Liquid CO₂ carriage by sea: An introduction

As awareness grows about the harmful effects of carbon emissions, we have seen a surge in demand for knowledge about how to carry CO₂ on seagoing vessels, in bulk. This article provides an overview of the challenges and opportunities involved.

Most other gases can be carried in a liquid form provided they are at or below their boiling point and either:

• Under pressure alone
• Under refrigeration alone
• Under a combination of pressure and refrigeration.

CO₂ however is different. To carry it in its liquid form, CO₂ requires both pressure and refrigeration to prevent it directly converting from its gaseous state into its solid state (dry ice). These combined pressure/refrigeration requirements are not fixed but are sometimes referred to as “high pressure”, “medium pressure” and “low pressure”. These constraints have important implications for vessel design and capacity.

CO₂ has been shipped by sea since the late 1980s, initially on converted dry cargo vessels. The existing fleet of CO₂ vessels comprises only four units (see table below). These have mostly been trading within Europe on short-haul business, supplying CO₂ to the food and drink industries.

Can any other types of gas ships carry CO₂?

The realistic answer is no. While there are six existing gas vessels of 10,000 CBM capacity with a rating that enables them to carry CO₂, they have never done so. The intake and sloshing-related challenges they would face makes CO₂ carriage impractical.

What about LNG carriers?

Again, the simple answer is no. LNG carriers are not able to withstand the pressure required to handle CO₂ cargo. There are some small-scale LNG carriers which have a relatively high-pressure rating, but still not high enough for this purpose.

Could further dry cargo vessels be converted for CO₂ carriage?

This would be impractical for cargo volumes of significant scale.