WHAT ARE HARD SPOTS IN PIPELINES?
Areas on the pipe surface with a hardness level higher than that of the surrounding metal are referred to as hard spots. They result from a localized thermal cycle producing a different (and harder) microstructure to the surrounding parent material. The specific thermal cycle determines what microstructure is present, see Figure 1. The microstructure expected in hard spots with high levels of hardness is martensite. The atomic lattice structure of martensite is body centered, tetragonal and has different mechanical, corrosion and magnetic properties to the steel phases of the unaffected base metal. In addition to these martensitic hard spots, other types of pipeline hard spots exist. These have a different microstructure to the adjacent pipe body, again caused by a local thermal cycle, but they are not fully martensitic. The hardness difference can be detected, but the levels of absolute hardness are lower.
Figure 1 – Schematic of a time temperature transformation diagram
The consensus is that pipeline hard spots usually occur during the plate/strip or pipe-manufacturing processes. They could also be produced during welding operations, either in the pipe mill or during construction. Consequently, hard spots present in pipelines nowadays have survived mill acceptance hydro tests and commissioning hydro tests.
Since hard spots have been present since pipeline construction, they could be considered stable features. However, they can be more susceptible to active cracking mechanisms and the threat of hard spots is increased when they exist in combination with other threats, e.g. bending strain, corrosion, etc. Pipeline hard spots therefore present a latent threat. Since there have been several pipeline incidents related to hard spots in North America in recent years, regulatory authorities are looking at this issue with increasing attention, making hard spot detection essential for pipeline operators.
Safety-relevant defects associated with hard spots are described below:
There are different types of hydrogen cracking, including hydrogen induced cracking (HIC), stress oriented hydrogen induced cracking (SOHIC) and sulfide stress corrosion cracking (SSCC). More details can be found here.
The relationship between steel hardness and susceptibility to hydrogen cracking has been recognized for many years and there is evidence suggesting that harder microstructures can be more susceptible to HIC and SOHIC.
STRESS CORROSION CRACKING
STRESS CORROSION CRACKING (SCC)
External stress corrosion cracking (SCC) is categorized as either high pH or near neutral SCC. The mechanisms and risk factors associated with these mechanisms are slightly different. Find more information here: high pH, near neutral SCC.
The exact effect of hardness on susceptibility to either near neutral pH SCC or high pH SCC is unclear. However, the presence of hard spots does appear to increase the risk of SCC. Contributory factors could include the different grain structure, the presence of residual stress near hard spots and galvanic differences between the hard spot and surrounding material.
METAL LOSS CORROSION
METAL LOSS CORROSION
Any form of metal loss corrosion (“normal” surface corrosion not associated with cracking) results from galvanic differences within a structure, a corrosive environment, and the presence/generation of local anodes and cathodes. The differing microstructures between hard spots and the surrounding matrix, together with the increased levels of residual stress to be expected in the vicinity of hard spots, result in galvanic differences between the hard spot and the surrounding matrix and hence potentially an increased susceptibility to metal loss corrosion.
In addition to the direct effects of galvanic differences, there is also an indirect impact of hard spots on coating adhesion. During coating operations, the shot blasting and surface preparation regime will be predicated on achieving the optimal surface roughness/key prior to coating. A surface preparation regime, which will give a good finish to the surrounding matrix, will not necessarily give the same finish to the hard spot, leading to potential lack of coating adhesion to it.
Fatigue cracking is mechanically induced cracking resulting from cyclic loading. More information can be found here.
Although fatigue cracks are normally thought to be independent of microstructure, there is still anecdotal evidence that hard spots can lead to increased susceptibility to fatigue. This may be attributed to the presence of dimensional sharp corners/stress raisers at the interface between the hard spot and the surrounding material or the influence of residual stress.
WHERE DO HARD SPOTS OCCUR?
Hard spots can be generated at different locations on the pipe surface, depending on their origination. In those cases, where welding is implicated, pipeline hard spots are generally concentrated near seam and/or girth welds and more likely on the outside surface. If hard spots emanated from the plate/strip or pipe-manufacturing process, they can occur at any position along the length and circumference of the pipeline, and they can be on the external surface or the internal surface of the pipe, as well as both.
Regardless of the mechanism, hard spots are generally a surface phenomenon, with the maximum hardness being present at either the pipe surface or sub-surface.
HOW TO MANAGE
A Holistic Approach
Management of pipeline hard spots is challenging. The main difficulty arises from the fact that hard spots can act as a focal point for other threats, as outlined above. Any management strategy for hard spots, therefore, needs to take into account all other threats and the increased susceptibility for each of them resulting from hard spots. This holistic approach is outlined below.
ACCURATE HARD SPOT INSPECTION AND DATA COLLECTION
The starting point of any coherent holistic integrity management strategy is data collection. Hard spots can occur at any location along the circumference or the length of a pipeline, making it imperative to use a robust, reliable inspection technology.
Debris can deposit and collect within the line at any stage of a pipeline system. In addition to cleaning pipelines at regular intervals, performing pipeline cleaning prior to in-line inspections (ILI) is essential. The sensor technologies used in pipeline inspections require cleanliness to achieve reliable and accurate ILI results. Therefore, a clean internal pipeline surface increases first run success, enables accurate integrity assessments, and reduces operational risk and potential costs.
- RoMat DMG
MAGNETIC FLUX LEAKAGE AND EDDY CURRENT (DMG)
A market-unique combination of Magnetic Flux Leakage (MFL) technology with Eddy Current (EC) enables reliable identification of hard spots along the entire pipeline. By combining full magnetic saturation with a well-defined lower magnetization, the detection, classification and sizing of hard spots is possible.
The susceptibility to and/or presence of hard spots is one aspect that should be taken into account as part of a pipeline management strategy, and should be integrated into the reporting, analysis, prioritization and assessment of any threats. Hard spots can therefore be considered as part of the bigger picture, aligned with other in-line inspection (ILI) data sets about features such as cracks, geometrical features, corrosion, and bending strains.
When considering the integrity threat of hard spots, in addition to assessing them as standalone threats, they must be considered in relation to other integrity assessments performed as part of the IMP.
- Bending Strain Assessment
BENDING STRAIN ASSESSMENT
Geohazards pose a major threat to pipelines. The resulting stresses can be hard to identify, difficult to predict and extremely challenging to control. Abnormal loadings can cause damage to, or even failure of, the pipeline from tensile fracture, buckling, or bending. In combination with threats like bending strain, the threat of hard spots is even increased.
More about our Bending Strain Assessment service
- Crack Assessment
The presence of hard spots can contribute to a higher susceptibility to environmentally assisted cracking due to local residual stresses. It makes sense that if we are managing the threat of hard spots, we need to ensure any cracking associated with those hard spots is assessed. To ensure the future integrity of a pipeline, it is important to not only reliably identify cracks and different crack types, but also to understand where cracking would be considered most likely to occur on a pipeline and what causes cracks; and hard spots are one such location.
More about our Pipeline Integrity Framework for Cracking
- Corrosion Growth Assessment
CORROSION GROWTH ASSESSMENT
As seen above, the threat of hard spots is increased when they exist in combination with other threats, such as corrosion. In addition, hard spots can also increase susceptibility to corrosion. For effective corrosion management, a detailed view of how corrosion has developed over time is important to increase the operator’s confidence in the future integrity of their pipelines. In addition to identifying active corrosion areas, identifying the causes is therefore also an indispensable input.
More about our Corrosion Growth Assessment service
- Verification of Material Properties and Attributes
VERIFICATION OF MATERIAL PROPERTIES AND ATTRIBUTES
Material property verification is the process of confirming or re-establishing a solid knowledge of the various material properties and attributes within a given pipeline segment. With this knowledge, it is possible to support safe pipeline operating pressure, as well as underpin critical integrity management processes such as fitness for service, remaining strength or predicted failure pressure assessments. Hard spots are more prevalent in specific vintage pipe types. These pipes also tend to have relatively poor material properties and it is therefore important to characterize those properties so that the threat can be managed.
More about Pipeline Material Verification
- Dent Assessment
When pipelines are subject to cyclic loading, dents can lead to local stress concentrations, resulting in cracking and fatigue growth. Dent fatigue life assessments address whether dents are susceptible to fatigue crack growth and whether this could affect the integrity of the pipeline during its operational life. If hard spots are combined with dents and geometric features the level of risk even increases.
More about our Dent Assessment service
- Threat Management and Management Plan
THREAT MANAGEMENT AND MANAGEMENT PLAN
Pipeline operators can make short-term decisions to ensure the performance, life and safety of their asset as soon as the in-line inspection data is available, analyzed and assessed. However, ROSEN's approach and our "big picture mindset" goes one step further and takes a more proactive, forward-looking approach. Bringing all the pieces together creates a robust, justifiable management plan that provides the optimal combination of activities (direct assessment, ILI, replacement etc.) to ensure safety and enable long-term decision-making.
We are convinced that this approach is also particularly important for the management of hard spots. Since hard spots can act as a focal point for several other threats, a management strategy that also considers other threats is even more important.
The availability of consistent and fully reconciled records is a basic requirement for the rapid and reliable assessment of the integrity of a pipeline asset. However, as the amount of data collected continues to grow, establishing a system of records in which all available data is readily accessible is becoming an increasingly critical issue for pipeline operators.
NIMA, our asset integrity management solution, is an intuitive and reliable framework that provides traceable, verifiable and complete records to enable better integrity management decisions.
Find out more about NIMA
Competence is an important aspect of managing pipeline assets. Therefore, learning, knowledge sharing, regular dialogue and continuous collaboration between stakeholders in the pipeline industry are essential to increase and maintain individual and organizational competence at a high level.
In response to this need, ROSEN has developed training courses, educational programs and qualifications specifically designed to address threats, including courses in Pipeline Risk Management, Pipeline Materials and Pipeline Integrity Management.
Find out more about Competence Training