PREDICTING FAILURE CONDITIONS OF SMAW GIRTH WELDED X70 PIPELINES SUBJECTED TO SOIL MOVEMENT

Pablo Fazzini 1, José Luis Otegui 2, Hernán Kunert 3

1. P.E., Failure analyst, GIE S.A fazzini@giemdp.com.ar

2. Ph.D. Professor, University of Mar del Plata, Argentina, jotegui@fi.mdp.edu.ar 3. M.Sc. Professor, University of Mar del Plata, Argentina, hgkunert@fi.mdp.edu.ar

Key words: pipeline, failure, landslides, vibrating wire strain gauges, soil – pipe interaction 1.- INTRODUCTION This paper discusses the results of fractographic and metallographic characterization and mechanical modeling of circumferentially welded API 5L X70 pipelines, failed under conditions of longitudinal traction and internal pressure at moderate to high room temperatures. Cases investigated include failures at heat affected zone of girth welds, in pipelines built in the previous few months:

1. Three failures in a 14“ diameter, 6.5 to 9.5 mm thick Natural Gas Liquids (NGL) pipeline in the Amazon basin. [1, 2, 3].

2. Failure in a 20”, 8.7 mm thick natural gas pipeline in high mountain: during rerouting maneuvers collapse of the pipe took place by longitudinal stretching. [4]

3. A circumferential leak at a 30“, 9 mm thick natural gas pipeline in a tropical, dry region, where the pipeline crossed a geotechnical fault. [5]

All cases are related to pipelines buried in unstable soils, see for instance Figure 1. Girth welds were all done with a SMAW procedure utilizing cellulosic consumables for the root, fill and cap. This is a typical procedure, even for today´s higher strength API pipe grades [6]. API 5L X70 pipes are commonly chosen when transport is an issue, as is the case of four of cases 1 and 2, due to their lower weight per unit length.

Fig. 1 As strength increases, it is increasingly more difficult to maintain the capacity of the pipeline to sustain large displacements. This is not only due to the lower capacity for strain hardening as

compared with lower grades, but also to the thinner pipes being installed. There seems to be a trend between the bending strength of the pipeline (which goes with the square of diameter and the thickness) and its ability to sustain longitudinal and lateral forces due to land movements in unstable grounds [6]. These and other similar failures prompted thorough Root Cause Failure Analyses (RCA), which results are briefly presented in what follows. Finally, tools for a risk management methodology are presented, which include monitoring soil stability and early detection of axial strains and numerical pipe-soil interaction studies utilizing nonlinear FEM methods. RCA is a joint effort by experts from the pipeline owners and specialists, which discusses all root causes of the failures that involve cracks in the heat affected zone (HAZ) of circumferential (girth) welds. The investigation of these failures is particularly relevant because they have similar patterns of initiation and occurrence. A total of 21 possible threats to the integrity of buried pipelines are mentioned in ASME 31.8S [14]. Note however that many of them are related to aged pipelines, in which service degradation over the years have an important role in life expectancy, reliability and structural integrity. Pipelines studied here are all relatively new, they failed well within the first part of the so-called “bath tub” reliability curve. These failures are related to the uncertainties in designing systems that cross land with many features mostly unknown. Those unknowns that most affected these lines are related to the stability of the soil, once the upper layers are dag and the containing layers of original vegetation are destroyed and attempted to be recovered by geotechnical restoration (Fig. 1). The purpose of these analyses was to identify which of these have been responsible for the failures under investigation. Proximate Cause(s) are the event(s) that occurred, including any condition(s) that existed immediately before the undesired outcome, directly resulted in its occurrence and, if eliminated or modified, would have prevented the undesired outcome. They are also known as the direct causes. Root Cause(s) are each one of multiple factors (events, conditions or exceeded barriers) that contributed to or created the proximate cause and subsequent undesired outcome. Typically multiple root causes contribute to an undesired outcome. The Root Cause Analysis (RCA) is a structured evaluation method that identifies the root causes for an undesired outcome and the actions adequate to prevent recurrence. Analyses include three main activities:

1. Review of the construction stage conditions and review of geological and geotechnical information in both failure sites.

2. Debate over the metallurgical tests and results and other information already available. 3. Performance of additional numerical models and mechanical tests; fracture mechanics,

small scale uniaxial and full scale biaxial tests on welded pipes identical to those failed. 2.- ROOT CAUSE ANALYSES OF FAILURES Experimental determinations and mechanical models were carried out to determine the failure mechanisms and the origin of the problems, and then possible mitigation methods. Materials, manufacture, transport, weld and post weld NDT quality issues were reviewed. Results eliminated material quality problems or material non conformity with standards as possible causes for the failures. Except for case 3, the verification of the quality of the x-rays allowed eliminating the possibility of previous defects (during manufacture, transport and weld of the tubes) which could be left undetected during post weld non-destructive testing. The inert conditions of the transported fluid and the lack of evidence of any previous defect on both inner and outer pipe surfaces eliminated previous damage due to corrosion as a possible cause for the failures. Potential cyclic loading that could have cause fatigue damage from hydrodynamic loads, cyclic pressure and thermal expansion was analyzed. The quantity and size of pressure fluctuations are too small to explain these circumferential cracks by mechanical fatigue.

Case 1 is related with three failures in a 14“, 6.5 to 9.5 mm thick NGL pipeline in the Amazon basin. In one incident the crack resulted in a circumferential through-wall leak, in the other incidents the crack resulted in the complete separation of the NGL pipeline, which also moved longitudinally more than 100 mm after fracture splitting. Case 2 is related with a failure in a 20”, 8.7 mm thick natural gas pipeline in high mountain: during rerouting maneuvers collapse of the pipe took place by longitudinal stretching. A circumferential ductile crack propagated at the section being cut for rerouting. Case 3 is related with a circumferential leak at a 30“, 9 mm thick natural gas pipeline in a tropical, dry region, where the pipeline crossed a geotechnical fault. A crack initiated at a high-low weld defect, and propagated into an embrittled portion of the HAZ, during a period of reduced gas flow after a few months of service. Figure 2 shows a typical crack propagating at the HAZ of the circumferential field weld, corresponds to case 3 but represents all cases under study. Secondary cracks are typical of API steels subjected to widespread yielding. As is common in these pipes made with a controlled thermal mechanical milling process, secondary cracks resulted from microstructural banding, see Figure 3 (a, b,c), with a particular sensitivity to a main secondary crack at mid thickness.

Fig. 2

Fig. 3 (a,b,c)

Geotechnical instability in all cases gave rise to a number of failures by crack propagation along Heat Affected Zone of (in-field) girth welds. Failure analyses defined longitudinal loads due to shallow ground movements, which is designated a geotechnical condition, as for the final fracture events. Soil loading finally induced the overload failures of the pipelines. Steadily increasing strains driving staged crack growth were induced by soil movement. In cases 2 and 3, steep slopes were the main contributor to this strain. In cases 1, during rainy seasons (4000 mm rain in a 4 month period) soil was saturated with ground water, which also produced relatively long spans of lack of support due to soil wash out under the pipe. Figure 4 shows polished and etched sections of different crack initiation sites. Both fractured sides were carefully matched to show the crack path and its interaction with microstructural features. In most failures there were no preexisting defects, the material underwent plastic deformations until exhausting its ductility, in the section of the ZAC of the circumferential weld between the curve and the replaced section. Ductile fracture propagation was always in accordance with the high toughness and low transition temperature of the material, and involved important local thickness reduction due to plasticity. Local inhomogeities o discontinuities helped locate crack initiation in each case. A relatively severe high – low defect associated with a hard spot in the weld metal – HAZ interface helped initiate the crack in Case 3, see Figure 5.

Fig. 4

Fig. 5

The problems associated with the operation of these pipelines are related not only to internal pressure, but also with the pipeline stability against horizontal or vertical displacement. In case 1, heavy rains (4000 to 7000 mm each year in the 4-month rainy season) create water flows along and across the pipes in the failed sections under study. In one incident in case 1, cracks resulted in a circumferential through-wall leak of about 10 inches in length. In the other two, the crack resulted in the complete separation of the NGL pipeline, which also moved longitudinally about 160 mm after fracture splitting. The 14” pipeline was constructed simultaneously at several “mini-spreads” along the Right Of Way (ROW). The ROW itself was the only available route for transporting pipes to construction sites, This 25 meter wide ROW was contained in a 3-km wide, government mandated corridor; to minimize environmental impact the pipeline was preferentially built along mountain ridges. Mitigation measures were constructed during and following installation of the pipelines, to control surface water runoff and stabilize the ROW. Girth-welded pipe sections were lowered into the trench and welded together to form an even longer pipe section. This is a standard methodology, which was replaced by special construction methods in very steep terrain, at river crossings, and at locations where the pipeline was laid along an existing road. These methods are consistent with general pipeline construction practice. Pressure, third party damage, seismic and geologic conditions were assessed in all case studies. The studies eliminated the probability of third party damage, while seismic and geologic effects at the time of the appearance of the leaks were involved in cases 2 and 3. As for case 1, studies defined longitudinal loads due to shallow ground movements, which is designated a geotechnical condition, as the cause for both crack initiation and final fracture events.

Steadily increasing strains driving staged crack growth were induced by soil movement in case 1, During rainy seasons soil is saturated with ground water, which also produced relatively long spans of lack of support due to soil wash out under the pipe.

3.- PREDICTING FAILURE CONDITIONS FOR PIPES A relatively thin-walled pipeline may be subjected to global bending as a result of soil movement or washout. But locally, weld discontinuities in the circumferential direction will primarily see tensile loading. All three failures in case 1 have a particular staged propagation path, which was the matter of much study and discussion, see for example Figure 6. Due to the high humidity of the region, the crack surfaces were always too corroded to allow a good

fractographic study. It is now believed that this characteristic is related to a very low strain rate during pipe elongation by slow landslides. Attempts to replicate experimentally this behavior showed a mostly ductile crack propagation by micro void coalescence, but with small regions of fast fracture. This is depicted in Figure 7. Another issue associated with this particular behavior is the large gradient in mechanical properties along the HAZ material. Typically, the coarse grained HAZ near the outer surface of the pipe is less tough than the rest of the HAZ, that underwent recristallization by successive weld passes. This was experimentally validated in fracture mechanics specimens, in which crack growth was much larger in that region of HAZ, see for example Figure 8.

Fig. 6

Fig. 7

Fig. 8

In response to these failures, small-scale and full-scale tensile strain tests were carried out, including biaxial tension on pressurized welded pipes [7, 8]. High strain rates promote failure in base metal, while fracture in HAZ (as seen in the failures) occurs at low strain rates (e.g., slow tensioning of the pipe). High toughness was confirmed in weld, HAZ and base metal, a typical X70 pipe steel as defined in Chen & Lambert analyses, [15, 16]. A low toughness surface layer was detected at the weld toe, where cracks actually initiated in Failures at case 1. Small-scale uniaxial tensile and bending tests under low and high strain and cyclic loads showed that uniaxial stress did not systematically produce through the thickness cracking- Most failures were ductile collapse in the HAZ and base metals. Not sufficient stress biaxilality was obtained, even in tests with severe lateral notches. 100 stress cycles did not initiate any fatigue cracks. Figure 9 shows a full sized specimen being tested under pressure and axial loading. A total of five uniaxial (tension) and biaxial (tension plus internal pressure) full-scale tests were carried out in both pipe thicknesses involved in case 1 Failures. Uniaxial tension of a pipe up to 2% plastic deformation was not able to produce any cracking. Experimental evidence indicates it could be related to the different properties of the coarse-grained HAZ and base materials. Results are conclusive, in that no HAZ cracking from the weld toe was possible unless the pipe was under pressure, and after a global plastic deformation of around 3%. As a result of this study, it is shown that welded pipelines have a very ductile behavior, Laboratory testing was able to duplicate through the thickness cracks from outer weld toes and a staged cracking mechanism seen in some of the failures. The mechanism of this staged cracking is still matter of speculation. It is now believed that staged crack growth, as seen in some of the analyzed failures, is only possible under conditions of very slow strain rates. This is due to the different behaviors of coarse grain HAZ and the rest of the tube material.

Fig. 9

Small-scale uniaxial tensile and bending tests under low and high strain and cyclic loads showed that uniaxial stress did not systematically produce through the thickness cracking. Fracture mechanics material tests showed that the HAZ was softer than the base metal, depending on strain rate. High strain rates promote failure in base metal, while fracture in HAZ (as seen in the failures) occurs at low strain rates (e.g., slow tensioning of the pipe) [9, 10]. High toughness was confirmed in weld, HAZ and base metal (typical X70 pipe steel as defined in Chen & Lambert analysis). A low toughness surface layer was detected at the weld toe, where cracks actually initiated in most failures. 4.- ASSESSING FAILURE CONDITIONS IN FIELD The longitudinal stresses due to soil movement caused an appreciable reduction in the in-service integrity of the pipelines, even though the affected tracts did not have severe weld defects or degradation in Heat Affected Zone, above admissible by the fabrication code. These and other similar failures prompted a risk management methodology that includes risk prediction tools, monitoring soil stability (e.g. inclinometers), and early detection of axial strains in places with high risk of soil movement). Figure 10 illustrates the movements upon the pipeline generated by a soil situation such as that of Fig. 1. Figure 11 shows length measurements in that same failed section, in order to determine the maximum strain that the pipe sustained previous to failure.

Fig. 10

Fig. 11

Identified sites are being instrumented with vibrating wire strain gauges. In order to quantify the strains and stresses due to soil-pipe interaction, three sensors were placed at 120 degrees in the perimeter of each pipe section. Multiple sensors were located In high risk zones, which allowed drawing conclusions not only on strains but also on rigid body displacements of the pipe sections. Differences with industry standard strain limits were analyzed and checked against several engineering criteria, defining safe work ranges. Ground movements, and the transference of loads that those imply, play an important role in the integrity of the pipelines located in potentially unstable areas. Buried pipelines are structures that interact with the soil as a foundation. When they are implanted in unstable areas, they are put under additional efforts transmitted by the soil movement. These efforts are quantified, placing strain gauges in the wall of pipeline. These instruments measure the deformations experienced in a determined point and a certain direction along the pipeline, indirectly obtaining stresses induced in that point. Most suitable are the Vibrating Strain Gages, due to the stability of the readings in the long term. A model of the movement of the ground is necessary, before the instrumentation of the critical areas. Then, a criterion to limit the acceptable deformation is needed, followed by a plan of interventions to reduce or to eliminate the deformation in the sectors in which threshold is surpassed. As an example, Figure 12 (a and b) shows different sites that were instrumented, the VWSG device (d) and the Sensor (c). As an example, Figure 13 shows the results of axial strains in a section of a pipeline buried in the Amazonian jungle. All three gauges arranged in a typical 120° configuration along the pipe circumference steadily increase axial strain readings (lines with dots) as rain flow (lower line in Fig. 12) also increases (discontinued line). Note that a single annual rainy season of around 2500 mm accumulated rain results in this case in a 500 microstrain increase in axial strain. At this rate, yielding would be reached after only 4 years service.

Fig 12 (a, b, c, d)

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5.- MODELLING SOIL – PIPE INTERACTION A comprehensive pipe-soil interaction study utilizing nonlinear FEM methods with proper modeling of pipe and weld resistance is being developed. This modeling includes criteria to be used for acceptable corrosion and external damage of pipes that are subjected to external soil loading. Figure 14 shows a lay out of the model, for the case of a right of way involving two pipelines, one 14” diameter, alongside the other, 32” diameter.

Fig. 13

Some characteristics of the finite elements model are:

• Model of multiple materials, steel pipes, mud elements and rock.

• Allows the definition of behavior of metals, rocks and mud, according to geotechnical measurements

• Allows defining conditions of contact mud – pipe. Parameters involved in the geotechnical characterization of the model include:

Geometry of mesh: • Length: between fixed points: streams or low pints, upper points at hill tops • Width: Right of way, or between pipelines, typically 20 m. • Depth: According to evidence of ground tests, until the rock including sliding planes.

Characterization of the ground and discretization of soil layers: • According to experimental data, from top to bottom of model. • Upper soil material is typically from old mud flow: more permeable, gravel in a muddy

matrix. The pipelines are located in this layer. • Muddy layer without gravel (residual deposits). • Muddy layer with gravel of greater hydraulic conductivity. • Transition layer between residual deposits and rock base. This material can experience

sliding differentials. • Rock, fixed edge.

The pipeline itself is modeled with pipe (or elbow) elements. Figure 15 shows an example of a kilometer - long stretch of pipeline being modeled between two fixed points.The ground behavior and soil-pipe interaction are modeled with the pipe-soil interaction elements. These elements have only displacement degrees of freedom at their nodes. One side or edge of the element shares nodes with the underlying pipe, or elbow element that models the pipeline. The nodes on the other edge represent a far-field surface, such as the ground surface, and are used to prescribe the far-field ground motion via boundary conditions together with amplitude references as needed. PSI elements do not discretize the actual domain of the surrounding soil. The extent of the soil domain is reflected through the stiffness of the elements, which is defined by the constitutive model as

described later. The constitutive behavior for a pipe-soil interaction is defined by the force per unit length, or “stress,” at each point along the pipeline, caused by relative displacement or “strain,” between that point and the point on the far-field surface.

Fig. 15

Conditions of drainage:

• State of ground saturation is important for slope stability

• Conditions based on seasonal conditions and works of drainage.

• Displacement differentials by layers: sliding of the lower layer would drag the upper ones Boundary conditions:

• Extreme fixed points

• External lateral displacements throughout both faces: induced by rotational sliding of the bordering slopes that affect on the produced ones by weight own throughout the track.

Scenes to model:

• Initial condition up to failure: focuses in the simulation of what happened, in order to validate models and coefficients.

• Present situation: is oriented to assess the effectiveness of interventions already made, with the validation by means of inclinometers, strain gauges, etc.

• Future scenes: focuses in simulating actions to take, based on the previous results, using the model as a prediction tool.

ASCE formulae, which are defined in the ASCE Guidelines for the Seismic Design of Oil and Gas Pipeline Systems, are provided for specifying constitutive behavior. The force per unit length, relative deformation (also elastic and plastic components) in the element local system is available. Element nodal force (the force the element places on the pipeline nodes, in the global system) is also available. With this output variables is always possible to show the pipe behavior.

6.- DISCUSSION OF RESULTS AND CONCLUSIONS All pipelines presented in this paper failed within the first two years of having been laid out, under conditions of longitudinal traction and internal pressure at moderate to high room temperatures. Cases investigated include failures in NGL pipelines in the Amazon basin, a gas pipeline in high mountain, and a gas pipeline in a tropical, dry region. Girth welds were done with a SMAW procedure utilizing cellulosic consumables for the root, fill and cap. Geotechnical instability in all cases gave rise to a number of failures by crack propagation along Heat Affected Zone of (in-field) girth welds. Root cause analysis and small-scale and full-scale tests show that all failed pipelines have a very ductile behavior. Laboratory testing was able to duplicate through the thickness cracks from outer weld toes. The longitudinal stresses due to soil movement caused an appreciable reduction in the in-service integrity of the pipelines, even though the affected tracts did not have severe weld defects or degradation in Heat Affected Zone, above admissible by the fabrication code. A comprehensive understanding of the cracking process of these failures required a detailed root cause analysis. In all cases the pipe yielded before the crack initiated, this was due to longitudinal loads by land movement. Crack propagation was caused by large external forces generated by soil movement and loss of pipe support. Progressive soil loading was identified as the likely substantial factor leading to crack propagation and inducing an overload failure of the pipelines. Incremental, stable tearing explains the type of staged crack growth involved in cracks in case 1. Cracks grew in a ductile manner, mostly by micro void coalescence. Further increase of crack growth required further increase in stress, the crack eventually stops until a new increase of stress reactivates crack growth.

Having verified the hypothesis that soil movement could alone be capable of yielding the pipe

and crack initiation is important in the evaluation of residual risks, since the number of potential

failure sites with pre-existing cracks would be reduced to sites where the pipe has been

yielded due to soil movement. These and other similar failures prompted a risk management methodology that includes risk prediction tools, monitoring soil stability (e.g. inclinometers), and early detection of axial strains (vibrating strain gauges in places with high risk of soil movement. Low tolerance to land forces by pipelines as diameter and thickness are reduced is the main reason for these pipe integrity studies. The ability of the currently employed ILI inspection tools to reliably detect pre-existing circumferential cracks at a size that allows effective future risk mitigation is still too limited to be considered in the near future.

A most promising way to assure pipeline integrity in possibly geotechnical unstable regions is performing a comprehensive pipe-soil interaction study utilizing nonlinear FEM methods with proper modeling of pipe and weld resistance. This modeling should include criteria to be used for acceptable corrosion and external damage of pipes that are subjected to external soil loading. A description of the model, and a detailed review of the analysis is presented in the last part of the paper. This modeling includes criteria to be used for acceptable corrosion and external damage of pipes that are subjected to external soil loading.

7.- REFERENCES

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gasopipeline de 30”. T.D. Williamson Services LA., 2008. [6]. API 5L Standard, Seam Welded Pipelines, American Petroleum Institute, 2001. [7]. Report GIE 2402-03-06 Expert assessment and definition of root causes of failures in

Camisea 14” Pipeline [8]. Draft Report. Tensile Strain Investigation, C-Fer Technologies 2007 [9]. J. Booman, M. Dasso, J.L. Otegui Fallas en Tuberías Enterradas y Simuladas en

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diámetro resistentes al gas amargo, Universidad autónoma de México 2004. [11]. R. M. Flores. Estudio de propagación de fisuras en materiales dúctiles, Le Roux 2002 [12]. Report GIE 2601-12-06 Rview of sections 4 & 5, Exponent interim report 20/12/2006, failures

NGL Camisea. COGA, 2007. [13]. Exponent Inc.: Pipeline Integrity Analysis of the Camisea Transportation System ,May 2007

[14]. ASME B31.8S - Managing System Integrity of Gas Pipelines, American Society of Mechanical Engineers, 14-Jan-2005.

[15]. Chen Y, Lambert S. : Analysis of ductile tearing of pipeline-steel in single edge notch tension specimens. International Journal of Fracture, Vol. 124, 3-4, pp. 179-199, 2003

[16]. Chen Y, Lambert S.: Numerical modeling of ductile tearing for semi-elliptical surface cracks in wide plates. Int´l Journal Pressure Vessels and Piping . Vol. 82, 5 , pp 417-426, 2005