Supporting Safer Decisions for Hydrogen Pipelines
Read the associated technical paper “Extension and Validation of a Transient Fireball and Jet-Fire Framework for Buried Hydrogen and Blended-Natural Gas Pipelines.”
As hydrogen blending and dedicated hydrogen pipeline service move from technical evaluation to project planning, pipeline operators are being asked to make decisions with long-term safety, reliability, and regulatory implications.
Many of those decisions can build on decades of natural gas experience. But hydrogen introduces a new consequence-assessment question:
How can operators estimate the hazards from a buried hydrogen or hydrogen-blended pipeline rupture and minimize the hazards?
Unintended rupture and ignition of high-pressure natural gas, hydrogen, or blended-fuel pipelines can generate transient jet fires and fireballs that impose severe thermal and mechanical loads on nearby people and infrastructure. Those loads can influence emergency response limits, facility siting, barrier design, and integrity management criteria for gaseous fuel systems.
Consequence models need to estimate how a rupture evolves, where thermal radiation may be most severe, whether overpressure should be considered, and how those hazards may change with fuel composition, crater formation, and ignition behavior.
Hydrogen Changes the Consequence Question
Natural gas pipeline rupture modeling is supported by extensive full-scale testing and established understanding of flame structure, radiation footprints, and near-field flow dynamics for methane-dominated systems.
Hydrogen does not behave exactly like methane. Hydrogen flames differ from methane in buoyancy, adiabatic flame temperature, laminar flame speed, and speed of sound; associated overpressures during early ignition can also be markedly higher.
Methane-based assumptions cannot always be relied on when evaluating hydrogen blending or dedicated hydrogen service. Existing tools may be based on methane-focused empirical correlations, full-scale experiments, or detailed CFD simulations that can be computationally intensive for routine engineering use.
A model that is physics-based enough to be credible and interpretable, but efficient enough to support consequence assessment across relevant gas compositions and operating scenarios is needed.
Modeling the Early Seconds of a Rupture
To support hazard-distance estimates for large cross-country releases involving natural gas, hydrogen, and blended compositions, C-FER’s work extends a transient jet-fire and fireball framework for pipeline-scale applications.
The model treats a rupture as an event that changes over time rather than as a single, steady flame. It brings together several stages that are important to consequence prediction:
- Outflow behavior, which describes rapid depressurization and discharge conditions after a pipeline failure
- Crater formation, which is relevant for buried pipelines because soil ejecta can modify the near-field geometry
- Jet-fire and fireball evolution, which reflects the transition from an impulsively started jet to a buoyant flame structure
- Thermal-radiation prediction, which estimates incident heat fluxes on surrounding assets
- Transient overpressure generation, which can arise from rapid combustion within the developing cap or fireball
Buried-pipeline ruptures are not the same as aboveground releases. Where a rupture forms a crater, interaction between opposing jets and the crater walls can entrain significant air, which can move the maximum heat intensity closer to the ground than a jet fire without a crater.
The framework also accounts for the fireball phase. In the model, the fireball is coupled to a time-varying, decaying jet, allowing the early flame structure and radiative behavior to respond to evolving source momentum and improving applicability to hydrogen, natural gas, and blended-fuel releases.
Evaluating Models against Published Experimental Data
The model was evaluated against published large-scale and medium-scale experimental data for natural gas, hydrogen, and mixed-composition jet fires.
For typical crater-derived rupture scenarios, the thermal-radiation model performed strongly. Excluding fireball-only scenarios, 96% of predictions were within a factor of two of measured results. For measurements greater than 10 kW/m², the model achieved a geometric mean bias of 1.01 and geometric variance of 1.10.
Predictive error increased for horizontal jet fire scenarios, which were not explicitly considered during model development. This effect was particularly evident for hydrogen jet fires. However, performance for horizontal natural gas jet fires was comparable to that observed for crater-fire scenarios. Extending the model to horizontal jet-fire consequence analysis would require additional development, including improved representations of buoyancy-driven entrainment, release-orifice behavior, radiant fraction correlations, and ground-reflection effects.
The work also developed an overpressure model for slightly delayed hydrogen jet ignition. When validated against experimental data from unconfined vapor cloud and jet ignition experiments, 93% of peak-overpressure predictions fell within a factor of two of the corresponding measurements. Furthermore, the model demonstrated a geometric mean bias of 1.00 and geometric variance of 1.16, which were identified in the paper as acceptable levels of accuracy for engineering risk assessment.
Why Hydrogen Consequence Modelling Matters
The value of consequence modeling is not only estimating the size of a potential hazard. Operators can use that information to support better decisions about risk, integrity, reliability, emergency response, and future hydrogen deployment.
The project identified accurate prediction of thermal and mechanical consequences as essential for risk-based design, consequence modeling, and integrity management of gaseous fuel systems. The resulting framework is an interpretable, intermediate-complexity tool suitable for consequence analysis, integrity management, and development of hazard-extent formulae.
For operating companies, those applications can help inform:
- Rupture consequence assessments
- Integrity management assumptions
- Risk-based design decisions
- Emergency response limits
- Facility siting and safe-distance evaluations
- Barrier design and mitigation planning
- Hydrogen blending and dedicated hydrogen service evaluations
The practical benefit is reduced uncertainty. A validated framework allows operators to compare scenarios, understand where hazards may be most significant, and evaluate potential consequences using evidence rather than relying only on assumptions extrapolated from natural gas experience.
Understanding Fireball, Jet-Fire, and Overpressure Hazards
A rupture and ignition event can involve more than one hazard mechanism. Thermal radiation from the jet-fire and fireball can expose nearby people, equipment, and infrastructure to high heat fluxes. Overpressure can also become relevant for hydrogen pipeline consequence modeling because hydrogen can exhibit higher flame speeds and pressure-generation potential than natural gas.
The framework combines fireball-jet-fire modeling with overpressure modeling so early ignition behavior can be assessed more directly. This combination is particularly necessary for hydrogen modeling and can be combined with other considerations, such as ignition delay, crater formation, structure vulnerability, and population vulnerability, for quantitative analysis of rupture consequences and damage extent.
Thermal radiation and overpressure do not affect decision-making in the same way. A model that can evaluate both gives integrity and reliability teams a clearer technical basis for assessing potential hazard extents and for prioritizing additional analysis where conditions fall outside the model’s strongest validation range.
Supporting Safer Hydrogen Deployment
Hydrogen adoption introduces new variables that need to be understood before making decisions about deployment, monitoring, and risk management. The modeling work does not remove the need for engineering judgment, site-specific assessment, or future validation. Instead, it provides a stronger basis for evaluating rupture consequences under defined conditions.
Consequence modeling for hydrogen and hydrogen-natural-gas pipelines needs to account for the event as it evolves. The release rate changes, the crater affects the flame, the fireball and jet-fire contribute to thermal radiation, and overpressure may need to be evaluated depending on ignition behavior and fuel composition.
Modelling can support safer planning for hydrogen infrastructure by helping operators evaluate hazard distances, integrity-management criteria, emergency response assumptions, and risk-reduction priorities.
Future Hydrogen Projects
Future work is needed in several areas, including alternative source conditions such as misaligned jets, refinement of radiative-fraction assumptions, more extensive validation against hydrogen releases and large unconfined hydrogen vapor cloud explosions, and further refinement of flammable-mass and turbulence effects in the jet-ignition overpressure model.
As hydrogen blending and dedicated hydrogen projects advance, reducing uncertainty around rupture consequences will remain central to safe deployment. By extending and validating a physics-based fireball, jet-fire, and overpressure framework for hydrogen and hydrogen-natural-gas pipelines, the work helps operators move from broad assumptions toward more evidence-based risk and integrity decision-making.
Contributor
Steven Middleton, MSc, PEng
Project Engineer
Steven Middleton is a Project Engineer for our Integrity Management & Structures Department
Steven specializes in the consequence evaluation field. Steven focuses on the quantitative simulation of hazardous industrial accidents and currently oversees the development of C-FER’s hydrogen and CO2 pipeline consequence evaluation infrastructure.
