Cracking

What are pipeline cracks?

Pipeline cracking refers to the formation of fractures or splits in pipeline material. These cracks are typically caused by mechanical stress, environmental conditions, or interactions with other defects, such as corrosion. Cracks are serious defects in pipelines because they can compromise the pipeline’s safety and can lead to sudden failure without warning. ^1,2,3

Illustration of a pipeline with the different locations for pipeline cracks marked.

Axial crack: A crack that runs lengthwise along the pipe, often associated with seam welds, mechanical stress, or corrosion.
Axial crack in seam weld: A crack that runs parallel to the pipeline axis, along the longitudinal weld seam, often caused by residual stress or corrosion.
Circumferential crack: A crack that encircles the pipe, typically caused by cyclic loading or stress concentration at welds or defects.
Circumferential crack in girth weld: A crack that runs around the circumference of the pipeline, at the welded joint between two pipe sections, typically caused by stress or welding flaws.
Lack of fusion: A manufacturing defect where weld metal fails to properly bond with the base material, creating a weak point that can lead to cracking.
Lamination: A defect in the pipe wall where layers of material separate, often due to issues during manufacturing, which can act as initiation points for cracks.
Stress Corrosion Cracking: A form of cracking caused by the combined effects of tensile stress and a corrosive environment, often leading to brittle fracture.

What causes pipeline cracking?

A pipeline crack can occur at any stage of a pipeline’s lifecycle, including during manufacturing, installation, or operation. These cracks do not form randomly but usually develop under conditions that place stress on the pipeline.³

One cause of a crack in a pipeline is stress from internal pressure. When a pipeline transports liquids or gases at high pressure, stress is created on the pipe walls. Experts refer to this stress as “hoop stress.” This stress can lead to cracks running along the pipe. Similarly, cracks can form in a circular pattern around the pipe if the pipeline is exposed to bending or heavy loads. External factors also play a major role. For example, corrosion can weaken the surface of the pipe and make it more susceptible to cracking. Physical damage, such as dents or scratches caused by machinery, can create weak spots where cracks can form. Over time, these small cracks can grow and cause leaks or even major failures.³

Why do cracks in pipelines pose a risk?

Cracks pose a high risk because they can grow under operational stress and eventually lead to a rupture. Unlike corrosion, which often progresses slowly, cracks can result in sudden failure. If a crack spreads rapidly, it may cause a running fracture, in which the damage extends far beyond the initial defect.³

Cracking also increases the likelihood of combined defects, such as corrosion interacting with a crack. This significantly raises the risk of pipeline failure. Early detection and assessment of cracks are essential to preventing leaks, environmental damage, and costly repairs.³

There are many types of pipeline cracking, all of which are caused by different mechanisms. Some examples of cracks and features that may appear crack-like are:

Stress Corrosion Cracking (SCC)

Circumferential SCC

Fatigue Cracks

Sour Cracking – HIC, Sulfide Stress Corrosion Cracking (SSCC) and others

Lamination

Hook Cracks

White-grey image showing a form of pipeline cracking.

Near Neutral pH SCC: Near-neutral pH SCC occurs on external pipe surfaces under areas of coating disbondment where the cathodic protection (CP) system is completely shielded. Similar to high pH SCC, surface cracks tend to form colonies along the axial direction of the pipe. Near-neutral pH SCC cracking typically occurs in association with pitting and general corrosion because it occurs under freely corroding conditions, i.e., under no CP polarization. Unlike high pH SCC, crack propagation through the pipe wall thickness is of a transgranular nature. These 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.³

Overall, however, the mechanism remains unclear and debated. It is nevertheless accepted that some level of stress cycling is required for initiation and growth.³

High pH Stress Corrosion Cracking – SCC: High pH SCC occurs on external pipe surfaces at locations where the coating has disbonded and where CP is partially shielded. These cracks generally form colonies that align axially with the pipe. Propagation through the pipe wall thickness is of an intergranular nature. These cracks are not usually associated with external pitting or general corrosion. Several factors contribute to the initiation and growth of high pH stress corrosion cracking, including high stress, pressure cycling, the development of a carbonate-bicarbonate environment, the partial shielding of applied cathodic protection, moderately elevated temperatures, and permanent or seasonal wetness in the soil.³

Circumferential SCC (high pH or near neutral) occurs when environmental conditions are favorable and peak stress in the pipe is axial. Axial stresses are typically caused by pipe bending to follow the ground profile or by additional loads imposed by ground movement, such as landslides, subsidence, etc..^1,3

Girth Weld Cracking

a. Cold Cracking: Also referred to as hydrogen-induced cold cracking, hydrogen cracking, and delayed cracking, this occurs when the following conditions are met during welding: the presence of diffusible hydrogen, stress, and a susceptible metal microstructure. Some examples for different welding processes are given below.³
b. Hot Cracking: Also referred to as hot shortness, centerline cracking, and hot tearing, hot cracking mechanisms generally depend on three factors: an inadequate supply of liquid metal at the solidification front due to low-melting-temperature impurities; shrinkage stress across the solidifying weld; and a susceptible weld size (depth-to-width ratio). Some examples for different welding processes are given below.³

2   Girth Weld Planar Features

a.    Cold Cracking: Cold cracking can occur in the heat-affected zone (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 because these regions are more susceptible due to the microstructure and the stress concentration present.³
b.    Lack of Penetration: Also known as lack of root penetration in multipass girth welds, this occurs when the root pass fails to penetrate the root region of the weld preparation.³
c.    Lack of Fusion: Lack of fusion in girth welds occurs when there is no union between the weld metal and the base material or previous weld passes. There are different types of lack of fusion in girth welds based on location. These include lack of root fusion, lack of sidewall fusion, and lack of inter-run fusion, for example.³

Fatigue cracks are aligned at right angles to the principal stress. They grow in response to stress or pressure cycling. Stress concentration occurs at the initial defect or the growing crack tip. Crack surfaces may show characteristic “beach marks,” which are 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.³

Environmentally assisted cracking can also occur inside pipelines. The most common conditions for this type of cracking are sour environments. Following aqueous corrosion reactions involving H2S on the internal surface of the pipe, atomic hydrogen is produced and absorbed by the pipe steel.³

When atomic hydrogen is trapped at irregularities in 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. This results in mid-wall blisters and cracking that runs parallel to the wall. These cracks may join at different levels through the pipe wall to create through-wall cracks (stepwise cracking – SWC). Surface blisters may also contain cracks. A variation of the HIC mechanism is stress-orientated HIC (SOHIC).³

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 to as sulfide stress corrosion cracking (SSCC).³

Laminations are unwanted discontinuities that lie parallel to the pipe surface and are usually marked by a concentration of non-metallic material. They are caused by the rolling out of inclusions, blowholes, or pipes in the parent material. Laminations are typically not significant, but they may mask cracks. Surface-breaking laminations can initiate fatigue cracks and hydrogen cracking in the pipe body. Laminations can create detrimental planar features and cracking in welds when the two coincide, such as SAW weld hot tears due to laminations.^1,3

Hook cracks occur in combination with nonmetallic inclusions or laminations at 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.³

Weld Cracking

Lack of Fusion

ERW and HFI Seam-Welded Pipe Planar Features

Toe Cracks

Porosity

Lack of Penetration

Picture of an axial crack in the seam weld of a pipeline.

In the weld metal of fusion welds (i.e., submerged arc welding (SAW) seams and multiple process girth welds), cracks can be both hot or cold. Hot cracks, which result from solidification mechanisms, can occur in the longitudinal and transverse directions, as well as “crazed.” These cracks usually form in the pipe mill for seam welds, or during pipeline construction for girth welds.³

Cold cracking in weld metal requires the presence of hydrogen, a susceptible weld metal microstructure, and stress, thus, cracks typically form in the longitudinal direction. While cold cracking generally occurs during or just after the welding process, it can also be delayed.³

There are types of environmentally assisted cracking that can readily occur in the weld metal. In EFW and ERW welds, “cracking” will most often be located at the midpoint of the bond line. Despite these features 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.³

Picture of lack of fusion cracking in pipeline material

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. It can be caused by a number of factors during manufacturing, including welding process parameters, contamination, and poor QA & QC.³

A lack of fusion in an ERW pipe appears as an axial, crack-like discontinuity at the midpoint of the weld bond line. Depending on its characteristics, it is also referred to as a cold weld, penetrator, or stitching. As with SAW welds, this discontinuity can be caused by a number of factors associated with process parameters, contamination, and so on.³

The toes of fusion welds are particularly susceptible to cold cracking due to the microstructure present in the HAZ, as well as the change in shape. This discontinuity in profile acts as a stress raiser, locally increasing the applied stress during formation. Later during service, it acts as a stress concentrator for fatigue cracking and similar phenomena.
Depending on location of the toe and the shape of the underlying weld, the crack may remain in the HAZ or propagate into the parent or weld metal. Of course, toe cracking is limited to fusion welds, because only these have the geometry where toes are present. Hence, toe cracking is not present in EFW or ERW pipe.^1,3

Porosity is a cavity-type discontinuity formed by the entrapment of gas in the weld metal during solidification. It can be present as isolated pores, clusters of pores, or elongated cavities, also known as wormholes. While it is not a form of cracking, it can add to the complexity of inspecting a weld.^1,3

Lack of penetration in pipeline material.

A 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. ^1,3

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