Hydrogen pipelines have specific challenges that must be considered during the early design stages. For safety, established guidance calls for the use of conventional and well-established materials, as well as conventional and precautionary pipe designs.
However, fast-developing demands for hydrogen will soon require higher pressures and volumes, with potentially longer cross-country pipelines, which create new challenges. Some experts have also suggested using existing oil and gas pipelines to distribute hydrogen instead. This idea comes with its own issues, including weakened structural integrity, stress and pressure limitations, material fatigue, pressure cycling, and hydrogen embrittlement (HE).
To further explore the possibility of using existing oil and gas pipelines to safely transport hydrogen, George E. Varelis, chief advanced analysis engineer for Worley, recently investigated the differences in design and performance requirements between conventional oil and gas pipelines and hydrogen pipelines.
“Hydrogen pipelines may not operate in the same manner as traditional oil and gas pipelines, and significant pressure fluctuations may be encountered,” Varelis said. “Cyclic loading conditions are a major concern in the design of new hydrogen pipelines, as well as for attempts to repurpose the existing oil and gas pipeline networks.”
Three objectives
The study evaluated the existing design rules and infrastructure challenges for hydrogen pipelines made of carbon steel (CS). Varelis focused on three key aspects: hydrogen-specific design guidelines for pipelines, pipeline bends (which are crucial and vulnerable parts of a pipeline), and comparing hydrogen pipelines and oil and gas pipelines to determine what revisions and upgrades are needed to refurbish oil and gas pipeline networks.
Among the researchers’ key considerations was the effect of hydrogen on carbon steels (CS), which have been widely used for onshore hydrogen piping and pipelines. The CS materials specified in accordance with current standards and codes can be used in gaseous hydrogen service provided the selected grade has been designed and qualified for use under the specific service conditions.
“Thus, the corrosion risk for the pipeline is considered to be low, with corrosion being avoided via operational controls,” Varelis said. “The primary threats for CS pipeline materials in hydrogen service are hydrogen embrittlement and hydrogen accelerated fatigue.”
As for hydrogen embrittlement itself, this phenomenon occurs when hydrogen atoms diffuse into the CS lattice structure, resulting in reduced ductility and fracture toughness, increasing susceptibility to stress-induced cracking.
“HE can facilitate crack initiation and accelerate crack propagation, ultimately compromising the structural integrity of the pipeline over time,” Varelis added. “Material variables that affect susceptibility to HE include alloy chemistry, microstructure, hardness, and strength level.”
Hydrogen-accelerated fatigue and degradation in fatigue endurance limits have been observed in tests of susceptible materials via both pre-cracked and smooth specimens in dry hydrogen gas environments.
“The impact has been observed in carbon steel of different grades,” Varelis explained. “The operation of a hydrogen pipeline in a cyclic nature will increase the risk of fatigue crack initiation and propagation.”
Although it’s generally accepted that existing pipeline networks can be repurposed for hydrogen use, they must meet the following requirements: structural integrity, pressure cycling, repair of all identified defects, and compliance with the latest health and safety regulations related to hydrogen transportation and storage. This is particularly important because the requirements for separation distance and cover depths may be stricter for hydrogen pipelines compared to conventional hydrocarbon pipelines.
Varelis conducted finite element analyses (FEA) studies on two representative example cases for hydrogen pipelines: a model of a typical pipe bend and a pipeline containing a typical expansion loop configuration. Both reflect a realistic operation profile for hydrogen pipelines. A rigorous FEA comparison showed that distribution of stresses in the hoop and axial direction of a pipe bend is significantly different than those of a straight pipe and depends heavily on the bend radius.
Mitigating impacts
Since carbon steel hydrogen pipelines face unique challenges that stem from the hydrogen’s embrittlement effects on the pipe material, for new hydrogen pipelines, a low-stress design option—and low yield strength material combination—is the preferred option for reducing embrittlement risks.
“Seamless pipes are also preferred over seam-welded pipes, as the longitudinal seam weld might be another area of potential defects affecting fatigue life,” Varelis noted. “The downside of selecting seamless pipes is their limited maximum outer diameter, which may not be enough if the pipeline internal volume is also intended to be used for hydrogen storage.”
However, unlike oil and gas pipelines, newly built hydrogen pipelines are expected “to experience a significant number of pressure daily fluctuations due to the way hydrogen is produced, stored, and distributed to off-takers,” he continued.
Overall, this work has demonstrated the need for detailed assessment of hydrogen pipelines containing bends and revealed the shortcomings of current design options for hydrogen pipelines.
“It is also demonstrated that the design selection process is predominantly fatigue-driven,” Varelis concluded. “As a result, the use of detailed numerical modeling at early design stages allows for a more precise estimation of stresses at critical locations, leading to better selection of cross-sectional characteristics and routing configurations.”
Mark Crawford is a technology writer in Corrales, N.M.
