A new class of ship is emerging to support offshore sequestration of CO₂ from industrial sources. Industry growth has led to the evolution of cargo management systems on liquified CO₂ (LCO₂) carriers to meet new challenges.

The CO₂ captured by hard-to-abate industries, such as steel, cement, and chemical production, can contain a range of impurities not found in the CO₂ transported by sea for the food and beverage industry.

Its specific impurity profile depends largely on the source, its temperature and pressure, and capture and dehydration technologies. This matters for transport to offshore sequestration sites: water, SOx and NOx can increase corrosion susceptibility; hydrogen can risk embrittlement.

During loading of LCO₂, vapour from the existing cargo heel in the vessel's tanks must be managed as the tanks are filled. Depending on the impurity specification of the stored and incoming CO₂ streams, operators may wish to prevent cross contamination of the vapour between ship and shore.

This could require a reliquefaction system capable of condensing the large volumes of vapour generated during loading – something that becomes very energy intensive and costly when loading rates are high.

Instead, liquefied-gas systems specialist TGE Marine has developed a solution that utilises a portion of the cold incoming LCO₂ cargo stream to condense displaced vapours from the cargo tanks.

The condensed CO₂ is returned onboard in a closed-loop arrangement, securing that vapour transfer to the terminal is from the terminal’s own gas composition while making effective use of the refrigeration already available in the cargo.

With the problem of cross contamination solved, bigger ships can collect CO₂ from more and varied industrial sites. This presents another challenge. For the CO₂ carriers currently in service, the CO₂ is stored under medium-pressure conditions: up to 19barg and down to -35^oC. This necessitates thick, high strength tank shells, so larger vessels are being designed for low-pressure LCO₂ storage: around 7 to 9barg and temperatures between -50^oC and -55^oC. This allows for increased tank diameter without increasing shell thickness.

“Cylindrical tanks can be used for medium-pressure storage. This is the easiest and best-known way of doing it, up to 20,000 to 24,000m³ capacity,” said Mathias Sorhaug, business development director for CO₂ shipping at DNV. “Beyond that, it’s typically low-pressure storage.”

Medium pressure has mainly been considered in Europe and low pressure in the Asia-Pacific. That situation is now evolving.

While the already-operational Northern Lights project transports CO₂ by ship to an onshore receiving facility before onward transport through offshore pipelines, other concepts are being developed elsewhere.

These include injection from floating or bottom-fixed offshore platforms, either with or without intermediate storage, and direct ship-to-well injection.

Offshore injection concepts in the North Sea generally favour larger vessels to reduce the impact of motions and maintain operational reliability in harsh weather conditions. Therefore, low-pressure CO₂ transport is often preferred.

Marine cargo-handling specialist MacGregor is developing a bow transfer system for injection from LCO₂ carriers that includes a unique LCO₂ coupler valve featuring moment-free Cardan suspension (gimbal) and a dedicated guiding pin. This equipment facilitates a secure connection between the LCO₂ carrier and the offshore injection unit’s hose-end valve, supporting offloading operations even in heavy seas.

The Cardan suspension is the primary load-bearing component of the bow transfer system. It supports the weight of the coupler valve, hose, and hose valve during connection and features an integrated swivel connection to the vessel's fixed piping system.

Once the LCO₂ hose connects to the coupler valve, the suspension enters a freewheeling mode to eliminate bending moments on the unit. It is specified to move sideways +/-35 degrees and forward: +60 degrees.

“Bolted to the hose valve housing, the guide pin mechanically aligns the flanges before connection, protecting flange surfaces and seals in heavy seas.

During the final pull-in stage, it enters the Cardan suspension capturing box to ensure proper flange alignment before the coupler valve claws clamp onto the hose valve,” says Øyvind Solli, development programs manager, MacGregor.

If exposed to normal atmospheric pressure, CO₂ flashes instantly into vapor and solid dry ice, so the seal can withstand the high pressure and extreme cold needed to maintain CO₂ in a liquefied state during transfer.

In testing it was subject to repeated pressure cycles and temperatures as low as -57^oC.

The bow transfer system handles a range of cargo pressures and injection methods, bypassing the need for onshore buffer and conditioning facilities. Eliminating this bottleneck could enhance the viability of more projects, says Solli.