CO2 containment sees Northern Light

Research into developing a suitable transportation solution for liquid carbon dioxide transportation received a major boost when the Norwegian government announced that it was planning to allocate funds to a carbon capture and storage (CCS) scheme in its draft 2020 budget bill.

One CCS scheme in line to receive government funding in Norway is the Northern Light Project, an initiative to carry out a full-scale demonstration project for carbon capture offshore in the Norwegian continental shelf. The project is a joint co-operation project between Equinor, Shell and Total, while DNV GL has provided technical support.

As part of the project, a liquified gas carrier for CO2 transportation design was produced. DNV GL has extensive experience with CCS schemes, having collaborated in initial vessel designs with Maersk in 2010, while the class society’s oil and gas arm signed an MoU with a leading CCS research institute, TCM, located near Bergen in 2018.

By combining a high-pressure low-temperature containment system with the design, the cost of the solution is reduced.

Subject to government approval and funding in 2020, a full-scale CO2 capture and storage chain could be established by 2023/24, with an initial storage capacity for up to 1.5 million tonnes of CO2 per year. This would be likely to require the construction of two specialised liquid CO2 carriers, each of which would transport CO2 from one of the project’s two initial capture sites in Norway to a dedicated onshore facility. The liquid CO2 cargo would then be transported by subsea pipeline to an offshore injection well, before being stored at a depth of 3,000 metres below the seabed.

The scheme is intended to have an eventual storage capacity of up to 5 million tonnes/year.

The project has wider applicability for alternative fuels, as commercial transportation of “green” carbon dioxide is seen as a prerequisite for the expansion of power-to-X solutions, to reduce the GHG footprint of synthetic methane or synthetic diesel fuels produced from hydrogen and carbon dioxide.

London Protocol

Unlike some other liquefied gaseous cargos, LCO2 is already included in regulations covering gas cargos. The IMO’s Maritime Safety Committee passed amendments to the International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (the IGC Code) in 2006 to include liquefied carbon dioxide in the code.

However, restrictions on the carriage of hazardous cargos have slowed progress on research into international CCS supply chains. International shipments of carbon dioxide are still currently restricted under the terms of the London Protocol. An amendment to article 6 of the London Protocol to address this restriction was passed in 2009, but has yet to be receive the necessary formal acceptance of two-thirds of contracting parties in order to come into force.

In October 2019, parties to the London Convention passed a resolution to permit trans-boundary exports of carbon dioxide under certain circumstances, removing a key obstacle to the development of international carbon capture and storage supply chains.

The removal of restrictions on trans-boundary shipments is likely to increase the economic viability of the pilot CCS scheme, as higher throughput volumes will lower the fixed costs associated with the project. This is particularly relevant where the Northern Lights project was concerned, as it would need to attract LCO2 shipments from a number of international sources around the North Sea in order to lift throughput at the undersea storage site close to the project’s 5 million tonnes/year capacity.

Equinor signed memoranda of understanding with seven counterparties in September 2019, including Air Liquide, ArcelorMittal and HeidelburgCement AG. The Northern Lights project is understood to have attracted interest from a number of other energy-intensive manufacturers across northwest Europe.

Technical challenges

In order to liquefy carbon dioxide, which sublimes directly between gaseous and solid (dry ice) states at atmospheric pressures, it is necessary to lower the temperature to around -50 deg C and a pressure of 7 bar, in line with the LPG containment systems.

The containment system approved by DNV GL reduced the refrigeration requirements, by achieving an operation design temperature in the range of -30 deg C. This required the Type C tank to withstand higher pressures of 19 bar.

Another characteristic of liquid CO2 is its higher density compared with other liquefied gases, such as LNG or LPG. Liquid CO2 has a density of around 1,100 kg m3, compared with 500 kg m3 for the other gases. This will increase the cargo weight, while the design features large single cylinder pressurized tanks. The decision to incorporate a large diameter single Type C cylinder cargo tank has also been taken to improve the economics of the liquified CO2 carrier design.

The tank designs provide a relatively high-volume efficiency applied on a typical ship designs typically used for LPG transportation.

The selected solution and the physical characteristics of liquid CO2 have implications for the loading and discharge phases of the solution. Particular care must be taken to control pressure drops, as this can provoke dry ice formation, with the possibility of abrasive effects on cargo pumps.

The design of the tank must be designed to resist variation in tank pressure experienced between laden and ballast voyages in addition to acceleration loads of liquids in full loaded condition.

An additional challenge confronting the ship designers were how to manage the boil-off gas (BOG) generated by the vessel during the voyages, as solutions involving atmospheric discharges are unlikely to be acceptable for vessels involved in CCS supply chains.

Innovative solution

The 7,500cbm capacity Type C containment vessel has been designed to withstand pressure accumulation during laden voyages and the corrosive effects of liquid CO2. The combination of high-density cargo, high design pressures and a large tank diameter has required innovative solutions to meeting the strength requirements of the cargo tanks.

The design has reconciled the competing demands of the economics of tank manufacture with meeting the safety requirements for the tank by replacing standard low temperature steel for pressure vessels with a “non-traditional” steel product.

The criteria for the design with the selected material were exacting: the design had to be so resistant to the development of fatigue cracks that fatigue cracks can be neglected in the design.

The safety level defined by the IGC code results in a safety level that can be described as: “Assuming a similar tank built by these principles in 1920. The same tanks shall have at least 100 additional years after 2019 before any cracks in the tanks can be expected.”

The design and material selection combine high tensile strength properties withstanding the higher extreme loading and resistance to fatigue cracks.

Design modifications

In order to control the dynamic loads and accommodate the new high strength steel, a number of design modifications were introduced. These included reinforcing the Type C tank design at various locations, but also softening the design at locations where larger deflections have been utilized to reduce the dynamic stresses in the tank.

Safety considerations

DNV GL noted that documenting the safety of the tank design against the development of fatigue cracks was a key criterion before the design could be awarded a pre-class approval.

This required exhaustive tests to ensure that the materials could meet the safety requirements against developing fatigue cracks.

In November 2019 the design was concluded to be able to withstand the dynamic loading without exceeding the stress levels that will create fatigue cracks. Accordingly, Equinor could finally receive the GASA Statement confirming a successful design approval from DNV GL.