Crack on Offshore Pipeline

Cracks may develop in pipelines at any time. During fabrication, cracks may originate from casting defects or plate rolling, and a family of cracks and crack-like defects may arise during the seam and girth welding operations. These features, along with handling and construction faults, are subject to a pre-commissioning pressure test.

During pipeline operation, existing defects may grow due to fatigue. Other in-service crack growth mechanisms include sour service cracking (HIC and SSCC) and external stress corrosion cracking (SCC).

Types of crack

-  Stress Corrosion Cracking (SCC)

External stress corrosion cracking on high-pressure pipelines is recognized in two forms: high pH and near-neutral pH. SCC cracks can initiate and grow in a range of conditions, including predominantly intergranular cracking in alkaline conditions and transgranular cracking in neutral pH environments. SCC can occur in a wider range of restricted aqueous environments at the pipe surface, and in extreme cases SCC has been confirmed on above-ground pipelines.

Stress Corrosion Cracking
The corrosion creates crack-like features aligned at right angles to the principal stress. In most cases, the product pressure in the pipeline creates the principal stress, so the cracks are aligned parallel to the axis of the pipeline. External stresses such as ground movement can give rise to cracks at almost any angle through to fully circumferential.

The threshold for SCC crack initiation is at or about the actual yield, so in the absence of a high residual stress or an externally imposed stress, SCC is not expected in operational pipelines. However, the threshold for crack initiation is reduced by stress or pressure cycling, and in cases where pipelines experience large diurnal fluctuations, the threshold stress for crack initiation may be below the mean operating stress. Some steels show a greater susceptibility than others. On occasion, this difference in material susceptibility has been the main factor in determining where high pH SCC has become an operational problem.

Temperature is also a key factor controlling the rate of high pH SCC crack growth. If all other conditions remain unchanged, crack growth rate increases with temperature. SCC risk can be minimized on new pipelines by careful coating selection and preservation of coating condition through the construction process. To reduce SCC risk, priority should be placed on the long-term adhesion performance of the coating and its resistance to adhesion loss from water uptake, cathodic disbonding, soil induced loading and impact or gouging.

-  Hydrogen Induced Cracking (HIC)

Sour service pipelines are vulnerable to HIC in the presence of water. It can occur in pipeline steels of any strength and is generally associated with non-metallic inclusions, particularly elongated manganese sulfides.

Diagram showing how Hydrogen induced Cracking (HIC) damages steels in sour service

Features within the pipewall appear as cracks, but features near the surface appear as blisters or bumps. Acid corrosion takes place on water-wetted areas inside the pipeline. Hydrogen is produced by this corrosion reaction, but in the presence of sulfide, scales on the steel surface rather than being liberated as a gas. The atomic hydrogen diffuses into the steel, forming blisters in the microscopic voids around non-metallic inclusions. The gas pressure in these blisters generates very high localized stress, which initiates cracking along lines of weakness in the steel.

HIC develops as flat cracks in the rolling plane of the pipe material. Crack colonies develop, and failure often occurs as colonies link together in a stepwise fashion. For this reason, HIC is sometimes called stepwise cracking.

Hydrogen Induced Cracking (HIC)

-  Stress-Oriented Hydrogen Induced Cracking (SOHIC)
A special form of HIC may occur when local stress concentration is very high in a sour service pipeline. High stress fields allow the hydrogen to accumulate without the need for inclusions or other interfaces. For example, some types of spiral-welded pipe exhibit highly stressed regions close to the seam weld, caused during the edge forming process. Stacked arrays of HIC can form in these regions, leading to rapid stepwise cracking failures.

Stress-Oriented Hydrogen Induced Cracking (SOHIC)
 -  Laps

These crack-like surface defects originate during the rolling process used to produce the plate or strip from which pipe is fabricated. Surface cracks in the hot slab become oxidized, which prevents them from welding to the adjoining metal during subsequent rolling. The cracks remain on the outer layer of the steel and are rolled over to become surface-breaking defects at a very shallow angle. They can occur in any position around the pipe.

Laps cracking
 -  Hook Cracks

These defects in the longitudinal weld occur during manufacture of the pipe, when inclusions at the plate edge are turned out of the plane of the steel during the welding process. They may pass the manufacturer’s initial hydrotest, but fail later due to metal fatigue. It is the turning out of the metal at the weld that gives the characteristic “hook” or “J” shape to the crack.

Hook cracks
 -  Girth Weld Cracks

Although girth weld cracks can occur in any position around the weld, they are most often found at the 6 o’clock mark inside the pipe, which is the position of maximum stress during movement of the internal clamp, when only the root bead has been made. The cracks are formed almost exclusively during construction because of inadequate fit-up and excessive stress.

Girth weld crack
 -  Fatigue Cracks

Metal fatigue is caused by repeated or fluctuating stresses whose maximum value is less than the tensile strength of the material. They start as minute cracks which grow steadily in reaction to pressure cycling, physical deformation of the pipeline and other mechanical stresses.

Fatigue cracks
 -  Narrow Axial External Corrosion (NAEC)

Although this is not strictly a crack, it is one of a number of defects associated with the seam weld, which are difficult to detect with standard metal loss tools because of their axial orientation. It is caused when the pipewrap “tents” over the seam weld bead, allowing moisture to enter and encouraging corrosion. The resulting loss of metal parallel to the seam can result in rupture. The PII Pipeline Solutions TranScan™ tool was developed specifically to locate NAEC and other seam weld defects.

Narrow Axial External Corrosion (NAEC)



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