Author: Mehdi Fardi
The Evolving Challenges of Pipeline Design and Assessment
From Barlow’s Equation to Advanced Fatigue and Fracture Mechanics Assessments for Hydrogen Pipelines
Over the past few decades, pipeline designs and methods for assessing pipelines have repeatedly had to be adapted to the industry's constantly changing conditions. With the introduction of hydrogen, the industry now faces new challenges. Our expert, Mehdi Fardi, Principal Engineer at the ROSEN Group, takes a closer look at these emerging challenges and discusses how the industry could solve them.
For decades, the pipeline industry has relied on fundamental engineering principles to design and assess the integrity of pipelines. Among these principles, some of the most essential equations include Barlow’s equation, which establishes a relationship between internal pressure, hoop stress, wall thickness, and pipe diameter. This equation has historically served as a cornerstone for pipeline engineers in determining safe operating pressures. In conventional pipeline design, the selection of pipe thickness follows either a prescriptive or a performance-based approach. However, regardless of the methodology, the design process traditionally begins with selecting the minimum pipe thickness based on pipe size, steel grade/strength, and design factor using Barlow’s equation. Once the pipe thickness is determined, pipelines are evaluated against various threats, including externally induced stresses/strain, external interference, and running ductile fractures.
While effective for conventional applications, this simplified approach does not fully account for the complex challenges encountered in extreme environments such as offshore, sour service, or geohazard-prone areas. Pipelines operating in these harsh conditions must withstand external factors such as seismic activity, landslides, mine subsidence, and frost heave, as well as internal threats like corrosion, fatigue, and failure mechanisms extending beyond simple hoop stress considerations.
As the industry started facing the integrity management challenges of the existing pipelines and the design and construction of new offshore and sour service pipelines, engineers recognized the need for more sophisticated assessment methods. Introducing fracture mechanics, fatigue analysis, and Engineering Critical Assessments (ECA) marked a significant advancement in pipeline integrity management. Offshore pipelines and risers are typically subjected to high strain during installation and experience cyclic loading in service, primarily due to thermal fluctuations and dynamic effects such as Vortex-Induced Vibration (VIV). These demanding conditions require rigorous ECA of girth welds to ensure their fitness for service under representative loading scenarios.
Sour service pipelines, which are exposed to hydrogen sulfide (H₂S), presented additional risks such as sulfide stress cracking (SSC) and hydrogen-induced cracking (HIC). Addressing these challenges required improved material selection, advanced stress analysis, and the implementation of mitigation techniques to enhance long-term structural integrity. These developments signified a shift from purely stress-based design methodologies to a more comprehensive fracture mechanics-based approach.
Hydrogen pipelines require a new approach to pipeline design and conversion
Today, as hydrogen emerges as a key energy carrier in the global transition to cleaner energy, the design of new pipelines or the conversion of existing pipelines faces unprecedented challenges. Hydrogen pipelines cannot be designed or assessed using traditional oil and gas methodologies; instead, they demand a fundamentally different approach rooted in fracture mechanics and hydrogen-material interactions. Hydrogen embrittlement – a phenomenon in which hydrogen atoms penetrate metal structures, reducing ductility and toughness – poses a significant risk to pipeline integrity. Unlike conventional oil and gas pipelines that typically exhibit ductile failure modes, hydrogen pipelines are far more susceptible to brittle fracture, even under relatively low stress conditions.
This paradigm shift necessitates reevaluating the skills and competencies required for pipeline design. Engineers must develop expertise in fracture mechanics, hydrogen-material compatibility, and novel testing and validation methods to ensure the safe transportation of hydrogen. Currently, a significant knowledge gap exists in the industry, and without immediate and comprehensive training, the deployment of hydrogen infrastructure could be hampered by safety risks, failures, and project delays.
To mitigate these risks and facilitate the transition to a hydrogen-based energy economy, global collaboration between academia, industry, and regulatory bodies is essential. Establishing standardized training programs, qualification frameworks, and research initiatives in hydrogen pipeline engineering will be critical in addressing these challenges. A proactive approach in competency development will not only ensure the safe and efficient operation of hydrogen pipelines but also prevent costly failures and accelerate the transition to sustainable energy solutions.
Excursion: A personal journey into fracture mechanics: Closing the circle from theory to hydrogen pipeline design
My first serious exposure to fracture mechanics and fatigue analysis came in 1997, during my Master’s degree, when I enrolled in a course titled Fracture Mechanics and Fatigue. At the time, I was already well-versed in elasticity, plasticity, plate and shell theory, and finite element analysis. However, fracture mechanics – particularly the assessment of structures containing cracks – presented a fundamentally different perspective.
What truly challenged my thinking was when the professor explained that fracture mechanics could be used proactively in the design of structures. This was a radical idea to us as students. Until then, our understanding had been grounded in the assumption that structures should be defect-free by design. The notion that no structure is truly without flaws – and that it is both practical and necessary to design for the presence of cracks – was unfamiliar and, frankly, counterintuitive.
At that time, only a few industries, such as aerospace and high-pressure equipment manufacturing, applied fracture mechanics principles in their design processes. In the pipeline industry, the use of fracture mechanics was limited and specialized. Techniques such as the Maxey equation had been developed specifically for pipelines, but even those were typically reserved for integrity assessments, not design. The general fracture mechanics frameworks outlined in standards like BS 7910 or API 579-1 were not widely adopted in pipeline engineering.
This paradigm began to shift with the emergence of hydrogen as a critical element in energy infrastructure. The introduction of hydrogen pipelines exposed the limitations of conventional integrity assessment methods. Traditional toughness tests like Charpy impact testing were no longer sufficient or valid for these applications. Moreover, the lack of full-scale test data meant the industry could no longer rely on established, pipeline-specific fracture mechanics models. As a result, we had to turn to the broader and more rigorous methodologies outlined in standards such as BS 7910 and API 579-1.
After nearly two decades of applying fracture mechanics solely for integrity assessments, I encountered a hydrogen pipeline design project that brought everything full circle. It was then that I realized the vision introduced to me in that 1997 course – using fracture mechanics as a design tool – was becoming a reality. Since that moment, I have fully embraced the approach and have conducted ongoing research into its applicability to hydrogen pipelines.
Continued innovation and adaption to new and emerging challenges
In conclusion, the evolution of pipeline design and assessment from simple stress-based methodologies to advanced fatigue and fracture mechanics-based approaches reflects the industry’s need to adapt to new and emerging challenges. The advent of hydrogen as an energy carrier underscores the necessity for continued innovation, rigorous material testing, and interdisciplinary collaboration to ensure the reliability and safety of next-generation pipeline infrastructure.