Co-Autor: Guillermo Carfi, Universidad Nacional Arturo Jauretche (UNAJ)

1. FAILURE CONDITIONS IN A PIPELINE

The failure of a buried pipeline (onshore or offshore) can be of two types: a leak or a rupture [1]. A rupture usually has much greater consequences. In the presence of a defect, the steel with which pipelines are constructed has two ways of failing:

• Plastic collapse of the remaining ligament, where the material is stressed until it reaches its yield strength. This is the criterion usually used in simplified design methods, where a Maximum Allowable Operating Pressure (MAOP) is defined, sufficiently lower than the material’s Specified Minimum Yield Strength (SMYS). This way, we account for the influence of defects known to be present in the material (especially in welds), when non-destructive testing (NDT) methods are not precise enough to accurately define the shape and size of the defects.
• Rapid propagation of a FRACTURE. This is how most in-service pipeline failures occur. The fracture (ductile or brittle) initiates from a stress concentrator generated by a defect. In this condition, the material is stressed until it reaches its TOUGHNESS. The usual way to measure it is through Charpy impact tests or, when greater precision is required, fracture mechanics tests (KIc, CTOD, J integral).

Fig. 1 Defect propagation leading to failure

The failure mode depends on the toughness and yield strength of the material, on the length (2c) and depth (a) of the defect, and on the applied stresses (internal pressure, external loads, temperature variations). In arc welding, strength and toughness may vary depending on whether it is base material, filler material, or heat-affected zone (HAZ). In addition, residual stresses and geometric stress concentrators introduced by the welding process must be considered in the analysis.

Fig. 2 Evaluation parameters: defect geometry, applied stresses, and material properties

A chain is only as strong as its weakest link. The three-link chain in Fig. 2 requires non-destructive testing (NDT) to allow defect detection and minimize the likelihood of underestimating defect size. Furthermore, it is necessary to ensure that the stresses used in the calculations do not compromise the results, in accordance with a proper definition of toughness and strength in the weakest areas of welded joints.

Partial Safety Factors will also be incorporated into these calculations for each element of the analysis (Fig. 2):

• The dimensions used for each evaluated defect (Fig. 1) consider the probability of underestimation, which depends on the NDT method used, and are affected by a factor >1.
• The material’s toughness and strength are the lowest determined from tests on base metal, weld, and HAZ, and are affected by a factor <1.
• The applied stresses correspond to the worst-case condition faced by each pipe section during installation and service.

The primary stresses in a pipeline are those due to internal pressure, and simplified calculation methods are based on the idealization of a perfect cylindrical pipe without external loads. In this case, Laplace’s equation reduces to Barlow’s Law for hoop stress:

Fig. 3 Stresses in a pipeline under internal pressure

Axial stresses are of two types: those due to longitudinal tension in the pipeline, and those due to bending. To calculate axial stress σm from tension, not only the pressure effect must be considered, but also the interaction with the soil in which the pipeline is buried. Near a free end (e.g., the cap during the hydrotest):

$$\sigma_{m} = \frac{\sigma_{t}}{2}$$

In most of the pipeline, buried in stable and reasonably rigid soil, axial displacement is null, so:

$$\sigma_{m} = \frac{\sigma_{t}}{3}$$

Finally, in cases of severe soil movement due to marine currents and tides in offshore pipelines, or during pipeline laying in the trench, bending loads appear on the pipeline. These loads generate axial stresses σm that vary greatly along the pipe perimeter and may reach values much higher than the stress components from internal pressure if the necessary precautions are not taken [2].

The propagation of a failure in a circumferential weld is controlled by axial stresses. During construction of buried onshore pipelines, the most relevant axial stresses are those due to bending of the line as it is laid into the trench, usually by the combined action of sideboom machines. During subsequent service, the most relevant axial stresses are, in decreasing order of importance: axial loads due to ground movement, internal pressure, external vertical loads, and variations in temperature and pressure.

2. QUALITY ASSURANCE FOR FIELD WELDS

Circumferential field welds are made under less-than-ideal conditions.

In the Oldelval Duplicar Plus Project, the joining of pipes in the new 525-kilometer oil pipeline between Allen and Puerto Rosales involved manual SMAW and semi-automatic welding, performed by 60 welders over 400 days. Field welding in Duplicar Plus was a complex and extensive process that used various methods to ensure the integrity of the new pipeline. Due to the tight schedule and strict quality requirements, automated welding processes were used for the Perito Francisco Pascasio Moreno Gas Pipeline (formerly Presidente Néstor Kirchner) and the Vaca Muerta Sur (VMOS) pipeline.

Repairing a circumferential weld in the field involves high costs and time. To maximize the accuracy in detecting and sizing potential discontinuities that could affect weld quality, while minimizing disruptions to the construction front, an automated ultrasonic testing method (AUS), also known as PAUT (Phased Array Ultrasonic Testing), is used.

The established acceptance standard for discontinuities detected using this technique is API 1104 [3], which in its Appendix A establishes the requirements for Engineering Critical Assessment (ECA). Similar procedures are defined by other standards as well, such as Annex J of the Canadian CSA Z662-15 standard [4].

ECA is defined as «an analytical procedure based on fracture mechanics principles that allows determining the maximum tolerable size of imperfections«. ECA is widely used in the pipeline industry due to its reliability and economic benefits when applied to fusion welds [5]. The most common defects in arc welds are treated as planar defects, which is particularly suitable for lack of fusion or penetration. Non-planar defects (slag inclusions or porosity) are also conservatively treated as planar defects.

The way ECA procedures define the acceptance limits of these assumed planar defects is through Failure Assessment Diagrams (FAD), a technique originally proposed by codes R6 and BSI 6493 [6], and first used in an analysis by GIE Group in 1995 [7].

In these diagrams, Fig. 4, the horizontal axis defines the proximity to plastic collapse failure, determined by the ratio Sr (stress ratio, also called load ratio Lr) between the equivalent stress (Von Mises, Tresca) and the yield strength or flow stress of the material (SMYS). The vertical axis defines the proximity to fracture failure, determined by the ratio Kr (K-ratio) between the applied effective stress intensity (KI) and the fracture toughness of the material (KIC). The analysis points of non-critical defects (those that DO NOT need to be repaired) fall into the SAFE zone.

Fig. 4 A FAD evaluates the predicted failure condition

The FAD shown in Fig. 4 is based on the original R6 document criteria, where the limit curve is the so-called log-sec, but there are other specific diagrams for each material and level of analysis. ECA is the basis for Fitness for Service (FFS) procedures for pipelines and pressure vessels where defects are found, standardized for example in API 579 [8] and BS7910 [9].

It is important to highlight:

• While collapse is related to axial and hoop stresses, fracture (in mode I, using fracture mechanics terminology) is governed by the stress normal to the defect (axial stress in our case).
• API 5L pipes (X60 and X70) used in the construction of modern pipelines have controlled chemical composition and thermo-mechanical treatment that provide high toughness, so analysis points are generally in the Sr > 0.7 zone, with ductile fracture mode (also called ductile tearing). Micromechanically, fracture propagates by microvoid coalescence, not cleavage.
• Both Sr and Kr depend on the dimensions of each analyzed defect, but the depth (a in Fig. 1) is the controlling parameter, and the hardest to measure with traditional NDT methods such as radiography.
• Real defects are often more complex than the semi-elliptical surface crack shown in Fig. 1; the dimensions a and 2c for each defect analyzed with the FAD come from a series of analyses, simplifications, and reclassifications.
• Embedded (internal) defects that do not reach the surface are less critical than surface ones. In automated field welds, the most frequent and largest defects are lack of fusion or penetration, mostly occurring on the pipe’s internal surface.
• Weld residual stresses are very important for brittle fracture conditions (Sr<0.4). When fracture mode is ductile, plastic deformations tend to redistribute residual stress fields, reducing their relevance.
• While the SMYS of the base pipe material is generally known with reasonable precision and conservatism, the fracture toughness KIC is usually not a required specification parameter and must be estimated through specific tests or conservatively inferred (via correlations with Charpy impact tests).
• In weld materials and especially in the HAZ, strength and toughness vary significantly depending on the position in the weld bead and welding parameters in that zone. If the corresponding tests were not conducted as part of material qualification (e.g., in the case of API 5L PS2 pipes), additional mechanical testing may be required. Recent developments in non-destructive estimation of these parameters (instrumented indentation testing) allow accurate mapping of strength and will soon enable toughness mapping as well.

3. CONCLUSIONS

The use of ECA procedures established in standards leads to conservative and safe results, as long as all the conditions under which they were developed are met. To keep these limits reliable, it must be ensured that:

• The quality of the pipes and welding procedures are not altered from those used in the evaluation.
• The precision and accuracy of the inspection method are maintained throughout the process.
• The applied axial stresses do not exceed the defined maximums.

The most severe loading condition on discontinuities in onshore field welds occurs during pipeline laying into the trench. The maximum axial stresses depend on lateral displacements, column height, and mainly on the number and position of sidebooms. ECA defines the maximum admissible dimensions based on the number of sidebooms used.

The maximum axial stress during laying is due to column bending and is highly dependent on sideboom position. Any deviation above the permissible parameters will cause an increase in axial stress beyond the limits defined in the ECA, with the aggravating factor that a defect propagated during laying would be hard to detect and costly to repair.

The yield strength SMYS and fracture toughness KIC of the materials where the analyzed defects are located (base metal, weld, and HAZ) must be estimated with reasonable precision and conservatism.

REFERENCES

1. Failure Analysis: Fundamentals and Applications in Mechanical Components. J.L. Otegui, Springer,2014, ISBN 978-3-319-03909-1.
2. Cañerías y Recipientes de Presión J.L Otegui,.E. Rubertis. EUDEM, 2012, ISBN 978-987-1371-96-9.
3. API Standard 1104 Welding of Pipelines and Related Facilities. American Petroleum Institute.
4. CSA Z662:19 Oil and gas pipeline systems. CSA Group.
5. S. Xu, W.R. Tyson, D.M. Duan: ECA of embedded flaws in pipeline girth welds–a review. Intl´ J. Press. Vess. & Piping, 172, Pp 79-89 (2019), https://doi.org/10.1016/j.ijpvp.2019.03.030
6. BSI 6493/91: Guidance on some methods for assessing the acceptability of flaws in fusion welded structures. British Standards Institution, 1991.
7. J.L. Otegui: Evaluación de estados tensionales y aptitud para el servicio del reactor de hidrogenación PBB R3202-3. Informe Técnico GIE-INTEMA, Mar del Plata, 1995.
8. API STD 579/ ASME FFS-1: Fitness For Service – American Petroleum Institute. Ed. 2021
9. BS 7910:2019: Guide to methods for assessing the acceptability of flaws in metallic structures. BSI Group.