Carbon capture and storage depends on moving large volumes of carbon dioxide from industrial emitters to offshore storage sites. The pipelines that do this look familiar from the outside, but the fluid inside behaves very differently from oil or natural gas. Designing them well means paying close attention to thermodynamics, impurities and fracture control from the very first feasibility study.
Dense phase or gas phase?
Carbon dioxide's critical point sits at roughly 31°C and 74 bar. Above that pressure it can be transported in a dense or supercritical phase, where it has a liquid-like density but remains compressible. Dense-phase transport allows much smaller pipe diameters for a given mass flow, which is why most large trunk lines are designed to operate there.
Gas-phase transport at lower pressures has its place, particularly for early or low-volume networks and for repurposed natural gas lines that cannot safely hold dense-phase pressures. The trade-off is larger diameters and a careful eye on keeping the fluid well away from the two-phase region.
| Aspect | Dense phase | Gas phase |
|---|---|---|
| Typical pressure | Above about 85–100 bar | Below about 35–40 bar |
| Pipe diameter for a given flow | Smaller | Larger |
| Compression and pumping | Compression then pumping | Compression only |
| Key design concern | Fracture propagation, depressurisation | Avoiding two-phase flow |
Impurities change everything
Captured CO2 is never pure. Depending on the source, it may carry water, nitrogen, oxygen, hydrogen, argon, sulphur compounds or trace amines. Even a few per cent of non-condensable gases shifts the phase envelope, raising the pressure needed to stay in dense phase and widening the two-phase region.
Water is the most important impurity to control. Free water combined with CO2 forms carbonic acid, which can corrode carbon steel rapidly. Specifications therefore set strict dehydration limits, and network operators increasingly agree a common specification so that multiple emitters can share one pipeline without one source's impurities creating problems downstream.
Running ductile fracture
A defining design issue for dense-phase CO2 is running ductile fracture. When a rupture occurs, the fluid decompresses but its pressure plateaus at a relatively high saturation pressure as it flashes to vapour. That sustained pressure can drive a crack along the pipe for long distances unless the steel has enough toughness to arrest it.
Engineers address this through a combination of measures:
- Specifying line pipe with sufficient Charpy toughness, often well above natural gas norms.
- Increasing wall thickness to lower the hoop stress at the arrest pressure.
- Installing mechanical crack arrestors at intervals along the line.
- Using decompression modelling that accounts for the actual impurity mix.
Older fracture arrest correlations were calibrated on natural gas and can be non-conservative for CO2, so full-scale burst testing data and updated models carry significant weight in modern designs.
Routing and dispersion
Carbon dioxide is heavier than air, colourless and, at high concentrations, an asphyxiant. A release can pool in low ground, hollows and valleys rather than dispersing upwards as natural gas would. Route selection therefore considers topography carefully, alongside the usual concerns of population density, environmental designations and ground conditions.
Dispersion modelling informs the risk assessment, defines consultation zones and guides the spacing of block valves. Terrain-aware modelling is particularly important where a route crosses valleys with nearby housing, roads or farms.
Depressurisation and operations
Planned blowdown of a dense-phase CO2 line is not trivial. Rapid depressurisation causes strong cooling, potentially taking the pipe wall below its minimum design temperature and forming dry ice in vent systems. Operating procedures, vent stack design and material selection all need to account for these low temperatures.
Flow assurance studies also examine transient behaviour during start-up, shutdown and changes in supply from different emitters, making sure the fluid stays in a single phase where the design requires it.
Offshore sections and storage interfaces
Many UK and North Sea projects end at offshore storage sites in depleted reservoirs or saline aquifers. Subsea sections bring the familiar challenges of on-bottom stability, free spans, trawl interaction and installation method, combined with the CO2-specific concerns above. Some projects consider reusing existing offshore pipelines, which raises the same questions of material records, condition and pressure rating that apply to hydrogen conversion onshore.
Getting the early stages right
The single most useful step for any CO2 transport project is a rigorous feasibility and concept phase. Agreeing the fluid specification, choosing between dense and gas phase, and establishing fracture control philosophy early prevents costly redesign later. With carbon capture clusters moving from plans to construction, the industry is building valuable experience quickly, and sharing that learning openly will help each new project avoid the pitfalls of the last.