ClassNK Releases Handbook for Onboard Carbon Capture Systems
ClassNK has published a new framework for shipowners and operators considering onboard carbon capture and storage. The handbook addresses a practical challenge: capturing carbon dioxide from ship exhaust is only the first step. The captured CO₂ must then be unloaded, transported, and either stored permanently or used elsewhere. Without this complete chain, the emissions reduction cannot be verified or counted.
The Onboard Carbon Capture Systems Implementation Framework was released on 23 October 2026. It provides guidance on evaluating the technical, infrastructure, regulatory and accounting requirements across the entire project lifecycle. For businesses exploring carbon capture as part of their decarbonisation plans, the framework offers a structured approach to assessing whether these systems can deliver measurable emissions reductions.
This matters because international rules for recognising captured emissions remain incomplete. Early adopters will need to navigate ship-specific approvals and demonstrate that captured carbon is genuinely removed from the atmosphere, not simply transferred to another part of the supply chain without proper oversight.
How onboard carbon capture systems work in practice
Onboard carbon capture and storage systems extract carbon dioxide from a vessel's exhaust gas. The CO₂ is then processed, temporarily stored in tanks on the ship, and transferred ashore at port. From there it must be transported to permanent geological storage, used in industrial processes, or managed through another verified route.
ClassNK's framework treats this as a complete value chain rather than isolated equipment. Consequently, the handbook covers onboard capture units, shoreside unloading facilities, transport infrastructure, and storage or utilisation sites. It also addresses regulatory approvals, monitoring and verification requirements, and chain-of-custody arrangements.
Chain of custody is particularly important. Shipowners need to prove what happens to captured carbon after it leaves the vessel. Without clear tracking and verification, the environmental benefit cannot be recognised in carbon reporting or used to meet regulatory targets such as those set out in Procurement Policy Note 06/21 for UK public sector suppliers.
The technology itself adds complexity to vessel operations. Capture equipment requires significant space and weight. It consumes energy to heat chemical solvents or operate membrane systems, pumps and compressors. This increases fuel consumption, which can partially offset the emissions saved through capture. Moreover, the captured CO₂ must be conditioned into a form suitable for safe storage and transfer, typically requiring liquefaction or pressurisation.
ClassNK's earlier carbon capture guidance and development timeline
The new handbook builds on technical work ClassNK began in 2023. In April that year, the classification society published Guidelines for Shipboard CO₂ Capture and Storage Systems. Those guidelines established safety provisions for shipboard installations and introduced class notations for vessels fitted with capture systems or designed to accommodate them in future.
ClassNK developed those initial guidelines through its involvement in the CC-Ocean project with Mitsubishi Shipbuilding and Kawasaki Kisen Kaisha. The project tested what ClassNK described as the world's first marine carbon-capture system during an actual voyage. ClassNK evaluated the installation from a safety perspective, examining how capture equipment integrated with existing ship systems.
The 2023 guidelines highlighted practical design consequences. Ships must accommodate carbon-absorption units and storage tanks. The capture process requires substantial additional energy for heating amine solutions and operating pumps. ClassNK included methods for estimating the size of major equipment and the additional energy demand.
In October 2025, ClassNK published a second edition of the guidelines. This version added provisions for membrane-separation systems alongside the earlier focus on chemical absorption using amine solutions. Membrane technology offers a different approach to separating CO₂ from exhaust gas, with distinct energy profiles and space requirements.
The October 2026 framework extends this technical foundation. It shifts focus from shipboard equipment alone to the full project infrastructure needed to make carbon capture viable. For operators, this means evaluating not just whether capture equipment can be installed, but whether the complete system can deliver verified reductions.
Regulatory gaps create approval challenges for early projects
International Maritime Organization standards for assessing CO₂ reductions from onboard carbon capture do not yet exist. According to ClassNK's Alternative Fuels Insight materials, this means flag-state approval remains necessary where shipowners seek regulatory recognition for captured emissions. Each project must demonstrate compliance through ship-specific risk assessments rather than standardised criteria.
Safety regulation is also under development. The IMO began drafting interim safety guidelines for ships fitted with onboard carbon capture systems at the Sub-Committee on Carriage of Cargoes and Containers' 12th session in September 2026. The proposed framework is expected to address hazards associated with high concentrations of CO₂, chemical exposure, ventilation, monitoring, storage and transfer operations.
Current planning targets finalisation of these guidelines at CCC 14 in 2028 and approval at Maritime Safety Committee session MSC 116 in 2029. Initially, the guidelines are expected to be non-mandatory. Therefore, early projects will continue to depend heavily on approvals from flag administrations and classification societies.
This regulatory uncertainty creates commercial risk. Businesses investing in carbon capture systems cannot yet be certain how captured emissions will be recognised in international reporting frameworks or national regulations. Similarly, there is limited clarity on how captured carbon will interact with emissions trading schemes or carbon border adjustment mechanisms as these develop.
For UK businesses, this matters when considering supply chain emissions. If a shipping provider claims reduced emissions through onboard capture, buyers need assurance that the reduction is real, measured, and recognised. Without established standards, verification becomes a bespoke exercise for each project.
Commercial considerations for businesses evaluating carbon capture
The energy penalty from running capture equipment affects operating costs. Capture systems consume power to operate compressors, pumps and heating systems. This increases fuel use, which raises voyage costs and generates additional emissions. The net reduction depends on capture efficiency and the energy required to achieve it.
Space and weight constraints also matter. Onboard capture units and storage tanks occupy areas that might otherwise be used for cargo or fuel. This can reduce revenue-earning capacity. Additionally, the weight of equipment and stored CO₂ affects vessel stability and fuel efficiency. Naval architects must account for these factors during design or retrofit.
Port infrastructure presents another constraint. Vessels fitted with carbon capture need access to shoreside facilities capable of receiving, storing and transporting captured CO₂. These facilities are not yet widely available. Consequently, route planning becomes more restricted, and operators may face higher port costs or limited scheduling flexibility.
The final destination of captured carbon determines whether emissions reductions can be claimed. If CO₂ is permanently stored in suitable geological formations, the reduction is clear. However, if it is used in processes that later release it back to the atmosphere, the benefit may be temporary or negligible. Chain-of-custody systems must track the carbon and verify its ultimate fate.
Monitoring, reporting and verification requirements add administrative cost. Operators must measure capture rates, energy consumption, storage conditions and transfer volumes. They must also maintain records suitable for third-party verification. For businesses already managing carbon reporting under frameworks such as the Streamlined Energy and Carbon Reporting regulations, this represents additional compliance burden.
Insurance and liability considerations are still emerging. Questions remain about responsibility if captured CO₂ leaks during storage or transfer, or if geological storage sites fail. Contractual arrangements between shipowners, port operators, transport providers and storage site operators need to allocate these risks clearly.
Summary of key facts
- ClassNK released the Onboard Carbon Capture Systems Implementation Framework on 23 October 2026, covering technical, infrastructure, regulatory and emissions-accounting issues across the full OCCS value chain.
- The framework builds on ClassNK's April 2023 Guidelines for Shipboard CO₂ Capture and Storage Systems, with a second edition published in October 2025 adding membrane-separation technology provisions.
- International Maritime Organization standards for recognising CO₂ reductions from onboard capture do not yet exist, requiring ship-specific flag-state approvals for early projects.
- Interim IMO safety guidelines for OCCS are under development, with finalisation targeted for 2028 and approval expected in 2029, initially on a non-mandatory basis.
- Capture systems increase fuel consumption through energy demand for compressors, pumps and heating, partially offsetting claimed emissions reductions.
- Effective emissions reduction depends on the complete system, including capture efficiency, onboard storage, shoreside unloading infrastructure, transport and permanent storage or verified utilisation.
- Chain-of-custody arrangements are essential to track captured CO₂ and verify its final destination, enabling recognition in carbon reporting and regulatory frameworks.
What businesses should consider before committing to onboard capture
Evaluating onboard carbon capture requires assessing the complete project, not just the shipboard equipment. Firstly, determine whether suitable shoreside infrastructure exists or can be developed along your operating routes. Without unloading facilities, captured CO₂ cannot be transferred ashore, making the system unviable.
Secondly, examine the energy penalty and its effect on operating costs. Request detailed performance data from equipment suppliers, including capture efficiency at different engine loads and the additional fuel consumption required. Calculate the net emissions reduction after accounting for increased fuel use. This determines whether the system delivers genuine environmental benefit.
Thirdly, assess regulatory recognition. Check with your flag administration and classification society whether captured emissions can be counted toward compliance targets. If you supply UK public sector clients, consider whether carbon reporting for PPN 06/21 compliance will recognise reductions from onboard capture, and what verification will be required.
Furthermore, establish clear chain-of-custody arrangements before installation. Identify who will receive the captured CO₂ at port, how it will be transported, and where it will be stored or used. Obtain commitments in writing. Verify that storage sites have appropriate permits and monitoring systems. Without these arrangements, you cannot demonstrate that emissions have been permanently removed.
Insurance and contractual risk allocation also need attention. Clarify liability for leakage, storage failure, or regulatory non-compliance. Ensure contracts with port operators, transport providers and storage sites allocate these risks appropriately. Review whether existing marine insurance covers carbon capture operations or whether additional cover is needed.
Finally, consider timing. Early projects will face higher costs and greater uncertainty while regulations develop. However, delaying may mean missing opportunities to learn, build relationships with infrastructure providers, or gain competitive advantage in tenders requiring lower emissions. Balance first-mover risk against the cost of waiting for standards to mature.
For businesses managing broader decarbonisation strategies, onboard carbon capture is one option among several. Fuel switching, operational efficiency improvements, and route optimisation may offer more immediate or cost-effective reductions. Additionally, structured net-zero programs help prioritise actions across your entire supply chain, ensuring investments deliver measurable progress toward your targets.
Where to find additional guidance and official resources
ClassNK's Onboard Carbon Capture Systems Implementation Framework is available through the ClassNK website. The classification society also maintains technical guidance on alternative fuels and decarbonisation technologies for maritime operators.
The International Maritime Organization publishes updates on regulatory development through its official website. This includes progress on interim safety guidelines for onboard carbon capture systems and broader work on greenhouse gas emissions reduction.
UK businesses can access guidance on carbon reporting and supply chain emissions through the Department for Energy Security and Net Zero. The department provides information on compliance requirements and policy development affecting emissions reduction across transport sectors.
For practical support with carbon measurement, reporting and reduction strategies across your supply chain, our net-zero resources offer guidance tailored to UK SMEs navigating compliance and tender requirements. We also provide training through SBS Academy on Scope 3 emissions and supply chain decarbonisation.