The Challenge

A transportation infrastructure that carries CO2 in large enough quantities to make a significant contribution to climate change mitigation will require a large network of pipelines spanning over hundreds of kilometres. Given that the most economical means of transporting CO2 is in the supercritical state due to its low viscosity and high density, a typical 100 km, 0.8 m diameter CO2 pipeline under such conditions would contain approximately 9000 tonnes of inventory. In the event of pipeline failure, for example a full bore rupture, based on the hyperbolic discharge behaviour synonymous with such failures8, a significant proportion of the inventory would be discharged in the first few minutes. At a concentration of 10%, an exposed individual would lapse into unconsciousness in 1 min. Furthermore, if the concentration is 20% or more, the gas is instantaneously fatal9. The ability of CO2 to collect in depressions in the land, in basements and in other low-lying areas such as valleys near the pipeline route, presents a significant hazard if leaks continue undetected. Hydrocarbons will eventually ignite or explode in such areas if, and when, conditions are “right”, but CO2 can remain undetected for a very long time. Also, CO2 will be mixed with potentially toxic substances whose natural dispersion might be impeded by the dense CO2 vapour layer close to the ground, further increasing hazards. In 1986 a cloud of naturally-occurring CO2 spontaneously released from Lake Nyos in Cameroon killed 1,800 people in nearby villages10.

There are several other hazards associated with the accidental release of CO2. It can act as an ignition source for nearby combustible materials due to friction induced static discharge. In 1953, such an incident resulted in 29 fatalities11. CO2 also reacts with water to form carbonic acid leading to the corrosion of carbon steel pipelines12. Supercritical CO2 widely considered to be the most economical state for pipeline transportation is a powerful solvent giving possible toxic contamination and sealing problems. Its release may lead to low temperatures resulting in brittle fracture of surrounding equipment12. High velocity solid CO2 discharge may pose the risk of erosion impact (supercritical CO2 with solid CO2 pellets is used commercially as a cutting media).There are several other hazards associated with the accidental release of CO2. It can act as an ignition source for nearby combustible materials due to friction induced static discharge. In 1953, such an incident resulted in 29 fatalities11. CO2 also reacts with water to form carbonic acid leading to the corrosion of carbon steel pipelines12. Supercritical CO2 widely considered to be the most economical state for pipeline transportation is a powerful solvent giving possible toxic contamination and sealing problems. Its release may lead to low temperatures resulting in brittle fracture of surrounding equipment12. High velocity solid CO2 discharge may pose the risk of erosion impact (supercritical CO2 with solid CO2 pellets is used commercially as a cutting media).

There are very few risk-based reference points in handling high pressure CO2 in large (1000 tonne) quantities against which estimated risks to persons can be compared to establish if a robust case for safety has been made13. In addition, there is very little understanding of the impact impurities on the phase equilibrium properties of CO2 and hence its discharge behaviour.

It is clear that the hazards associated with CO2 pipelines are quite different compared to those posed by hydrocarbon pipelines, presenting a new set of challenges. As such, any confidence that existing experience with operating hydrocarbon pipelines can be wholly extended to CO2 pipelines is dangerously misplaced.

The state of the art review14 on CO2 pipelines commissioned by the Research Council of Norway, Gassco and Shell Technology Norway lists many of these challenges, with modelling ranking as top priority area for further research.

‘Without a clear understanding of the hazards associated with the failure of CO2 pipelines, CCS can not be considered as a viable proposition for tackling the effects of global warming. The development of reliable validated pipeline outflow and dispersion models are central to addressing this challenge. This proposal deals with this missing link.’

8. Mahgerefteh H, Denton G, Rykov Yu G, Proc IChemE Hazards XX Symp, Manchester, UK, 15-17 Apr 2008
9. Pohanish, PR, Greene SA, Hazardous Materials Handbook, Carbon Dioxide, 330-331, Van Nostrand Reinhold, 1996
10. Krajick K, Defusing Africa’s Killer Lakes, Smithsonian 34 (2003) 46
11. Barrie J, Brown K, Hatcher PR, Schellhase HU, Proc 7th Int Conf Greenhouse Gas Control Technol, Vancouver, Canada, 5-9 Sept 2004
12. Mahgerefteh H, Atti O, AIChE Jl 52 (2006) 1248
13. Connolly S, Cusco L, Loss Prevent & Safety Perform in Process Ind Symp, IChemE, 2007
14. Oosterkamp A , Ramsen J , State-of-the-Art Overview of CO2 Pipeline Transport with Relevance to Offshore Pipelines, Polytech Report POL-O-2007-138-A, 2008