Update of Understanding of Near-Neutral pH SCC Crack Growth Mechanisms and Development of Pipe-Online Software for Pipeline Integrity Management

2016 ◽  
Author(s):  
Weixing Chen ◽  
Jiaxi Zhao ◽  
Jenny Been ◽  
Karina Chevil ◽  
Greg Van Boven ◽  
...  
Keyword(s):  
Service Life ◽  
Crack Initiation ◽  
Crack Growth ◽  
Oil And Gas ◽  
Stage Ii ◽  
Variable Pressure ◽  
Pipe Surface ◽  
Neutral Ph ◽  
Wide Range ◽  
Online Software

This paper is aimed to introduce Pipe-Online Software that has been developed recently for crack growth and remaining service life prediction for pipelines experiencing near-neutral pH stress corrosion cracking and corrosion fatigue. The software was developed based on the latest understanding of the physical, chemical and mechanical processes involved during crack initiation, early crack growth and coalescence, and stage II crack growth. In each stage of cracking, governing equations were established based on extensive experimental simulations under realistic conditions found during pipeline operation in the field and vast amounts of field data collected, which include pipeline steel properties, crack geometries, field environmental conditions, Supervisory Control and Data Acquisition (SCADA) data of oil and gas pipelines. The model has considered a wide range of conditions that could lead to the crack initiation, crack dormancy and crack transition from a dormant state to active growth. It is concluded that the premature rupture caused by stress cracking at a service life of about 20–30 years commonly found during field operation could take place only when all the worst conditions responsible for crack initiation and growth have been realized concurrently at the site of rupture. This also explains the reason why over 95% of near-neutral pH cracks remain harmless, while about 1% of them become a threat to the integrity of pipeline steels. It has been found that crack initiation and early stage crack growth are primarily caused by the direct dissolution of steels at constrained areas. The rate of dissolution can be high at the pipe surface because of various galvanic effects, but decreases to a low value as the cracks approach a depth of ∼ 1.0 mm, leading to a state of dormancy as generally observed in the field. In stage II crack growth, the software has considered loading interactions occurring during oil and gas pipeline operations with underload-type variable pressure fluctuations. The software has provided predicted lifetimes that are comparable to the actual service lives found in the field. This forms a sharp contrast with the predictions made by existing methods that are generally conservative or inconsistent with the field observations.

Stress Corrosion Crack Initiation in X-65 Pipeline Steel Under Disbonded Coating With Cathodic Protection

2008 ◽  
Author(s):  
Bingyan Fang ◽  
Abdoulmajid Eslami ◽  
Richard Kania ◽  
Robert Worthingham ◽  
Jenny Been ◽  
...  
Keyword(s):  
Crack Initiation ◽  
Cathodic Protection ◽  
Pipeline Steel ◽  
Seasonal Fluctuation ◽  
General Corrosion ◽  
Cathodic Current ◽  
Pipe Surface ◽  
Neutral Ph ◽  
Wide Range ◽  
Disbonded Coating

A novel testing setup has been used in this study to simulate crack initiation in X65 pipeline steel exposed to near-neutral pH soil environment. This test setup was designed to simulate synergistic interactions of cathodic current with soil environments underneath the disbonded coating on the pipe surface. It was found from the simulations that the local environment underneath the disbonded coating can be very acidic or alkaline, instead of near-neutral pH as commonly believed, depending on seasonal fluctuation in CO2 level and cathodic current. There exists a wide range of corrosion conditions on the steel surface up the gradient of cathodic current underneath the disbonded coating. General corrosion was found to increase as CP current diminishes. Pitting corrosion in terms of number of pits and size of pits was found to be the most severe at locations where cathodic protection was nearly diminished. These locations had also developed some crack like-defects, which were usually elongated in a direction perpendicular to the loading axis and appeared to be formed from a linkage of neighboring pits and by enhanced corrosion at stress raisers.

Retarding Crack Growth by Static Pressure Hold for Pipeline Steel Exposed to a Near-Neutral pH Environment

2016 ◽  
Author(s):  
Mengshan Yu ◽  
Weixing Chen ◽  
Karina Chevil ◽  
Greg Van Boven ◽  
Jenny Been
Keyword(s):  
Growth Rate ◽  
Cyclic Loading ◽  
Crack Growth Rate ◽  
Crack Initiation ◽  
Crack Growth ◽  
Growth Curve ◽  
Pipeline Steel ◽  
Early Stage ◽  
Pipe Surface ◽  
Neutral Ph

From extensive investigations for over 30 years since the discovery of near-neutral pH stress corrosion cracking (NNpHSCC), the physical processes of crack initiation and growth have been determined, despite that some details in various aspects of crack initiation and growth are still to be understood. The growth curve is a function of crack growth by direct dissolution of steels at localized areas on pipe surface during initiation or at the tip of a crack during early stage of crack growth (the dissolution growth curve), and by a process involving the interaction of fatigue and hydrogen embrittlement (corrosion fatigue, the hydrogen enhanced fatigue growth curve) in Stage II after crack initiation and early stage of crack growth. For the latter case, recent research shows that crack growth rate can be substantially enhanced by variable amplitude cyclic loading. One of the most severe scenarios of cyclic loading in terms of crack growth rate is the underload type of pressure fluctuations that is often found within 30 km downstream of a compressor station. This investigation is aimed to evaluate pressure scenarios that could reduce or retard crack growth during pipeline operation. Specifically, the effect of pressure holds was investigated. Different periods of static hold were performed to an X65 pipeline steel exposed to a near-neutral pH solution. It was found that a static hold at the maximum load for one hour immediately after a large depressurization-repressurization cycle (underload cycle) yielded the lowest crack growth rate, which was about one third of that of constant amplitude fatigue without the static hold. Static holds for a period shorter or longer than one hour have yielded higher crack growth rates. This observation can be applied to field pipelines during operations to retard crack propagation.

Application of Load Sequence to Control Crack Growth in Steel Pipelines Under Near Neutral pH SCC

2016 ◽  
Author(s):  
Olayinka Tehinse ◽  
Weixing Chen ◽  
Jenny Been ◽  
Karina Chevil ◽  
Sean Keane ◽  
...  
Keyword(s):  
Growth Rate ◽  
Crack Growth Rate ◽  
Crack Growth ◽  
Oil And Gas ◽  
Pressure Fluctuations ◽  
Operating Pressure ◽  
Variable Amplitude ◽  
Neutral Ph ◽  
Novel Approach ◽  
Load Sequence

Pipelines are designed to operate below a maximum operating pressure in service. However, there are pressure fluctuations during operation. The presence of pressure fluctuations creates a drive for crack growth in steel pipes. In order to prevent catastrophic failure of pipelines, there is need for better understanding of the contribution of pressure fluctuations to crack growth rate in steel pipelines. Analysis of pressure fluctuation data in oil and gas pipelines shows that there are different types of fluctuations in a pipe due to friction loss with distance from the pump or compressor station. All these fluctuation types show a form of variable amplitude loading classified in this research as underload, mean load and overload. Studies of some structural systems shows that underload can cause acceleration of crack growth while retardation of crack growth is observed after an overload. This research aims to apply pressure fluctuations to manage integrity of steel pipelines through a novel approach of load sequence involving underload and overload in near neutral pH environment. Clear knowledge of the effect of load interaction involving load sequence of underload and overload is vital to control crack growth in steel pipelines under near neutral pH environment. The result of crack growth rate under different load sequence on X65 steel indicate that increase in overload ratio of 2, 3 and 4 caused an increase in crack growth rate of 1.68E−3, 1.89E−3 and 2.31E−3 mm/block respectively. These results are compared with results from other tests under variable amplitude without load sequence. Analyses were carried out on the morphology of the crack tip and the fracture surface after the test.

Growth of Surface-Type Stress Corrosion Cracks in Near-Neutral pH Environments Under Disbonded Coatings

2010 ◽  
Author(s):  
Afolabi T. Egbewande ◽  
AbdoulMajid Eslami ◽  
Weixing Chen ◽  
Robert Worthingham ◽  
Richard Kania ◽  
...  
Keyword(s):  
Growth Rate ◽  
Crack Propagation ◽  
Stress Corrosion ◽  
Crack Growth ◽  
Oil And Gas ◽  
Preliminary Test ◽  
Surface Type ◽  
Operating Variables ◽  
Neutral Ph ◽  
Growth Crack

Near-neutral pH stress corrosion cracking (NNPHSCC), which occurs when ground water penetrates under the pipe coating, causes longitudinal cracks to develop on the surface of pipelines. Such cracks grow over time and can ultimately lead to pipeline failure. NNPHSCC is currently managed by in-line inspection or hydrostatic testing for oil and gas pipelines respectively. These procedures are enormously expensive and have to be repeated at predetermined intervals. Re-inspection intervals are currently determined by empirical models, which have been found rather imprecise. A major flaw in currently applied models is that they assume that once a NNPHSCC crack is formed, it grows at a constant rate that is independent of pipeline operating variables and both pre- and in-service history of the pipeline material. This is not necessarily true as pipeline history, the nature of the service environment and operating factors, among several other factors, have a strong influence on the rate of NNPHSCC crack propagation. Most existing models also treat NNPHSCC cracks as long through thickness cracks rather than surface type cracks typically observed in the field. This research proposes to provide an empirical model that more accurately predicts the growth rate of near-neutral pH SCC cracks in near-neutral pH environments by studying the growth rate of surface type flaws while also accounting for the influence of operating factors, environmental factors, coating disbondment and cathodic protection on the rate of crack propagation. This paper reports some preliminary test results obtained using a long specimen with three semi elliptical surface flaws located in three reduced sections to simulate field observed NNPHSCC cracks. Preliminary results suggest that: 1) crack grows much faster at the open mouth, which was attributed to hydrogen effects; 2) crack dormancy can occur under certain combined mechanical factors; 3) although the benign mechanical loading cannot lead to a direct crack growth (crack dormancy), it causes damage to the crack tip, which makes the crack more susceptible to crack growth upon a more aggressive condition is encountered.

Near-Neutral pH SCC Initiation and Early Propagation of X70 Pipeline Steel

2002 ◽  
Author(s):  
Baotong Lu ◽  
Jingli Luo ◽  
Brian McCrady
Keyword(s):  
Crack Initiation ◽  
Applied Stress ◽  
Stress Level ◽  
Pipeline Steel ◽  
Surface Condition ◽  
Test Duration ◽  
Pipe Surface ◽  
X70 Pipeline Steel ◽  
Neutral Ph ◽  
Dog Bone

The near-neutral pH SCC initiation and propagation behavior of X70 pipeline steel was investigated using dog-bone specimens with two different surface conditions, the original external pipe surface and a ground surface. The tests were conducted in NS4 solution de-aerated with N2/5%CO2, under low frequency cyclic load (v = 0.1Hz, R = σmin/σmax = 0.5). The fractography shows that the transgranular crack morphology of near-neutral pH SCC is reproduced under the current test condition. In line with the crack initiation mechanisms, the cracks observed are divided into two types, pit cracks and non-pit cracks. Non-pit crack initiation is found to be the dominative mechanism for the original external pipe surface but the contribution of pit cracks depends on the test duration. The test results indicate that the crack initiation rate increases and the crack initiation life decreases with increasing applied stress level. The crack propagation rate is essentially independent of the applied stress level. Finally, the effects of surface condition and non-metallic inclusions on the SCC mechanism of pipeline steels are discussed.

Some Factors Affecting Initiation of Stress Corrosion Cracking of an X80 Pipe Steel in Near-Neutral pH Environment

2018 ◽  
Author(s):  
Jidong Kang ◽  
Darren Bibby ◽  
James A. Gianetto ◽  
Mark Gesing ◽  
Muhammad Arafin
Keyword(s):  
Yield Strength ◽  
Stress Corrosion ◽  
Stress Corrosion Cracking ◽  
Oil And Gas ◽  
Pipe Steel ◽  
Critical Role ◽  
Corrosion Cracking ◽  
Pipe Surface ◽  
Near Surface ◽  
Neutral Ph

Near-neutral pH stress corrosion cracking (NNpHSCC) continues to be a concern for existing high pressure pipelines used to transport oil and gas in Canada. Although several studies have focused on the role of pipe steel microstructure on the initiation and growth of NNpHSCC, most used specimens machined from sub-surface locations that did not preserve the original pipe surface, which is the material that ultimately exposed. In the present work, a series of test specimens were designed to preserve the external pipe surface and allowed shallow 0.05 mm root radius surface notches with depths from 0.1, 0.2 and 0.3 mm to be machined and tested. All specimens were machined in the hoop (transverse) direction from a 1067 mm diameter, 12.5 mm thick X80 pipe. The specimens were subjected to a constant load of 95% of the specified minimum yield strength (SMYS) (equivalent to 80% of the actual pipe hoop yield strength) using proof rings for extended durations, e.g., 110, 220, 440 or 660 days. The results show that there was no apparent SCC developed on the smooth specimens with the original surface even after being tested for up to 660 days. In contrast, SCC were found to have initiated at the machined notches, irrespective of their depth after testing for 220 days. To provide further understanding of specimen design, the same SCC testing conditions were applied to smooth round-bar test specimens machined in the hoop direction of this same pipe close to the external surface and the mid-wall locations. While minor SCC initiation was found in the near surface specimens, significant SCC was observed in the specimens taken from the mid-wall location. This finding suggests that the heterogeneous or variable microstructure through the pipe wall thickness plays a critical role in SCC initiation for the X80 pipe investigated. It also suggests that careful attention must be paid to the design of test specimens as well as the location that they are removed from a test pipe in order to realistically assess the SCC susceptibility of pipe steels.

Stress-Corrosion Crack Initiation in X-52 Pipeline Steel in Near-Neutral pH Solution

2010 ◽  
Author(s):  
B. Fang ◽  
R. Eadie ◽  
M. Elboujdaini
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Pipeline Steel ◽  
High Stress ◽  
Acute Angle ◽  
Stage I ◽  
Immersion Method ◽  
Low Frequencies ◽  
Neutral Ph ◽  
Stress Ratios

Specimens from a failed X-52 pipeline that had been inservice for 34 years were pitted using the passivation/immersion method developed by the authors to simulate pitted pipelines observed in service. The resulting pitted samples were then cyclically loaded in an aqueous near-neutral pH environment sparged with 5% CO2 / balance N2 gas mixture at high stress ratios (minimum stress/maximum stress), low strain rates and low frequencies which were close to those experienced in service. It was found that the majority of cracks initiated from the corrosion pits and were less than 0.5 to 0.6 mm deep and were generally quite blunt. These cracks were transgranular in nature and designated as Stage I cracks and were typical of cracks found in most crack colonies. However, the further growth of these short, blunt cracks was significantly influenced by the distribution of the nearby non-metallic inclusions. Inclusions enhanced the stress-facilitated dissolution crack growth, which is the crack growth method proposed by the authors in a related paper. When the orientation of the inclusions was at a small acute angle to the orientation of the pits or cracks, and the inclusions were in the same plane as crack initiation or advance, these inclusions would enhance crack growth, or even trap hydrogen which further resulted in the formation of clusters of tiny cracks, which appeared to be caused by hydrogen. The hydrogen-produced cracks could be eaten away later by the stress-facilitated further dissolution of the blunt cracks. If these cracks can grow sufficiently however they pose an integrity risk, as they can initiate long cracks (near-neutral pH SCC). These hydrogen-caused cracks in Stage I were rare. It was nevertheless suggested that cracks deeper than 0.5 to 0.6 mm in the field should be removed to reduce or avoid the threat of rupture. If active corrosion and hydrogen generation can be prevented then smaller cracks are innocuous.

Application of the R-Curve Concept to Fatigue Crack Growth

1978 ◽  
Vol 100 (4) ◽  
pp. 416-420 ◽  
Author(s):  
D. P. Wilhem ◽  
M. M. Ratwani
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Crack Extension ◽  
Stress Ratio ◽  
Cyclic Stress ◽  
Crack Growth Resistance ◽  
Resistance Curve ◽  
Cyclic Stress Ratio ◽  
Wide Range

Crack growth resistance for both static (rising load) and for cyclic fatigue crack growth has been shown to be a continuous function over a range of 0.1 μm to 10 cm in crack extension for 2024-T3 aluminum. Crack growth resistance to each fatigue cycle of crack extension is shown to approach the materials ordinary undirectional static crack resistance value when the cyclic stress ratio is zero. The fatigue crack extension is averaged over many cycles and is correlated with the maximum value of the crack tip stress intensity, Kmax. A linear plot of crack growth resistance for fatigue and static loading data shows similar effects of thickness, stress ratio, and other parameters. The effect of cyclic stress ratio on crack growth resistance for 2219 aluminum indicates the magnitude of differences in resistance when plotted to a linear scale. Prediction of many of these trends is possible using one of several available crack growth data correlating techniques. It appears that a unique resistance curve, dependent on material, crack orientation, thickness, and stress/physical environment, can be developed for crack extensions as small as 0.076 μm (3 μ inches). This wide range, crack growth resistance curve is seen of immense potential for use in both fatigue and fracture studies.

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