What is pipeline cracking?
Cracks can develop in pipelines at any stage – during manufacturing, fabrication, installation or throughout operational life. There are many forms of cracking, all caused by different mechanisms. The morphology of cracking is highly variable, and there may be many anomalies present in a pipe body and seam weld that would not behave as cracks but create a crack-like indication in crack inspection data. Thus, it is difficult to reliably identify the different cracking types based on inspection data alone. The solution to identifying and assessing pipeline cracks starts with an understanding of the cause of the cracking.
Some examples of pipeline cracks and features that may appear crack-like in ILI data are given below.
Near Neutral pH SCC
Near Neutral pH Stress Corrosion Cracking
Near-neutral pH SCC also occurs on external pipe surfaces under areas of coating disbondment but where CP is fully shielded. As for high pH SCC, the surface cracks generally form colonies in the axial direction of the pipe.
Near-neutral pH SCC cracking typically occurs in association with pits and general corrosion, as it occurs under freely corroding conditions, i.e. under no CP polarization. In contrast to high pH SCC, the crack propagation through pipe wall thickness is of a transgranular nature. The cracks tend to be wider, with corroded crack walls and filled with corrosion products.
It is thought to be most prevalent in high latitudes where there is a marked seasonal change in the carbon dioxide content of the soil. Some investigations have also associated anaerobic microbial activity with the initiation of near-neutral pH SCC; overall, however, the mechanism remains unclear and debated. It is nevertheless accepted that some level of stress cycling is required for initiation and growth.
High pH SCC
High pH Stress Corrosion Cracking - SCC
High pH SCC occurs on external pipe surfaces at locations of coating disbonding and partially shielded CP. The pipeline cracks generally form colonies that align axially with the pipe. Propagation through pipe wall thickness is of inter-granular nature. The cracks are not usually associated with any external pitting or general corrosion. A number of factors contribute to the initiation and growth of high pH stress corrosion cracking, including high stress, pressure cycling and development of a carbonate-bicarbonate environment, partial shielding of the applied cathodic protection, moderately elevated temperatures, and permanent or seasonal wetness in the soil.
Fatigue pipeline cracks are aligned at right angles to the principal stress. Cracks grow in response to stress or pressure cycling. Stress concentration occurs at the initiating defect or at the growing crack tip. Crack surfaces may show characteristic “beach marks” that were formed at each stage of crack growth. Fatigue cracking in pipelines is typically associated with areas of stress concentration such as dents and seam weld flaws.
Sour Cracking - HIC, Sulfide Stress Corrosion Cracking (SSCC)
Environmentally assisted cracking can also occur internally in pipelines. The most common conditions for these are sour environments. Following aqueous corrosion reactions in the presence of H2S on the pipe internal surface, atomic hydrogen is produced and absorbed in the pipe steel.
When atomic hydrogen is trapped at irregularities within the pipe steel – e.g. at inorganic inclusions (manganese sulfur) – and allowed to recollect with other trapped atomic hydrogen, this leads to the formation of molecular hydrogen and local buildup of pressure, which results in mid-wall blisters and cracking parallel to the wall. Pipeline cracks may join up, at different levels through the pipe wall, to create through-wall cracks (Step-Wise Cracking – SWC). Surface blisters may also contain cracks. A variation of the HIC mechanism is Stress-Orientated HIC (SOHIC).
In parallel events, once atomic hydrogen is absorbed within the microstructure, it can also diffuse into solid solution, leading to local crystal embrittlement and ultimately stress corrosion cracking in the presence of residual or applied tensile stresses. This mechanism is referred as Sulfide Stress Corrosion Cracking (SSCC).
Laminations are unwanted discontinuities lying parallel to the pipe surface that are usually marked by a concentration of non-metallic material. The rolling-out of inclusions, blowholes or pipes in the parent material causes them. Typically, laminations are not significant, but they may mask cracks.
Surface-breaking laminations can be initiation points for fatigue cracks and hydrogen cracking in the pipe body. Laminations may cause detrimental planar features and cracking in welds when the two coincide, such as SAW weld hot tears due to laminations.
Hook cracks occur in combination with non-metallic inclusions or laminations in the edges of the strip used for ERW welds. These features become partially incorporated into the weld as aligned discontinuities during the forming process, leading to the characteristic hook appearance.
LACK OF FUSION
Lack of Fusion
Lack of fusion is a planar (i.e. crack-like) discontinuity in which there is a lack of union between the weld metal and the parent metal or weld metal.
Lack of fusion can be caused during manufacturing by a number of factors associated with welding process parameters, contamination and poor QA & QC.
ERW AND HFI SEAM-WELDED PIPE PLANAR FEATURES
ERW and HFI Seam-Welded Pipe Planar Features
Lack of fusion in ERW pipe appears as an axial, crack-like discontinuity at the midpoint of the weld bond line. It is also referred to as cold weld, penetrator or stitching, depending on its characteristics. As with SAW welds, it can be caused by a number of factors associated with process parameters, contamination, etc.
In the weld metal of fusion welds (i.e. SAW seam and multiple process girth welds), cracks can be both hot and cold. Hot cracks from solidification mechanisms can be in both the longitudinal and transverse directions as well as “crazed.” These cracks are usually formed in the pipe mill for seam welds or during pipeline construction for girth welds.
Cold cracking in weld metal requires the presences of hydrogen, a susceptible weld metal microstructure and stress, so cracks typically form in the longitudinal direction. While cold cracking generally occurs during or just after the welding process (it can be delayed), there are types of environmentally assisted cracking that can also readily occur in the weld metal.
In EFW and ERW welds, “cracking” in most cases will be located on the mid-point of the bond line. Despite such features actually being related to fusion, the term cracking is still used (e.g. hook crack), and from an integrity perspective, cold welds, stitching and penetrators are all assessed as cracks.
The toes of fusion welds are particularly susceptible to cold cracking due to the microstructures present in the HAZ, as well as the change in shape. This discontinuity in profile acts as a stress raiser to locally increase the applied stress during formation or, later during service, as a stress concentrator for fatigue cracking and the like.
From the toe location, and depending on the shape of the underlying weld, the crack may remain in the HAZ, or it can propagate into the parent or weld metal.
Toe cracking is of course limited to fusion welds, as only these have the geometry where toes are present. Hence, toe cracking is not present in EFW or ERW pipe.
Porosity is cavity-type discontinuity formed by the entrapment of gas in the weld metal during solidification. It can be present as isolated pores, multiple pores in a cluster or elongated cavities also known as wormholes. It is not a form of cracking but can add to the complexity of inspecting a weld.
LACK OF PENETRATION
Lack of Penetration
Lack of penetration is a planar (i.e. crack-like) discontinuity where the full thickness of the joint is not welded. In a double-sided SAW weld, lack of penetration occurs between the inside and outside weld passes.
Circumferential Stress Corrosion Cracking
Circumferential SCC (high pH or near neutral) occurs where the environmental conditions are right and the peak stress in the pipe is axial. Axial stresses are usually created by the pipe bending to follow the ground profile or additional loads imposed by ground movement (landslides, subsidence, etc.).
Girth Weld Cracking
Circumferential Crack in Girth Weld
Also referred to as hydrogen-induced cold cracking, hydrogen cracking and delayed cracking, it requires the following during welding: the presence of diffusible hydrogen, stress and a susceptible metal microstructure.
Also referred to as hot shortness, centerline cracking and hot tearing, hot cracking mechanisms generally depend on three influencing factors: inadequate supply of liquid metal at the solidification front as a result of low melting-temperature impurities, shrinkage stress across the solidifying weld and a susceptible weld size (depth to width ratio).
GIRTH WELD PLANAR FEATURES
Girth Weld Planar Features
Cold cracking can occur in the HAZ or weld metal at both the root and cap of the weld bead. Cold cracking often originates in the HAZ of the root and cap due to these regions being more susceptible in terms of microstructure present and the stress concentration.
LACK OF PENETRATION
Also known as lack of root penetration in multipass girth welds, this is caused when the root pass fails to penetrate into the root region of the weld preparation.
LACK OF FUSION
Lack of fusion in girth welds is caused by a non-union between the weld metal and the base material or previous weld passes. Different types of lack of fusion exist in girth welds based on location. These include, for example, lack of root fusion, lack of sidewall fusion and lack of inter-run fusion.
Where is Cracking?
Cracking can be anywhere, but it needs some specific boundary conditions to initiate. Therefore, similar to other typical pipeline anomalies, a fundamental question is the time dependency of crack-like anomalies. For cracking, a time dependency is hard to observe. Therefore, cracks in pipelines can be categorized into environmentally assisted active cracking and mill-related or dormant planar anomalies.
Cracking can be found at any location of the pipeline, be it in the pipe body or in the welded areas. But pipeline cracks are always detected perpendicular to the main local stress direction of the pipe material. A pressurized pipeline experiences so-called hoop stress, which creates an environment for axial cracking. Likewise, an axial load on the pipeline supports the occurrence of circumferential cracking.
Another factor is the exposure of the pipeline surface. A corrosive environment or other impact on the surface of the pipeline – mechanical damage, for example – also support the initiation of cracking due to micro-embrittlement. Finally, the pipeline material and its mechanical properties are contributing, as well. Higher strength steels will be more susceptible to environmentally assisted cracking mechanisms associated with hydrogen embrittlement eg SSCC, or hydrogen embrittlement due to CP overprotection. The presence of hard spots can also contribute to higher susceptibility to environmentally assisted cracking due to local residual stresses.
HOW TO MANAGE
An Holistic Approach
Just like every threat has its unique characteristics, so does every pipeline. Approaching crack detection and assessment for pipelines with a “big picture” mindset allows operators to adopt the most effective way to “take control of cracks.”
The ROSEN Group has created a Pipeline Integrity Framework for Cracking that is a consolidation of current industry best practices and the most advanced pipeline crack inspection solutions with the knowledge of subject-matter experts. It outlines all the key elements needed to develop a comprehensive and justifiable integrity management program. It is a systematic, collaborative approach effective for assessing even the most challenging forms of cracking.
This approach does not advocate employing the fanciest or most expensive crack detection technology; instead, it suggests an added-value approach to ensure that objectives and needs are understood. It includes pre-inspection elements that answer critical questions to allow for optimal system selection.
The framework continues as a flexible guide through the entire process from inspection to integrity, ultimately resulting in a proper threat management plan. It is modular and adaptable, ensuring a common understanding and allowing operators to choose which elements are relevant to them in reaching their objectives and make the decisions needed for safe and efficient pipeline operation.
Find out more about our Framework
ACCURATE INSPECTION DATA Collection
In-line inspection, of course, is a major part of the pipeline integrity framework. ROSEN uses the latest generation of crack detection technologies. Using liquid-coupled ultrasonic or dry-coupled electromagnetic acoustic technologies supported by the unwavering magnetic flux leakage technology, RoCD provides reliable crack detection and accurate crack sizing. The technology also establishes appropriate baseline standards for the successful and effective management of pipeline integrity.
Axial Cracking Detection Technologies
•Patented measurement principle for high-resolution electromagnetically generated ultrasound
•Highly dependable detection and accurate continuous sizing of
axial crack anomalies
•Reliable detection of coating disbondment, a precursor of cracking
•Preferred service for gas or liquefied gas pipelines
More about our ROCD EMAT-C service
•Ultrasonic shear wave technology for the detection and sizing
of axial-oriented cracking
•Highly dependable detection and accurate continuous sizing
of crack anomalies
•Preferred service for liquid pipelines from water to gasoline
to crude oil
•Increased sensitivity for crack detection using tailored probe
design and a high-resolution setup
•Full data recording; no data reduction for confidence and
•Circumferential magnetic flux leakage technology
•Precise long-seam categorization and assessment using magnetic saturation
•Extra-high sensor density and high sampling rate support crack identification and location
•Serves also as the supporting technology by collecting data that can increase POI of crack features
More about our ROCORR MFL-C service
Circumferential Crack Detection Technologies
Basically, any of the above-mentioned technologies can be modified to detect and size circumferential cracking as well. Since the determination of the axial load is also important, supporting technologies like axial stress detection and bending strain analysis are recommended. Circumferential Crack Detection Services are used frequently but not as often as axial crack detection services.
Collecting the data is half the battle – using it properly and gaining the most valuable information from it is the trick. Operators have to make the inspection data work for them. This includes proper reporting and analysis along with further assessments of the data.
Reporting and Analysis
Close collaboration of expert data evaluators and senior integrity engineers with extensive experience in dealing with cracks in pipelines ensures credible results and that efforts are focused on the critical areas. Properly visualizing data in reporting software based on fully analyzed data covering the entire pipeline provides easy asses to the information at hand and is best for reviewing potentially harmful anomalies.
Find out more about VIRTUALYZE
Immediate Crack Prioritization
A ranking of possible crack features identified by the inspection system allows the operator to make decisions. It highlights failure risks, recommends further field investigation and identifies pipeline compliance requirements. Once the preliminary in-line inspection results become available, it is directly possible to calculate indicative defect failure pressures to ensure that immediate integrity threats are identified and prioritized.
In combination with a susceptibility analysis, and ILI data evaluator confidence, the results will be used to drive the selection of sites for initial in-field investigations. The process ultimately prioritizes features that need immediate attention and identifies where further field verification is necessary.
Complete Crack Assessment
For a complete crack assessment, final results from in-line inspections, any testing and in-field work are combined, and the features assessed, to determine the impact on the immediate and future integrity of the pipeline. The future integrity assessment considers fatigue and environmental (e.g. SCC) growth mechanisms where applicable. A summary of all previous activities – including root cause analyses and metallurgical testing – provides a comprehensive list of mitigation and repair actions.
Defect Critical Size
Using the chosen assessment method (e.g. API 579, BS 7910, MAT-8, ln-Sec, CorLASTM), calculations are completed to identify defect sizes that would be unacceptable. The output informs the minimum sizing requirements of the ILI system to locate critical pipeline cracks.
It also highlights the impact of conservative assessment inputs, such as an assumed fracture toughness. This can be extended to a Critical Crack Defect Manual, which defines acceptance curves, response times and pressure reduction requirements to assist in-field decision-making.
Root Cause Analysis
A root cause analysis can range from diagnosing the types of cracking present on a pipeline to a full investigation of crack-induced failures. Materials, corrosion and welding experts experienced in all conceivable types of cracking are on hand to accurately diagnose the cracking type. State-of-the-art laboratory testing is available to support investigations when necessary.
To ensure a comprehensive integrity management strategy, the consequence of failure must be combined with the threat of cracking to determine overall risk. A key and unique input is our approach to susceptibility modelling: it starts with industry good practices but is continuously modified and developed for each pipeline based on the results of ILI and field verification activities to produce a detailed bespoke model.
Elements of an operator’s existing risk assessment can often be adapted by adding the latest data. It is also possible to develop a completely new model. Generally, a consequence assessment is completed and combined with the results of the optimized susceptibility analysis to produce an overall analysis and report.
The in-line inspection data is available. The data has been analyzed and assessed. Operators can make short-term decisions to ensure the performance, lifetime and safety of their asset. But the Pipeline Integrity Framework for Cracking goes one step further. It includes the management “step,” which takes a more proactive, forward-looking approach. This element brings the framework full circle and creates a result that is greater than the sum of all the parts.
Pulling all the pieces together creates a robust, justifiable integrity management plan that delivers the optimum combination of activities (direct assessment, ILI, hydrotesting, recoating, replacement) to ensure safety. A plan like this allows operators to take the right maintenance steps at the right time in order to extend the lifetime, safety and performance of their asset.
A basic prerequisite for the quick and reliable assessment of an asset’s integrity is the availability of consistent and fully aligned datasets. Additionally, more and more regulations now require that all pipeline records be traceable, verifiable and complete.
However, with the amounts of collected data steadily growing, the establishment of a system of record where all available data is readily accessible is becoming an increasingly critical issue for pipeline operators.
Find out more about NIMA
Competence is a key consideration in managing pipeline assets. The risks they present – and the safety of people and the environment – are becoming more imminent. Plus, standards and regulations explicitly require all personnel to be competent and qualified in their respective fields of responsibility.
Understanding this need, ROSEN has developed training courses, education programs and qualifications specific to addressing threats. Specifically, training for the management of cracking in pipelines is available for various integrity topics and for the application of reporting software.
Find out more about Competence Training