Crack propagation is a problem in pipelines conveying gas or liquids with high vapour pressures. Fractures can propagate in either the fully brittle or fully ductile modes for long distances, and in theory, could propagate almost indefinitely. The problem of fracture propagation was recognised in the gas industry over forty years ago and extensive worldwide research has led to the establishment of a number of models, which describe propagation behaviour for gas pipeline systems. These models have been very successful in defining toughness requirements for pipe material, which ensure fracture arrest. In the literature on CO2 pipelines, many authors have indicated that ductile fracture propagation may be an issue and indeed, the requirement to consider fracture propagation in CO2 pipelines is included in the federal regulations in the USA.

In ductile fracture propagation, the fluid saturation pressure, resulting from the sudden decompression of the fluid provides the crack driving force. In order to arrest a ductile fracture therefore, the arrest pressure must be greater than the saturation pressure i.e. either the arrest pressure must be raised or the saturation pressure must be lowered. Maxey (1986) indicates that the saturation pressure can be lowered by lowering the operating temperature or by removing impurities with lower critical temperatures than CO2. In particular, hydrogen has been shown to have a detrimental effect in terms of fracture propagation.

Conversely, it has been shown that the arrest pressure can be raised by increasing the wall thickness, increasing the toughness, decreasing the pipe diameter or increasing the yield strength. In the design of gas pipelines, the fracture arrest pressure is generally controlled by specifying a required material toughness. In light of this, the material requirements for the Sheep Mountain pipeline specified a minimum Charpy toughness to avoid the possibility of long running fractures. However, for example, both the CRC and Central Basin pipelines were not designed with sufficient toughness to arrest propagating ductile fractures and therefore crack arrestors were installed along these pipelines; every 5.8 km on average on the CRC pipeline and every 0.4 km on the Central Basin Pipeline.

Unlike natural gas pipelines, for CO2 pipelines, it is also possible to increase the arrest pressure by increasing the wall thickness and/or increasing the pipe material strength, assuming that the diameter is controlled by required flow and therefore cannot be decreased. These might be considered expensive options; however, it could be particularly beneficial for offshore pipelines which tend to be of increased wall thickness and pipe grade compared to onshore pipelines.

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In the gas industry, there has been less work conducted on fracture propagation in offshore pipelines. The prediction is more complicated than for onshore pipelines because of the interaction of the escaping fluid with the water, which reduces the hoop stress in the pipe wall. It is considered that, for gas pipelines, the methods for calculating fracture arrest conditions are conservative when applied to offshore pipelines. However, this has not been confirmed for CO2 pipelines offshore. It is also highlighted that the installation of crack arrestors offshore would increase the cost of construction considerably and should be avoided.

In conclusion, fracture propagation is a credible threat to the CO2 pipelines and the level of toughness required has to be defined for CO2 carrying impurities.

References:

Maxey, W. A. (1986) “Long Shear Fractures in CO2 Lines Controlled by Regulating Saturation Arrest Pressures”, Oil and Gas Journal 84(31): 44