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.

Fatigue Performance of Sour Deepwater Riser Welds: Crack Growth vs. Endurance

2011 ◽  
Author(s):  
Colum M. Holtam ◽  
David P. Baxter
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
Pipeline Steel ◽  
High Stress ◽  
Fatigue Loading ◽  
Stress Intensity Factor Range ◽  
Valuable Insight ◽  
Stress Ratios ◽  
Fatigue Endurance

Steel catenary risers (SCRs) are increasingly used in deepwater oil and gas developments. SCRs can be subject to low-stress high-cycle fatigue loading, for example from wave and tidal motion, vortex induced vibration (VIV) and operating loads, and corrosive environments (internal and external). When the production fluids are sour, higher fatigue crack growth rates (FCGRs) are expected and therefore shorter overall life compared to performance in air, as a result of the interaction between fatigue crack growth and sulphide stress cracking. Successful design of risers is critically dependent on the availability of appropriate experimental data to quantify the extent to which fatigue lives are reduced and rates of fatigue crack growth are increased. Historically there has been a discrepancy between experimental sour fatigue endurance data and fracture mechanics-based estimates of the corresponding stress-life (S-N) curves. This paper summarises the results of recent sour FCGR tests on C-Mn pipeline steel. Tests were performed under conditions of increasing applied stress intensity factor range (ΔK), on specimens containing shallow initial flaws and at very high stress ratios (R), to obtain data close to threshold. In many cases it is material behaviour at these low values of ΔK that dominate the fatigue life (e.g. VIV loading). The FCGR data are then compared to sour fatigue endurance data, both published and from a TWI Joint Industry Project (JIP). The observed environmental reduction factor (ERF) for endurance tests is compared to that expected from the difference in fatigue crack propagation rates, to examine whether FCGR data might provide an alternative means of predicting ERFs. This paper offers valuable insight into current best practice methods for generating sour FCGR data when qualifying girth welds for sour service, and the relationship between fatigue crack growth and fatigue endurance.

Blunt Crack Initiation and its Transition to Sharp Cracks in Pipeline Steel in Near-Neutral pH Solution

2012 ◽  
Author(s):  
B. Fang ◽  
R. L. Eadie ◽  
M. Elboujdaini
Keyword(s):  
Crack Growth ◽  
Pipeline Steel ◽  
High Stress ◽  
Crack Tip ◽  
Crack Mouth ◽  
Blunt Crack ◽  
Neutral Ph ◽  
Aspect Ratios ◽  
Risk Of Failure ◽  
Mouth Width

This paper reviews our research into pipeline stress corrosion cracking (SCC) in near-neutral pH (NNpH) environment to help understand the mechanisms on pit-to-crack transition and early growth to contribute to pipeline integrity management so that the risk of failure can be avoid or reduced. Pitted specimens by using two different techniques (passivation/immersion and electrochemical methods) were cyclically loaded in NNpH environment sparged with 5% CO2 / balance N2 gas mixture at high stress ratios (minimum stress/maximum stress), low strain rates and low frequencies which are close to the operational pipelines in the field. Blunt cracks initiation was seen first and associated with the pit geometry, and most of the blunt cracks were observed to have initiated from the corrosion pits that had the pit depth to surface width aspect ratios greater than 0.5. The blunt crack growth was engendered by anodic dissolution, which was facilitated by stress. So it was called as stress facilitated dissolution crack growth. These blunt cracks had considerably large crack tip width to crack mouth width aspect ratios, and the majority were below 0.5 to 0.6 mm deep, and considered dormant. Once cracks surpassed the critical value, around 0.5 to 0.6 mm, the cracks would be reactivated and the crack tip width to crack mouth width ratios became significantly smaller. Meanwhile, more hydrogen would be trapped in the plastic zones. Thus, hydrogen would play an important role in the crack propagation. So in this stage, cracks tended to become sharp and the mechanism was referred to hydrogen facilitated cracking. The observations from the field can be interpreted very well by using the proposed models. It was proposed that two different mechanisms are responsible for the early-stage crack growth and sharp cracks be removed to reduce the risk of failure in pipelines.

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.

Effects of Loading Spectra on High pH Crack Growth Behavior of X65 Pipeline Steel

CORROSION ◽  
10.5006/3472 ◽  
2020 ◽  
Vol 76 (6) ◽  
pp. 601-615 ◽  
Author(s):  
Hamid Niazi ◽  
Karina Chevil ◽  
Erwin Gamboa ◽  
Lyndon Lamborn ◽  
Weixing Chen ◽  
...  
Keyword(s):  
Free Surface ◽  
Crack Initiation ◽  
Crack Propagation ◽  
Strain Rate ◽  
Crack Growth ◽  
Pipeline Steel ◽  
Crack Tip ◽  
Growth Behavior ◽  
Crack Growth Behavior ◽  
Secondary Crack

The effects of mechanical factors on crack growth behavior during the second stage of high pH stress corrosion cracking in pipeline steel were investigated by applying several loading scenarios on compact tension (CT) specimens. The main mechanism for stage 2 of intergranular crack propagation is anodic dissolution ahead of the crack tip which is highly dependent on crack-tip strain rate. The maximum and minimum crack growth rates were 3 × 10−7 mm/s and 1 × 10−7 mm/s, respectively. It was observed that several factors such as mean stress intensity factor, amplitude, and frequency of loading cycles determine the crack-tip strain rate. Low R-ratio cycles, particularly high-frequency ones, enhance secondary crack initiation, and crack coalescence on the free surface. This mechanism accelerates crack advance on the free surface which is accompanied with an increase in mechanical driving force for crack propagation in the thickness direction. These findings have implications for pipeline operators and could be used to increase the lifespan of the cracked pipelines at stage 2. For those pipelines, any loading condition that increases the strain rate ahead of the crack tip enhances anodic dissolution and is detrimental. Additionally, secondary crack initiation and coalescence could be minimized by avoiding internal pressure fluctuation, particularly rapid large pressure fluctuations.

Designing Light Alloys for Fatigue Crack Growth Resistance

2011 ◽  
Vol 690 ◽  
pp. 393-396
Author(s):  
Anastasios G. Gavras ◽  
Diana A. Lados
Keyword(s):  
Fatigue Crack ◽  
Fatigue Crack Growth ◽  
Crack Growth ◽  
High Stress ◽  
Compact Tension ◽  
Fatigue Crack Growth Resistance ◽  
Reduction Strategies ◽  
Stress Ratios ◽  
And Performance ◽  
Wrought Aluminum

Fatigue crack growthresistance is critical to the design and performance of structural components.This study focuses on understanding the microstructural mechanisms of fatigue crack propagation in commonly used structural materials, cast and wrought aluminum and titanium alloys, with various microstructures resulting from changes in chemistry or heat treatment (A535-F, 6061-T6, and mill- and b-annealed Ti-6Al-4V).Stress ratio effects were evaluated by conducting fatigue crack growth tests on compact tension specimens at low, intermediate, and high stress ratios, R=0.1, 0.5, and 0.7, respectively. Initial flaw size effects were also studied by performing small crack growth tests at R=0.1. Data reduction strategies compensating for closure and Kmax effects on crack growth and design curves will be presented.

A Fracture Mechanics Investigation on Crack Growth in Massive Forming

2005 ◽  
Vol 482 ◽  
pp. 339-342 ◽  
Author(s):  
Gernot Trattnig ◽  
Christof Sommitsch ◽  
Reinhard Pippan
Keyword(s):  
Crack Initiation ◽  
Crack Growth ◽  
Hydrostatic Stress ◽  
Crack Opening ◽  
Crack Depth ◽  
High Stress ◽  
Symmetry Plane ◽  
Stress Triaxiality ◽  
Opening Displacement ◽  
Massive Forming

To understand the crack growth in massive forming and to consequentially avoid crack growth in workpieces, it is necessary to investigate its dependence on the crack depth and thus on the state of hydrostatic stress. Prior work shows that the crack opening displacement (COD) for shallow cracked tension specimens with low stress triaxiality is twice as high as for deep cracked specimens with high stress triaxiality. This work examines the crack growth in compression specimens with pre-cracked cylindrical upsetting samples. The compression samples were cut in the stress symmetry plane in order to observe crack initiation and crack growth by a single specimen technique. In this way it is possible to observe blunting, crack initiation and crack growth inside the upsetting specimens. The resulting COD does not differ significantly from the values achieved in tension samples with short surface cracks.

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