Authors: Khan Thran, Simon Slater
Relevance of Hard Spots Regarding Engineering Critical Assessment
How to deal with the threat of hard spots
In a Nutshell:
Hard spots are there... and we know it. Of all the existing threats putting pipelines at risk today, hard spots have become one of the more heavily scrutinized in the last several years. This article explains how ROSEN experts successfully assist operators with managing the threat from hard spots using a rigorous and structured approach based on the RoMat DMG in-line inspection service.
A hard spot is defined as a localized area with increased hardness compared to the surrounding base metal [1]. The presence of hard spots, especially in combination with other threats such as geometric anomalies, manufacturing features, etc. have a significant impact on integrity assessment and the subsequent response. Up until recently, the industry has had a fairly rudimentary understanding of hard spots. Assessing this treat brings certain challenges, including but not limited to:
- What types of hard spots exist, and which are actual integrity threats?
- How do we confirm the presence and understand the nature of hard spots?
- How do we assess the integrity of hard spots?
- How do we focus on the right features and get the most of our integrity budget?
- How do we address the threat of hard spots? What are appropriate remediation actions?
Obtaining reliable data
The first step in any decision-making process is reliable data. Based on data published in the period from the 1960s to the early 2000s, pipes manufactured prior to 1970, especially A.O. Smith flash-welded pipes, are known to be particularly susceptible to hard spots. In 2021, there was a significant change to gas regulations, CFR 49 192. A newly created clause, CFR 192.632 – Engineering Critical Assessment for Maximum Allowable Operating Pressure MAOP Reconfirmation, explicitly states, "If a pipeline has segments that might be susceptible to hard spots based on assessment, leak, failure, manufacturing vintage history, or other information, then the ILI program must include a tool that can detect hard spots". Addressing that, ROSEN's RoMat Dual MaGnetization (DMG) service, which is based on standard magnetic flux technology, can serve as the starting point of a coherent holistic integrity management strategy. As hard spots possess different metallurgical properties – magnetic permeability, specifically – compared to the surrounding metal, combining full magnetic saturation with a well-defined lower magnetization enables accurate and precise hard-spot classification and sizing.
Performing an in-line inspection (ILI) is only the tip of the iceberg. Understanding the ILI results and differentiating the types of hardening material features that can exist in the pipeline is vital to ensuring the correct response is implemented.
Delivery of the RoMat DMG service
The use of RoMat DMG can support established integrity management processes and address intended regulatory guidance concerning the existence of hard spots as part of MAOP reconfirmation. The service is based around a framework with RoMat DMG as the cornerstone, as shown in Figure 1 below.
The framework starts with a susceptibility analysis, the objective of which is to interrogate available data pertaining to the pipeline and produce diagnostic plots to identify areas of increased susceptibility. If ILI information about cracks, metal loss, coating condition, etc. is already available, this data can be aligned to give more content to the susceptibility assessment.
Data evaluation process
Following RoMat DMG in-line inspection, data evaluation produces a feature listing that contains the reported hard spots and associated details. This listing is reviewed through an iterative process between evaluation and integrity engineers. During this stage, the ILI results are interrogated from an integrity perspective and subsequently integrated with other available information. This process enables a thorough understanding of the data reported, thus driving added value in the integrity assessment.
Following delivery of a final report, in-field excavations are critical when it comes to validating the ILI system and investigating/remediating the threat of hard spots as necessary. Hence, a prioritized feature listing from the integrity engineering team is incorporated into the RoMat DMG service to assist operators with dig selection. Field results are provided to ILI vendors, which allows for continuous system improvement with respect to probability of detection (POD), probability of identification (POI) and sizing. The integration of real-world findings with ILI signal data also helps to define what constitutes a threat and how to manage hard spots effectively.
Advancements in DMG technology
Our understanding of hard spots and the DMG technology has now advanced such that we can detect and identify three different types of hardening material features, as discussed below. The figures below show examples of the types of hard-spot features the industry has been aware of and that were the basis of the development of the RoMat DMG service. The hardness of this feature type ranges from 220 HBW to 400 HBW.
Extensive validation efforts have yielded an awareness and a characterization of two other types of hardening material features. The following features - shown in the figure below – were reported by RoMat DMG, and they are different from what we had expected in that they consistently show a unique circumferential "saw tooth" pattern on the internal surface, with no indication observed on the external surface.
The hardness measured on the internal surface of these features ranges from 190 HBW to 240 HBW. The internal hardness of these features, albeit elevated compared to base metal, is significantly lower than the threshold suggested for "hard-spot defects" by API 5L [1] of 327 HBW.
Another group of features reported by RoMat DMG has distinct hard spots on both the external and internal surfaces, as shown in the figure below; these are through-wall.
The hardness measured on the outside surface of these features can reach maximum values of up to 350 HBW and 240 HBW on the internal surface. Based on the measured hardness and dimensions, these features should, according to the criteria defined in API 5L [1] be classified as "hard-spots defects".
Addressing the threat from hard spots
Before discussing appropriate remediation options, it is important to consider the potential threats associated with hard spots. Hard spots are generally created during the manufacturing process (localized quenching/weld repairs); thus, they have likely survived mill acceptance and commissioning hydrostatic testing. Accordingly, it is tempting to assume that hard spots are non-injurious stable defects and that, therefore, hard spots present a latent threat.
Susceptibility to hydrogen cracking
One of the major concerns associated with hard spots is susceptibility to hydrogen cracking. Hydrogen can be present on the internal surface of the pipe from the transported product or on the external surface from the surrounding soils or the chemical reaction because of the cathodic protection system. Hence, for pipelines transporting products that do not contain a source of hydrogen, and without the presence of interacting features, internal hardening material features are less likely to be a threat. As replacing natural gas with hydrogen is becoming more prevalent, the threat from internal hardening material features will need to be considered as that topic matures.
External hardening material features, when interacting with other treats, such as third party damage, surface-breaking defects, etc., can increase the severity of such threats due to stress concentration and steel embrittlement. Therefore, DMG results may be aligned with other ILI and field data to identify any coincidence of hard spots with other threats. Moreover, coating disbondment information from EMAT inspection can help to pinpoint areas where hydrogen may migrate to the steel microstructure. With the presence of hard spots, these areas can become conducive to cracking. Figure 6 below demonstrates the alignment of DMG (top image) and EMAT ILI results (middle image), together with pipe-book information (bottom image). This exercise facilitates the identification of interacting features, if any, and the corresponding pipe properties.
What are the appropriate response criteria?
Currently, there is limited regulatory guidance regarding an appropriate response in terms of acceptance criteria or remediation. According to API 5L, hard spots with hardness exceeding 327 HBW shall be classified as a defect. Consequently, many operators rely on this manufacturing standard and use DMG-reported hardness of 277 HBW [2] as the threshold for in-field investigation and subsequent replacement if hardness exceeding 327 HBW is confirmed by in-field measurements. However, for features with ILI-reported hardness below 277 HBW or measured hardness below 327 HBW, what are the appropriate response criteria?
Prioritization should be given to hardening material features interacting with other threats as well as features located in areas with disbonded coating and a source of hydrogen. Besides cutting out and replacing a section of pipe, other available mitigation options include installing type B sleeve, grinding out the features and monitoring. Ultimately, response actions are dependent on the operator's risk tolerance and their IMP procedures. Tailored to the individual needs of the customer, ROSEN's RoMat DMG service framework assists the operator in managing uncertainty, maximizing the benefits of the in-line inspection and mitigating the threat as necessary.
Additionally, by combining multiple technologies, RoMat DMG can also collect metal-loss (RoCorr MFL service), pipe grade (RoMat PGS service), and mapping data. Figure 7 shows an example from the RoMat PGS service, with each data point representing the measured yield strength (YS) of an individual pipe along the pipeline and color coding used to illustrate the different populations identified. Hard-spot data reported by the RoMat DMG service has been overlaid with these populations, with their locations shown by the dashed vertical lines. All but one of the reported hard spots are contained within population "A1". This example shows how integrating multiple datasets can reveal risk factors and integrity threats that are populations-specific, thereby supporting integrity management going forward.
Conclusion
As part of their integrity management plan and/or ECA approach for MAOP reconfirmation, operators rely on ILI systems to accurately detect, classify and size pipeline anomalies. Moreover, understanding ILI capability and limitations as well as threat susceptibility is equally important. RoMat DMG is capable of discriminating hard spots and other types of hardening material, with ongoing continuous improvement and development.
Data integration provides a comprehensive picture
DMG technology will undoubtedly continue to mature and rise to meet the demands of protecting against the threat of hard spots, particularly as regulatory bodies provide updated and comprehensive guidance on safe pipeline operations. Moreover, while identifying the presence of hard spots in the pipeline system is vital to any IMP, it is only part of the bigger picture. Ultimately, data integration provides a more comprehensive picture of the potential threat from hard spots specifically – and of asset integrity generally - for the purpose of MAOP verification.
References
[1] API 5L, 46th Edition, April 2018, Section 9.10.6: “Any hard spot larger than 50 mm (2.0 in.) in any direction shall be classified as a defect if its hardness exceeds 35 HRC, 345 HV10, or 327 HBW, based on individual indentations.”
[2] With the RoMAT DMG tolerance of ± 50HBW for pipe body features, the lower bound of 327 HBW could be 277 HBW from ILI.