Impact Assessment of Hydrogen on Transmission Pipeline BPDs in IGEM/TD/1

As part of the LTS Futures HyTechnical project, IGEM requested that DNV GL undertake an assessment of the possible impact of hydrogen transmission on BPDs, to support the development of supplements to the existing suite of natural gas standards to accommodate the possible future use of hydrogen. The current state of knowledge of the behaviour of large scale, high pressure, hydrogen releases is limited in comparison with the considerable body of data from research and operational experience of natural gas, but is adequate to undertake an impact assessment to take account of the different gas outflow and fire characteristics of 100% hydrogen vs. natural gas.
Calculations of the BPDs for 100% hydrogen pipeline fires on an equivalent basis to those in IGEM/TD/1 for natural gas have been performed with a degree of confidence in the results and demonstrated that the equivalent BPDs for 100% hydrogen are approximately 10% smaller than for natural gas. The results are presented graphically in this report.
However, hydrogen introduces the potential for substantially higher overpressures than natural gas, due to the higher flame speed and wider flammable limits, if delayed ignition is a credible event. The overpressure estimates presented in this report are intended to be scoping calculations, to put the likely overpressures into context. The results suggest that significant overpressures are possible at the BPDs, but there is a lack of evidence to support the estimation of the overpressures following delayed ignition of a large, turbulent, hydrogen release in the open (in contrast to explosions in confined or congested regions) and there is a high degree of uncertainty in the predictions presented here. It is therefore recommended that large scale pipeline rupture experiments are performed, similar to those undertaken previously for hydrogen, natural gas and natural gas/hydrogen mixtures, but with ignition engineered to take place after a short delay, in order to measure the overpressures and provide the means to validate or refine the predictions made.
The analysis has highlighted limitations in the original method of calculating BPDs in IGEM/TD/1, which reflects the techniques available at the time, approximately 40 years ago. Since then understanding of the hazards from pipeline failures and the ability to model the consequences and predict the associated risks to people in the surrounding area have advanced very considerably, facilitated by software tools and documented in standards such as IGEM/TD/2. These methods allow the highly transient nature of a high pressure gas pipeline rupture release to be modelled more accurately and for the thermal effects of fires on people and buildings to be calculated, taking account of the time-varying thermal dose.
For these reasons, a simple comparison of the possible overpressure effects of delayed ignition of a 100% hydrogen release at the BPDs can be misleading and implies that the overpressure hazards could be more severe than those for fires, which may not be the case. Example calculations have been performed for a representative pipeline case which indicate that using current methods, the predicted thermal hazard distances for 100% hydrogen pipeline fires (house burning and escape for people) are substantially greater than those estimated for overpressures following delayed ignition for similar levels of vulnerability. This report addresses buried pipelines only – the potential for more severe explosion overpressure effects for hydrogen releases may be more significant for Above Ground Installations (AGIs), especially where congestion or confinement may be present. It is recommended that similar studies are conducted to quantify the effect of hydrogen conversion on the consequences and risks associated with hydrogen releases at AGIs.
Finally, it is stressed that the analysis in this report does not consider the relative risks for 100% hydrogen and the equivalent natural gas pipelines. There remain uncertainties in the failure frequencies for steel pipelines transporting hydrogen and particularly the probability of immediate and delayed ignition. The likelihood of delayed ignition of a large, turbulent, high pressure hydrogen gas pipeline rupture release may be very low, due to the wider flammability limits and lower minimum ignition energy for hydrogen compared with natural gas. Additional research is currently ongoing or planned to address the gaps in knowledge for 100% hydrogen, which should allow more robust comparisons of the relative risks to be made in the future.