Particular challenges with operation and design of CO2 pipelines
Any large-scale implementations of CCS in the future will most likely involve transportation of large quantities of dense phase CO2 by pipeline — from the plant with CO2 capture to the storage site. In addition, there will in many cases be a network of smaller pipelines, collecting CO2 from various emitters to the main pipeline going to the storage site. See for example the planned Yorkshire and Humber CCS project in the UK.
Traditionally, high-pressure pipelines are laid as far from public infrastructure as possible to reduce the risk of e.g. leakage, which most often results from third-party impact on the pipeline.
However, it is expected that due to the location of the CO2 sources, a distributed CO2 pipeline network in a future CCS system will in some cases be close to areas with public activity. In addition, due to the thermodynamic properties of CO2 and CO2 mixtures, transportation will raise challenges not seen with natural gas or hydrogen transport pipelines. These factors necessitate care with respect to pipeline design.
Fracture propagation control in CO2 pipelines
Pipelines are sometimes at risk of accidental impacts or loads causing a hole or flaw or crack through the pipeline wall. Examples of such incidents are excavation or road work, geomechanical events such as earthquakes and avalanches, and corrosion/wear caused by internal or external conditions.
The fracture race
A sudden rupture, flaw or hole in a pressurized containment or structure will lead to an outflow of the contents and a rapidly falling loading or driving force on the stress concentrators caused by the flaw or hole. If the initial pressure is high enough and the flaw is long or severe enough, a fracture will rapidly propagate in the structure; a so-called «running fracture». For steel materials operating above the ductile-to-brittle transition temperature, the velocity of this running fracture is determined by pressure at the proximity of the tip of the moving fracture. This pressure is again determined by the speed of sound in the pressurizing medium. That is, the speed of the fracture is determined by the speed of the depressurization wave moving away from the tip of the propagating fracture.
If the pressurization wave moves faster than the fracture, the pressure at the crack tips will eventually be below what is needed to drive the fracture forward, and the fracture will stop (arrest). However, if the depressurization speed of the medium at the pressure close to the crack tip is lower than the velocity of the fracture, the fracture will continue to propagate until e.g. some mechanical hindrance (crack arrestor) increases the local fracture resistance. This race between the depressurization wave and the fracture, we call the «fracture race» and is illustrated in the figure below.
For liquids, the speed of sound is roughly 1000 m/s, while the speed of ductile fractures typically is on order of 100-300 m/s. Thus, long running (ductile) fractures never take place in high pressure liquid pipelines. However, when dense-phase CO2 is depressurized, a gas-liquid (two-phase) mixture will form. The speed of sound in such a mixture can be lower than 100 m/s while the pressure is sufficiently high to drive the running fracture, so that the fracture will not arrest. This situation is illustrated in the animation below, where one can see the profile opening of the pipe in green color, and the pressure profile in the blue line during a simulation (pure CO2) using the coupled code developed in Task 2.1. One can clearly see in the animation that no crack arrest will take place.
In a situation where the saturation pressure is slightly lower – either due to higher pipe pressure or lower temperature, the same pipeline material is able to arrest the running fracture. This is shown in the animation below.