Greg Van Boven

Crack Growth Modeling and Life Prediction of Pipeline Steels Exposed to Near-Neutral pH Environments: Stage II Crack Growth and Overall Life Prediction

This investigation was initiated to provide governing equations for crack initiation, crack growth, and service life prediction of pipeline steels in near-neutral pH (NNpH) environments. This investigation develops a predictive model considering loading interactions occurring during oil and gas pipeline operation with underload-type variable pressure fluctuations. This method has predicted lifetimes comparable to the actual service lives found in the field. This is in sharp contrast with the predictions made by existing methods that are either conservative or inconsistent with the field observations. It has been demonstrated that large slash loads (R-ratio is 0.05), often seen during gas pipeline operation, are a major life-limiting factor and should be avoided where possible. Oil pipelines have shorter lifetime because of their more frequent pressure fluctuations and larger amplitude load cycles. The accuracy of prediction can be improved if pressure data with appropriate sampling intervals are used. The sampling interval error is much larger in the prediction of oil pipelines than gas pipelines because of their different compressibility but is minimized if the pressure sampling rate for the data is at or less than one minute.

Statistical Analysis on Underload-Type Pipeline Spectra

AbstractIn this investigation, analyses of pressure fluctuations during oil and gas pipeline operations are performed. The analyses are performed in a way to capture all the variables of pressure fluctuations and their magnitudes in terms of crack growth rate. It is found that pipeline spectra can be categorized into three main types: underload-dominant, mean load-dominant, and overload-dominant spectra, depending on their locations with respect to a compressor or pump station. The underload spectra—typical of pressure fluctuations at the discharging sites—are the most severe in terms of crack growth as these spectra are subjected to the highest pressure level and the largest magnitude and frequency of pressure fluctuations. The underload spectra are further analyzed in terms of loading and unloading frequency, maximum stress-intensity factor, stress-intensity factor range, numbers of minor cycles between two adjacent underloads, and their potential for crack growth. Special attention is paid to the diffe...

Correlating Corrosion Field Data With Experimental Findings for the Development of Pipeline Mitigation Strategies

Coating disbondment on pipelines is a common phenomenon that leads to exposure of the pipeline metal to ground water solutions, promoting a corrosive environment which is associated with stress corrosion cracking (SCC). This investigation aims to understand the corrosion behavior and rate of pipeline steel under coating disbondments of varying sizes based on field data and experimental studies. In the analysis of the field data, dig reports provided by a Canadian gas transportation company were analyzed for cases of anaerobic corrosion under tape coatings. The analyzed field data provided a correlation between the tape coating disbondment size and corrosion rate found under the coating. The experimental studies aimed to understand the field findings. The analyses were performed on X-65 pipeline steel coupons placed in a vertical coupon holder with a PMMA shielding. To imitate the variation in the disbondment size, the gap size between the metal coupons and the shielding was varied (2 mm, 5 mm, and 10 mm, and infinite). The general corrosion rates were obtained through weight loss calculations. The experimental results were compared and correlated with dig-report data from the field for a development of cathodic protection and pipeline mitigation strategies.Copyright

Surface Crack Growth Behavior of Pipeline Steel Under Disbonded Coating at Free Corrosion Potential in Near-Neutral pH Soil Environments

Crack growth behavior of X65 pipeline steel at free corrosion potential in near-neutral pH soil environment under a CO2 concentration gradient inside a disbonded coating was studied. Growth rates were found to be highest at the open mouth of the simulated disbondment where CO2 concentrations, hence local hydrogen concentration in the local environment, was highest. Careful analysis of growth rate data using a corrosion-fatigue model of the form ΔKα/Kmaxβ/fγ, where (1/fγ) models environmental contribution to growth, revealed that environmental contribution could vary by up to a factor of three. Such intense environmental contribution at the open mouth kept the crack tip atomically sharp despite the simultaneous occurrence of low-temperature creep and crack tip dissolution, which are the factors that blunt the crack tip. At other locations where environmental enhancement was lower, significant crack tip blunting attributed to both low-temperature creep and crack tip dissolution was observed. These factors both led to lower crack growth rates away from the open mouth.

Developing Cathodic Protection Based on Disbondment Geometry

Coating disbondment on pipelines is a common phenomenon that leads to exposure of the pipeline metal to ground water solutions, promoting a corrosive environment which is associated with stress corrosion cracking (SCC). This investigation tracks the corrosion behavior of X-65 steel at different coating disbondments at Open Circuit Potential (OCP) conditions through weight loss tests, as well as SEM analysis. For the weight loss test, X-65 coupons were placed into a vertical coupon holder with varying gap sizes between the coupons and the shielding (2 mm 5 mm, and 10 mm) simulating the coating disbondment. The results of the tests suggest that the corrosion rate inside the gap is dependent on the mass transfer rate of CO2. For larger gaps, the corrosion rate inside the gap decreases almost linearly with position (highest at the top of the disbondment) and is dependent on the iron carbonate film deposition. For smaller gaps (≤5 mm) a galvanic effect was observed between the coupons at the top of the disbondment (anodic) which showed a high degree of general corrosion, and the coupons at the bottom of the disbondment (cathodic) which had very low corrosion rates. The experimental observations were related to the corrosion scenarios found in the field. Cathodic protection strategies with respect to the geometry of disbondment are also discussed.Copyright

Effect of Pressure Sampling Methods on Pipeline Integrity Analysis

AbstractA computing program has recently been developed to predict corrosion fatigue crack growth in pipeline steel in near-neutral pH environments. Supervisory control and data acquisition (SCADA)...

Depressurization-Induced Crack Growth Enhancement for Pipeline Steels Exposed to Near-Neutral pH Environments

Pressure fluctuations are recognized as the driving force for the crack growth of pipeline steels in near-neutral pH environments; however, the crack growth mechanisms are still not fully understood. Difficulty in understanding the crack growth mechanisms is present due to two dilemmas between laboratory testing and field findings: high frequency study in the laboratory versus low frequency pressure fluctuations in the field; constant amplitude cyclic laboratory tests versus random pressure fluctuations in the actual spectra. To bridge the dilemmas, the crack growth behavior of X60 pipeline steel was investigated in near-neutral pH solution at frequencies as low as 1×10−5 Hz under variable amplitude cyclic loading. Special attention was given to the loading scheme consisting of minor cycles with R ratios (minimum stress/maximum stress) as high as 0.9 and underloads with a relatively lower R ratio of 0.5. It was found that the constant amplitude crack growth rate in near-neutral pH solution in the frequency region below 1×10−3 Hz decreases with decreasing loading frequency, and it reaches a constant value at very low frequencies. This crack growth rate-frequency relation is opposite of that found in the high loading-frequency regime, where crack growth rate was found to increase with decreasing loading frequency. Crack growth rate was observed to increase by a factor of up to 10 when the underload plus minor cycle loading scheme, as mentioned previously, was applied. Based on the findings obtained from the investigation, recommendations of pressure control were also made to minimize the crack growth during pipeline operation.Copyright

The Role of Electrochemical Conditions in Near-Neutral pH SCC Initiation Mechanism(s)

In order to improve our understanding of near-neutral pH SCC initiation mechanism(s), a comprehensive test setup was used to study the electrochemical conditions beneath the disbonded coatings in cracking environments. In this setup the synergistic effects of cyclic loading, coating disbondment, and cathodic protection were considered. Our previous results showed that there can be a significant variation in the pH of the localized environment under the disbonded coating of pipeline steel. The pH inside the disbondment can change significantly from near-neutral to high pH values, strongly depending on the level of cathodic protection and CO2 concentration. Both of these variables affected the electrochemical conditions on the steel surface and therefore the initiation mechanisms. This work highlights the role of electrochemical conditions in near-neutral pH SCC initiation mechanisms.Copyright

Developing a Predictive Model of Near-Neutral pH Stress Corrosion Cracking of Underground Pipelines

Near neutral pH Stress Corrosion Cracking (NNpHSCC) associated with external corrosion of pipelines is an issue facing industry today. Determining areas of NNpHSCC susceptibility is crucial to developing Integrity Management Programs and inspection dig schedules. This research involved collecting pertinent field data (inspection dig reports, failure reports, loading histories) and developing a predictive model to help identify areas and lines most susceptible to NNpHSCC. The predictive model focused on the loading history (in this case, SCADA data) patterns to classify different groups of loading conditions. Hydrogen has been identified and established in previous literature to be a major contributor to NNpHSCC. Different Hydrogen Enhancement Factors (HEF) were applied based on how the mechanisms of hydrogen embrittlement react to the respective loading conditions. The predictive model illustrated a dormancy behaviour, similar to the one seen in field conditions and a mechanically activated growth dependent on both hydrogen and previous loading scenarios. A correlation was shown between a limited field sampling and the predicted values. Further improvements and calibrations can be made with the gathering of more field data and continued experimental validation. Once this validation has been performed, this model has the possibility to illustrate what loading conditions increase a segments susceptibility to NNpHSCC.Copyright

Achieving Maximum Crack Remediation Effect from Optimized Hydrotesting

Hydrostatic testing is a key method for managing SCC in oil and gas pipelines. Benefits are achieved by eliminating defects above a critical size for the hydrotest pressure and hence achieving a post-test period without operating failure. Other benefits are related to temporary growth retardation after hydrotest because of crack tip blunting. Conversely, benefits of such a test could be offset by stable flaw growth in the previously dormant population and growth of cracks during hydrostatic loading. Although this type of growth behaviour has been previously analyzed, it was only assessed from tests in air, which neglects the effects of corrosive environments.Recent research has shown that crack growth can occur during hydrotests at much smaller crack dimensions than those originally analyzed. The adverse effect of hydrotesting is negligible if it initiates crack growth only on large-size cracks that are near the final stage of pipeline life. However, benefits of hydrotest would become uncertain if hydrostatic loading induces crack growth when crack dimensions are relatively small. This reduction in pipeline remaining lifetime by hydrostatic testing can be significant considering the fact that several hydrotests may be performed in the lifetime of a pipeline. Although hydrotests would usually re-condition the crack tip so that a lower crack growth rate can be expected for a limited time, it is questionable whether the reduced growth rate would compensate for the loss of life because of the crack growth during hydrotesting. Therefore, the overall benefits of hydrotesting must be evaluated from the viewpoint of life extension and pipeline safety.Effects of hydrostatic tests in this investigation were analyzed based on two competing morphological conditions at the crack tip before, during and after hydrostatic testing, that is, crack tip blunting and crack tip sharpening. Crack tip sharpening is related to the hydrogen effects and mechanisms of cyclic loading, while the crack tip blunting is attributed to low temperature creep of the pipeline steels. From the investigation, strategies aimed at achieving maximum crack remediation are proposed.Copyright