Obtaining Materials Properties Using an In-Situ Method: Case Studies

As oil prices continue to remain low due to high market supply, producers are pulling back on spending as evidenced by revised earning forecasts and reduced budget spend. While this is expected to continue in the short term, the business case for non-destructive testing (NDT) services relies on cost-avoidance strategies as operators are faced with tough decisions. Despite these short-term trends, the market for the oil and gas industry is predicted to grow 7% per year through 2033 reaching a total of $7.5 billion. The North American share of this market is estimated to be 35%, which is expected to rise to approximately $2.5 billion in 2033. While these numbers are estimates, the overall picture is clear: demand for NDT services and technologies is still significant and growing.

This article will first explain some of the key drivers in the increased application of nondestructive testing methods and technologies. It sets the context for the continued demand for nondestructive solutions to improve asset integrity management in the oil and gas space.

We then explore two use cases that illustrate how automated ball indentation (ABI®), a unique nondestructive testing method, has been utilized to solve asset integrity challenges. We start with a case describing how the indentation method was used after a catastrophic gas pipeline failure to validate Fitness for Service (FFS) for similar pipelines in the same area. The second use case illustrates how the technology was used to validate flanges for service in offshore applications involving low temperatures.

Growth Key Factors

Growth in the NDT services market is driven by several key factors.

Aging Infrastructure — While a challenge worldwide, this problem is most acute in North America and the EU. Growing energy demand will continue to drive motivation to keep existing assets in service. This will continue to increase demand to validate the material properties, ensuring that these assets are safe to operate and capable of achieving adequate product throughput and productivity levels.

NDT Technology Growth — Today, NDT technologies and methods, such as optical scanning, ultrasound scanning, x-ray scanning, and eddy current solutions, are used to identify anomalies and flaws in materials at the surface and slightly below. Automated ultrasonic testing (AUT), phased array ultrasonic testing (PAUT), computed radiography (CR), and digital radiography (DR) are utilized for weld inspection, corrosion detection, and corrosion under insulation detection. Mechanical testing methods like automated ball indentation (ABI®) can provide information about material properties, such as tensile properties, material grade, and fracture toughness. This data helps operators to understand the impact of anomalies and flaws on materials properties and to make decisions about repairs, maintenance, and equipment operating capabilities. These and other NDT methods can offer operators options and insights at a lower cost with quicker response than traditional destructive testing.

Integration with AI and Predictive Analytics in Digital Twins — Digital Twins are increasingly becoming part of the discussion on capital asset management. As digital twins leverage AI and ML algorithms, they seek to incorporate inspection data. NDT methods are becoming more able to automatically detect and classify defects (cracks, corrosion, etc.) and predict failure and degradation trends based on historical data patterns. Having more physical testing data available allows these systems to become more accurate and their analytics more trustworthy and valuable for making decisions more quickly.

Regulatory Focus — A greater emphasis has been placed on traceable, verifiable, and complete (TVC) data for pipelines in recent years by PHMSA. Under PHMSA's "Gas Mega Rule" (49 CFR §192.607), non-destructive testing (NDT) methods are explicitly permitted for verifying pipeline material properties (e.g., toughness, grade, yield strength) when original traceable records are missing. This applies to older pipelines, many of which lack verified documentation of their material properties and ability to continue in operation. This also applies to verification of maximum allowable operating pressure (MAOP) for pipelines that lack adequate documentation. API standards and recommended practices encourage the use of TVC for assessing susceptibility to cracking and selecting the appropriate inspection technologies, and reinforce the need for high-quality input data in integrity assessments.

Decarbonization may also add to the demand for NDT as operators consider repurposing natural gas pipelines for hydrogen blending and carbon capture and storage systems (CCS).

Use Case 1: Fitness for Service — Natural Gas Pipeline

A catastrophic failure occurred in a natural gas plant on a cold winter night shortly following the leak of some liquid natural gas into a nearby natural gas line. The combination of cold temperature and high strain rate near a crack in the natural gas pipe resulted in the destruction of approximately a 12-meter section of a 508-mm (20-inch) diameter pipeline into several hundred small pieces.

The plant operator was concerned that the pipeline steel might have poor tensile and fracture toughness properties since the fracture surfaces of many of the small pieces indicated a brittle fracture. Furthermore, the plant operator was concerned about potential failures in three identical gas plants utilizing the same grade of pipeline steel. The remaining pieces from the exploded pipeline section were not sufficient to machine destructive tensile and fracture toughness specimens. Hence, the ABI® technique was used to measure the tensile and fracture toughness properties from multiple tests on a few pipeline pieces.

The ABI®-measured tensile and fracture toughness results provided the basis for the fitness-for-service assessment of the remaining natural gas pipeline sections of the plant. Although the pipeline piece containing the crack was not found at the time, the ABI® tests on several small pieces confirmed that the pipeline steel material met the mechanical properties specified for the seamless carbon steel pipe at the time of construction.

Multiple ABI® tests were conducted on a block machined from a small piece at several low temperatures. All ABI® tests were conducted using a 0.51-mm (0.020-inch) diameter tungsten carbide indenter at a speed of 0.01 mm/s (0.0004 in/s), or a strain rate of 0.014/s, to a maximum indentation depth of 0.076 mm (0.003 in). Stress-strain curves and fracture toughness values were measured from the individual ABI® tests.

The reference temperature, T₀, defined in ASTM Standard E1921-97 as the test temperature corresponding to a median fracture toughness level of 100 MPa√m (90.9 ksi√in), was determined from the ABI® tests at several low test temperatures. The ABI® tests determined a T₀ value of -24°C for the base metal of the pipe. The ABI®-determined T₀ value demonstrated that the pipeline material had a good static fracture toughness of 100 MPa√m at a low temperature of -24°C, which is lower than the normal pipeline operating temperatures in winter. However, this data must be coupled with an examination of crack defects. While the material had good static fracture toughness values, the existence of a small crack that developed during pipeline service, sudden exposure to a low temperature from a leak of liquid natural gas near the pipe and dynamic loading at a high strain rate resulted in brittle fracture.

It should be noted that all carbon steels have a low/brittle fracture toughness shelf with a median value of 30 MPa√m, regardless of their much higher fracture toughness values at higher operating temperatures. Furthermore, dynamic fracture toughness values are lower than the static values at the same temperatures (i.e., the fracture toughness median curve is shifted to the right by an amount depending on the yield strength of the steel material). The tensile and fracture toughness values of the small pieces demonstrated that the pipeline material for that heat was adequate (fit-for-service). The corrective action involved repairing the valves of the liquid line and replacing the exploded natural gas pipeline section.

Organizations are using the fracture toughness master curve to assess the fitness-for-service of their steel structural components and their welds. The ABI®-measured fracture toughness and its master curve does not require destructive specimens (i.e., hot tapping is eliminated for operating pipelines), and it is nondestructive and localized (the latter feature is highly desirable for small welds and heat-affected zones). Moreover, the ABI® technique provides both tensile and fracture toughness properties from each single ABI® test in a cost-effective and speedy manner.

Use Case 2: Evaluation for Fitness for Service for A350 Steel Flanges

The two goals of this project were (a) to measure the tensile and fracture toughness properties of A350 steel flanges and (b) to estimate the Charpy V-Notch (CVN) impact energy at -50°F (-46°C) from automated ball indentation (ABI®) tests conducted at room temperature. All miniature tensile and 0.45T CT fracture toughness specimens were machined from a large diameter flange. Mechanical properties testing included six miniature tensile tests and nine 0.45T CT fracture toughness specimens tested at various temperatures.

In order to assess the acceptability and the structural integrity of the A350 flanges according to ASTM requirements and according to API Standard RP 579 (Recommended Practice for Fitness-for-Service), fracture toughness specimens were tested at room temperature and at three low test temperatures. All tensile and ABI® tests determined that all the flanges met the yield and ultimate tensile properties. Six of the 0.45-inch-thick (0.45T) compact tension (CT) specimens failed by cleavage when tested at the test temperatures of -50°F (-46°C), -94°F (-70°C), -148°F (-100°C) and produced a valid reference temperature (T₀) of -175°F (-115°C) per ASTM Standard Test Method E1921. The multiple ABI® tests conducted on a sample machined from the same flange produced a reference temperature of -99°F (-73°C). These results show that the T₀ determined from the ABI® tests at low temperature is conservative by 42°C for the same A350 flange.

In-situ (field) ABI® testing of the desired flanges (intended for offshore service) can be conducted only at room/ambient temperature; therefore, three to five ABI® tests were conducted on each flange at room temperature in order to determine a very conservative T₀ for each flange. The room-temperature ABI® tests produced T₀ values ranging from -30°C (-22°F) to -22°C (-8°F) for all flanges. Per ASTM Standard E1921, and since all T₀ values were within 20°C for all twelve (12) flanges, they are considered to be of the same A350 material, and the average T₀ for all flanges can be taken as -26°C. This T₀ value of -26°C is very conservative by 89°C (as compared to the T₀ of -115°C from the destructive fracture toughness specimens). This is consistent with the results of the previous PRCI Report L52280 (2007), where 15 ABI® tests conducted at room temperature determined a T₀ of -15°C (i.e., conservative by 71°C).

The very conservative ABI®-determined T₀ values of -30°C to -22°C were used to calculate the fracture toughness values at -46°C (-50°F), and these values were then used in Equation F.66 of API 579-1/ASME FFS-1 2007 "Fitness-For-Service" to calculate lower bound estimates of the CVN values for all 12 flanges. The estimated CVN values ranged from 24.6 ft.lb to 28.4 ft.lb (33.6 J to 38.8 J), which are higher than the minimum CVN requirement of 15 ft.lb (20 Joules) at -50°F (-46°C). Hence, all 12 A350 steel flanges were determined to be fit for offshore service.

Conclusion

In a business environment where demand for accurate, cost-effective, fast material testing methods and technologies continues to increase, it is vital that asset owners and operators are aware of the various options at their disposal. Deciding when and how to repair or conduct maintenance on pipelines, pressure vessels, storage tanks, and other infrastructure presents tactical and strategic challenges for asset integrity groups.

Knowing the physical properties of their assets, particularly those assets that operate in high-risk locations or extreme environments, is a must for asset owners and operators. This data can help support specific decisions about a pipeline section or pressure vessel. It can also feed into analytical systems that use AI or ML to predict failure risks throughout their asset bases. Deterministic data on material grade, tensile strength, and fracture toughness enables AI and ML systems to refine their algorithms so that they direct the asset operators to the highest priority repair, remediation, and maintenance cases.

As testing technology continues to evolve becoming more portable, tougher/more ruggedized, more cost-effective, and easier to use, it will allow asset integrity leaders to deploy NDT more broadly and frequently across their asset bases. This will result in safer and more productive assets.