at which actual failure would occur. For purposes of illustration in the next paragraph the value of SMTS is arbitrarily taken as 1.2 times SMYS and the wall stress in a burst test at failure likewise has been taken as 1.4 times SMYS.
With these assumptions the above image indicates that adopting a maximum allowable stress (MAS) at 72% SMYS can be quite conservative. Relative to SMYS the ratio of MAS/SMYS is 1.39, whereas relative to SMTS as defined above the ratio of MAS/SMTS is 1.67. Finally, relative to actual failure in a hydrotest that is taken to occur at the actual ultimate tensile stress (UTS) considered to be 1.5x SMYS, the ratio of MAS/UTS is 1.94. As implied from the arithmetic that underlies these ratios, the MoS is proportionally larger relative to the ratio of design factors considered. For example, if a DF of 0.5 was adopted then the MAS is 50% SMYS, in which case the ratio of MAS/SMYS = 2.00, whereas the ratio of MAS/SMTS = 2.40, and the ratio of MAS/UTS = 2.80. It is also evident that the maximum hoop stress is well less than SMYS, such that a pipeline remains nominally elastic in service.
Viability of SMTS = 1.2xSMYS
As the above paragraph has made use of arbitrary values of SMTS (at 1.2xSMYS) and the UTS (at 1.4xSMYS) it is necessary to explore the viability of these arbitrary assumptions relative to 1) code-based trends between SMTS and SMYS and 2) empirical trends between the UTS and the actual yield stress (AYS).
The next image presents trends in SMTS versus SMYS referenced to API 5L  and/or ISO 3183 in contrast to the above-assumed value of 1.2, with SMTS plotted as a function of SMYS. The trend created by connecting successive values of the specified (SMTS,SMYS) data pairs is shown in this image as the heavy dashed trend. Also shown in this image is the one-to-one trend (i.e., SMTS = SMYS), which is shown as the short dashed straight line. This dashed straight line is paralleled by a second straight-line determined as SMYS = SMYS + 10 ksi (or 68.9 MPa), which is shown as a dot-dashed line. Finally, this figure shows a polynomial best-fit to the specified (SMTS,SMYS) data pairs from Gr A up through X80, which is shown as the dotted trend.
It is apparent from this image that SMTS for the higher-strength Grades can be characterized as SMTS = SMYS + 10 ksi (68.9 MPa) – which is akin to the definition of the flow stress used in Modified B31G. It is further apparent that the consistent trend between SMTS versus SMYS for the higher-strength Grades reflects the flow stress in Modified B31G . Finally inspection of the specified values of SMTS relative to SMYS indicates that the assumed value of SMTS = 1.2 is viable for X52 and below, but for higher grades this value is less appropriate. For example, the ratio of SMTS/SMYS decreases to 1.14 for X70, and 1.1 for X100.
Viability of UTS = 1.4xSMYS
Consider next the viability of the MoS quantified relative to failure, wherein the burst pressure at failure is quantified relative to the UTS taken as 1.4xSMYS. The adjacent figure trends the flow response typical of pipe steels for Grades from Gr A25 up through X100. Values of the UTS / AYS from this figure provide insight into the ratio of the failure stress in a burst test relative to SMYS. The y-axis in this image is the value of the engineering stress normalized by the 0.2% AYS, which is plotted as a function of the corresponding plastic strain on the x-axis.
Inspection of this plot indicates that the vintage production led to higher hardening rates and correspondingly higher values of the UTS. This is evident in comparing those trends specific to Gr B. For the vintage production the ratio of UTS/AYS is about 1.60 whereas for the modern production the value of this ratio is about 1.25. This difference is somewhat reduced in comparing the results for the vintage and modern production of X52.
In regard to the value of the UTS earlier taken at 1.4xSMYS to illustrate the conservatism embedded in the design basis for pipelines it is evident from this plot that such a value is viable for the vintage grades X52 and below, but is less relevant for the modern production as the Grade increases. The form of this plot is also instructive regarding other resistance metrics that are used in line-pipe applications. For example this plot indicates that the uniform strain – which is relevant in strain-based design – decreases as the Grade increases. So also does the total strain to failure, and the strain hardening exponent.
Summary – and a Question that Begs to be Asked
It is clear from this discussion that WSD as prescriptively implemented in Codes like ASME B31.4 and B31.8 embeds a significant margin of safety, which in practice for some steels reduces the maximum allowable stress to much less than one half of the failure stress. It is equally apparent that the extent of this conservatism diminishes as Grade has increased, and as quality controls on production have been implemented. In this context care must be taken when implementing procedures and processes developed relative to production practices adopted in past decades.
What is less obvious is that the MoS is specific to the design basis adopted. While their intent is the same – a safe serviceable pipeline – the factors that aggregate into the MoS differ, so the MoS as discussed herein is unique to WSD as prescribed in Codes like ASME B31.4 and B31.8.
Recognizing that the lowest margin evident in the above discussion is 39% above the maximum allowable stress, which ensures that the pipeline remains nominally elastic at stresses well below the minimum strength specified the question begs to be asked – why and how do pipelines fail? The discussion in the Materials and Construction (M&C) Defects Chapter provides some insight into this based on the presence of preexisting defects. It also makes clear that fracture can intervene at stresses that remain elastic, and implies that this can occur at stresses less than the MAS. It follows that while Codes, Standards, and Regulations provide conservative strength, pipelines can fail for reasons unrelated to strength. Care also must be taken in this context to specify for weldability in balance with toughness, and provide layers of protection against corrosion-related mechanisms, mechanical damage, and other causes of failure.
- Zhu, X.K. and Leis, B.N., “Average Shear Stress Yield Criterion and its Application to Plastic Collapse of Pipelines,” International Journal of Pressure Vessels and Piping, Vol. 83, 2006, pp. 663-671.
- Anon., “Specification for Line Pipe,” API 5L, 45th Edition, 2012.
- Kiefner, J. F. and Vieth, P. H., “A Modified Criterion for Evaluating the Remaining Strength of Corroded Pipe,” Final Report on Project PR 3-805 to the Pipeline Research Committee of the American Gas Association, December 22, 1989.
Reality in this context is that failure stress in a pipeline is a function of at least the UTS and the strain-hardening exponent (n) . But, within any comparative circumstance the role of n self-cancels, such that this dependence on n can be ignored in relative comparisons. ↑