Failures by SCC in buried pipelines

British Corrosion Journal, 2001

where the metal is in contact with diluted solution under disbonded coating. It seems already known that such cracking involves hydrogen evolution and permeation at the crack tip, as previously observed in nuclear pressure vessel steels and other low alloy steels under variable loading. This cracking mechanism is much more influenced by loading conditions (crack tip strain rate) than by the solution chemistry. With reference to pipeline steels, several authors claim that it is difficult to reproduce in laboratory the actual cracking morphology observed in field. In this paper, results of tests where the fracture surface observed in serviced pipes has been reproduced are presented and discussed. The reason for the present results is that attention has been given to mechanical factors, much more than to chemical parameters.

Pipeline steels are often susceptible to SCC in two basic forms of cracking, namely Intergranular and Transgranular. Intergranular cracks initiate and propagate at grain boundaries and usually form at high-pH (8.5-10.5). They are initiated at the outer surface of pipe and the cracking results from the generation of a carbonate-bicarbonate solution under disbonded coatings. On the other hand, transgranular cracks cut through the grain and is usually manifested at near-neutral-pH SCC (5.5-8.5). Transgranular cracks in high-pressure gas pipelines has been found to be associated with dilute solutions of near neutral pH in regions where coatings are disbonded. This environment could develop from ground water in the presence of carbon dioxide (CO2). However, it is not understood how cracks initiate under such a coating disbondment environment, and whether there are any microstructural features responsible for the initiation process. A characteristic of both types of cracks is the presence...

Threshold stresses and crack growth rates for in-service SCC cracking of two Electrical Resistance Weld (ERW) seam welded pipes from two 45 year old oil pipelines were experimentally assessed. Seventeen high-pH SCC tests were carried out, in both base and ERW weld metals, at two temperatures (73 C and 45 C). Tapered specimens were used for base metal, constant section specimens were developed for ERW tests, in which original surface conditions were preserved. It was found that susceptibility of the ERW seam welds is much higher than those for base materials, so that the welds define the length of the pipe that is susceptible to SCC. Threshold pressure estimates for SCC initiation were defined from tests at elevated temperature, service temperature and literature correlations. Fabrication residual stresses were also measured and taken into consideration. SCC threshold pressures for these lines are controlled by the ERW welds; the pipe tracts which are considered to be susceptible to SCC are those which undergo a service pressure of at least 2.4 MPa. For the case under study this represents about 70 % of the length of the pipeline.

Corrosion, 2007

Buried carbon steel fuel transmission pipelines, protected by external coatings and cathodic protection, are known to experience transgranular stress corrosion cracking (TGSCC). Failure analysis of failed pipelines and laboratory tests have indicated that hydrogen plays an important role in the overall failure mechanism. However, the role of various groundwater constituents in TGSCC and the source of hydrogen are not well understood. The objective of this research was to further understand the mechanism of TGSCC by examining the environments in which TGSCC can occur and near-neutral pH can be maintained. Specifi cally, this research focuses on the production of hydrogen through the interaction of iron with carbonic acid (H 2 CO 3) and bicarbonate (HCO 3 −) as based on the Colwell-Leis mechanism. The Colwell and Leis mechanism attributes the hydrogen generation, in groundwater solutions, to the decomposition of bicarbonate and reaction with metal ions to form metal carbonate. Results from coupon exposure tests and slow strain rate tests indicate that TGSCC on pipeline steels can be produced in simple bicarbonate solutions. Slow strain rate tests in simple bicarbonate solutions with 5% carbon dioxide (CO 2) produced cracks that have similar crack morphologies to what is found in the fi eld. These results suggest that the environmental conditions for TGSCC and intergranular stress corrosion cracking (IGSCC) on pipeline steel surface are related and may change from one to another by changes in the applied cathodic potential.

The increasing energy development activities in the world impose the viability of pipeline for the safe and economical working. Pipelines mainly transport chemicals and hazardous fluids hence need to have very good safe track record and proved to be a safe and economical mean of carrying them. To lay down this platform, extensive research and development program has lead to appropriate design, selection of suitable materials and good operational practices. The optimization of selecting and evaluating coatings for feasible use in northern pipelines is of great importance with respect to the extreme environments to which the coated pipe may be subjected. Still failure occurs in pipelines due to some instants. Major cause of this failure includes crack formation and it's propagation over the surface of the pipe. Key cause behind these unwanted cracks is induced due to stressed corrosion cracking (SCC). The reasons behind SSC include the crevice loads, hydrogen embrittlement, residual stresses and film induce cleavage. The main objective of this article is to explore mechanisms and morphology of cracks propagation due to stress corrosion cracking to the pipe wall and its effect over the pipe span.

Journal of Natural Gas Science and Engineeringning , 2022

This paper presents an updated review of the external corrosion and failure mechanisms of buried natural gas and oil pipelines. Various forms of external corrosion and failure mechanisms such as hydrogen-induced cracking (HIC), hydrogen embrittlement (HE), corrosion fatigue (CF), stress corrosion cracking (SCC) and microbiologically influenced corrosion (MIC) for oil and gas pipelines are thoroughly reviewed. The factors influencing external corrosion and possible forms of environment-assisted cracking (EAC) of pipeline steels in the soil are also reviewed and analyzed in depth. In addition, the existing monitoring tools for the external corrosion assessment and the models for corrosion prevention and prediction, failure occurrence, and remaining life of oil and gas pipelines, are analyzed. Moreover, the articles on external corrosion management, reliability-based models, risk-based models, and integrity assessment including machine learning and fuzzy logic approaches, are also reviewed. The conclusions and recommendations for future research in the prevention and prediction of external corrosion are presented at the end.

Volume 2: Pipeline Integrity Management, 2012

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 a...

Volume 2: Integrity and Corrosion; Offshore Issues; Pipeline Automation and Measurement; Rotating Equipment, 2000

Neutral-pH stress corrosion cracking (SCC) is a topic of significant interest to pipeline operators in the Canadian Energy Pipeline Association (CEPA). In recent years there has been a shift in laboratory SCC research toward investigating cracking phenomena under more realistic environmental and physical loading conditions. To support this move, CEPA has been active in collecting pressure-time data from its varied membership of liquid and gas pipeline companies. Histories of pressure fluctuations in operating systems have been characterized in terms of loading cycles, R-values (the ratio of minimum to maximum stress) and strain rates. These parameters are relevant to the design of laboratory experiments for the investigation of SCC. An overview of the data is presented for both liquid and gas pipelines in various service applications.

Engineering Failure Analysis, 2015

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