Understanding Hydrogen’s Impact on Pipeline Integrity

A current area of focus in industry is how hydrogen can accelerate the growth rate of pre-existing cracks, much more significantly than natural gas, while at the same time degrade material properties such as fracture toughness and ductility that otherwise act as safety barriers to the severity or intensity of pipeline failure outcomes. This has the potential to compromise the safety and reliability of aging pipeline infrastructure but when coupled with conservative assumptions in absence of proven data, the viability of hydrogen pipeline projects can be significantly impacted.

While there are various emerging models that can predict the fatigue response of pipelines to hydrogen, there is a cautionary level of conservatism built in and not every model can handle the variable changes that Pipeline Operators need to assess viability. There is a growing need to understand the limitations of different models, underlying assumptions, and being able to recognise when you have the adequate data to apply less conservatism. While conservatism is an industry best practice, if applied too broadly in response to limited data, it can cloud the perception of hydrogen being a viable long term energy solution.

Key Considerations for Accurate Life Prediction

There are two crack growth models codified in international pipeline standards – the ASME B31.12 and IGEM TD/1 Ed 6 Supp 2 models. Both are conservative upper bounds of hydrogen-assisted crack growth but remaining life predictions are significantly impacted by a few key inputs that can lead an Operator to believe their pipeline cannot tolerate hydrogen.

Having justifiable and reasonable answers to the following questions will not only provide the most accurate and representative remaining life prediction, but will provide Clients with pragmatic next steps to take for repurposing their assets.

1. What population of cracks are known or suspected to exist?

2. How much hydrogen is in the system? <10%? 100%?

3. What does the pressure profile look like? Is it stable? How often do outages happen?

4. What are the material properties of the pipeline and how do they degrade in hydrogen?

Assessing the Initial Crack Population: This difficult question to answer is a key input into a remaining life assessment. An inaccurate or unsubstantiated assumption on length and depth can result in unmanageable life predictions (<10 years in some cases). Crack ILI tools can detect and size cracks along a pipeline but also tend to be the most expensive inspection technology on the market, over 5x the cost of conventional ILI. Alternatively, operators can predict the population of defects that would have just survived a pressure test by calculating a critical crack curve. Both approaches are data-informed and justifiable starting points, instead of arbitrarily selecting a crack size that may or may not be reasonable.

Hydrogen Concentration – Choosing the Right Model: A third crack growth model, known as ASME Code Case 220, is not yet codified in standards but is capable of handling varying hydrogen partial pressure, residual stress, and load ratio and was developed largely as a result of industry feedback to ASME B31.12. ASME B31.12 and IGEM models assume 100% hydrogen concentration as a conservative upper bound for growth rates.

The following graph shows the difference between the three models, and the flexibility Code Case 220 can provide for hydrogen blends [Figure 1]. Following the dotted lines, the two 100% hydrogen-calibrated models are conservative compared to the partial pressure model, resulting in differences of hundreds of years of fatigue life. In other words, the Code Case 220 model should be used for blended hydrogen pipeline

Figure 1: Fatigue Crack Growth Curve

Operating Envelope: Knowing how much a pipeline system cycles can be critical. Large, infrequent pressure cycles, such as outage windows, account for the vast majority of fatigue damage to a pipeline. Once a crack population is defined, the maximum amount of permissible daily pressure cycling can be calculated, which assists Operators in establishing the future operating philosophy of the repurposed system.

Material Properties: The effects of hydrogen degradation on important material properties such as fracture toughness (a measure of the ability to resist crack growth) are well-studied. Conservative assumptions can made to account for the degradation effects of hydrogen (i.e. 50% reduction) and as research evolves, it is becoming clear that with the wide range of variables in steel making, predicting how much degradation is difficult. Physically removing a pipe specimen and testing in a hydrogen environment provides the most clarity will likely improve remaining life predictions substantially.

Optimising Hydrogen Repurposing through Data-Driven Insights and Testing

Fundamentally, these simplified hydrogen crack growth models were designed to streamline the energy transition for Operators. It is critical to have sound, justifiable assumptions to reflect the actual operating environment and to also commit to upfront costs on material testing to eliminate the wide uncertainty associated with hydrogen degradation. When blending concentration is known or you are trying to define a realistic operating envelope, leveraging the right model for the circumstances is important because it can help to show the net present value of hydrogen repurposing projects to be favourable.

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