Any discussion about Onboard Carbon Capture and Storage (OCCS) inevitably begins with a fundamental question: where does the equipment go, what does it weigh, and how does it change daily operations? These three factors – space, weight, and operations – are not footnotes but the hard reality that determines whether an OCCS project is feasible at all.
The capture technology itself is only one part of the system. Add absorber columns, heat exchangers, regeneration units, piping, control systems, and not least the storage tank for the captured CO2. Each of these components demands space and adds weight – on vessels that are typically already optimised for maximum space utilisation.
Those who discuss OCCS solely in terms of capture rate miss the decisive point: the real challenge lies not in the chemistry of separation but in the physical integration on a floating asset with limited volume, strict stability calculations, and ongoing operations.
Many vessels were not designed for additional process equipment. The engine room is typically densely packed, and open deck areas are rarely truly available – they are needed for access, maintenance, life-saving appliances, or cargo operations.
A typical OCCS system for a medium-sized cargo vessel requires, depending on capture rate and technology, between 200 and 600 cubic metres for the process plant alone. The CO2 intermediate storage must be added: on a two-week voyage with a 30 % capture rate, a vessel with a 15,000 kW main engine produces several hundred tonnes of liquefied CO2. That corresponds to a tank volume comparable to the ballast water tank of a small bulker.
The space requirement also depends heavily on the chosen capture technology. Amine-based systems require large columns with significant height. Membrane processes are more compact but achieve lower capture rates. Calcium-based processes in turn need fixed-bed reactors and material handling systems. None of these solutions is genuinely space-saving.
Retrofit projects are particularly critical: the technology must be integrated into an existing arrangement without compromising escape and rescue routes, without blocking access to existing systems, and without restricting sightlines from the bridge. This demands extensive 3D modelling and frequently requires compromises on system size.
Additional weight from OCCS installations directly affects a vessel's stability and available cargo capacity. The process plant itself can weigh between 50 and 150 tonnes depending on configuration. Add the weight of stored CO2, which increases continuously during the voyage. This weight change is not static – it shifts the vessel's centre of gravity and must be accounted for in stability calculations for every phase of the voyage.
For vessels already operating near their maximum deadweight, every additional tonne of OCCS equipment means one tonne less cargo. On a Panamax bulker, this can make the difference between profitable and uneconomical operation. The calculation becomes more complex when one considers that the CO2 tank starts empty at departure and is full at the destination port – an asymmetry that must be reflected in every voyage plan.
Operationally, OCCS fundamentally changes life on board. The plant requires energy drawn from the main power supply – typically 10 to 25 % of exhaust gas energy. The chemicals for amine-based systems must be stocked, monitored, and regularly replaced. Exhaust gas routing becomes more complex because part of the exhaust stream must be directed through the absorber. And in port, the CO2 transfer to terminals or barges must be organised – a process that is not yet standardised in most ports today.
The crew must also be trained for operating the plant: emergency shutdown, leak detection, handling of absorption agents, and CO2 transfer. These are not trivial additional tasks but new core processes that must be integrated into Safety Management Systems.
A serious assessment always considers OCCS as a complete system and never in isolation through the capture rate alone. The professional approach begins with a detailed stock-take: how much space is genuinely available? What weight margins does the vessel have? What operational constraints arise? And above all: is there actually a functioning CO2 offtake chain in the ports of call?
A professional assessment also includes an honest examination of economics. The capital expenditure for an OCCS plant ranges from single-digit to low double-digit millions depending on size and technology. Ongoing costs for energy, chemicals, and maintenance are additional. These costs are offset by potential savings on EU ETS allowances and FuelEU penalties – but only if the plant operates reliably and regulatory recognition is secured.
Integrating an OCCS system into an existing vessel requires careful coordination across multiple technical disciplines. At its core, there are four integration areas: exhaust gas connection, process plant, CO2 storage, and auxiliary systems.
The exhaust gas connection is the first critical point. The exhaust stream must be diverted after the exhaust gas boiler but before the funnel. This requires additional dampers, piping, and frequently exhaust gas pre-cooling to maintain the process temperature of the absorber column. On vessels with SCR catalysts or exhaust gas scrubbers, the exhaust routing becomes particularly complex as multiple systems compete in the same exhaust train.
The process plant itself – typically an absorber-stripper system for amine processes – must be mounted vibration-resistant whilst providing sufficient access for maintenance. The columns can reach heights of 8 to 15 metres, which on many vessels is only possible through deck penetrations or installation on deck. Deck-mounted systems must be secured for sea conditions and protected against weather.
CO2 liquefaction and storage presents its own requirements. Liquid CO2 is typically transported at -20 to -50 °C and 7 to 20 bar. The tanks must be pressure-rated, insulated, and equipped with pressure relief systems. Tank positioning directly influences vessel stability and must be closely coordinated with the stability booklet.
Auxiliary systems include cooling water supply, electrical supply, control and monitoring technology, and safety installations such as CO2 detectors, ventilation systems, and emergency shutdowns. Each of these systems must be integrated into the existing vessel automation – an effort that is frequently underestimated.
The energy balance deserves particular attention. Amine-based capture systems require substantial heat quantities for absorbent regeneration. This heat is typically provided from exhaust gas or via an additional steam generator. The resulting energy consumption can reduce the overall propulsion efficiency by 10 to 20 % – an effect that must be honestly reflected in every economic calculation.
For vessel operators, OCCS represents a fundamental change to the operational profile. The plant does not run in the background – it becomes an integral part of vessel operations, affecting planning, maintenance, and port logistics.
Voyage planning must now account for whether the destination port offers CO2 transfer facilities. If not, either a detour to a suitable port must be planned or the capture plant must be shut down for certain voyages. Both have economic consequences.
Maintenance planning becomes more complex. In addition to existing engine room systems, a complete chemical process is added that requires its own inspection intervals, spare parts, and specialist knowledge. The availability of spare parts for OCCS plants is currently still limited, as it is a young technology with few installed units.
For fleet managers, the question of standardisation arises. If different vessels in a fleet are equipped with different OCCS technologies, parallel maintenance and training requirements emerge. A fleet strategy based on uniform systems reduces complexity – but simultaneously limits flexibility in technology choice.
Classification societies have begun developing guidelines for OCCS installations. DNV, Lloyd’s Register, and Bureau Veritas already offer Class Notations or Approval in Principle for certain systems. Nevertheless, a uniform, internationally recognised standard is still lacking. This means for operators that class engagement must begin early in the project – ideally during the concept phase.
The OCCS pilot projects in shipping to date provide valuable insights into the practical limits of the technology. Projects such as the collaboration between Mitsubishi Shipbuilding and K Line have demonstrated that technical feasibility is fundamentally given – but under very specific conditions.
A recurring finding is that space requirements are regularly underestimated during the planning phase. Pilot installations integrated into container modules achieve flexibility in positioning but bring their own problems: restricted maintenance access, longer pipe runs, and higher pressure losses.
Port infrastructure has emerged as the greatest practical bottleneck. Even in advanced ports such as Rotterdam or Antwerp, CO2 transfer facilities are still under development. For vessels regularly calling at smaller ports, this infrastructure is entirely absent. This currently limits the economically sensible deployment of OCCS to certain routes and trading areas.
On a positive note, the energy efficiency of capture systems shows progress in recent pilot projects. Newer absorbents and optimised process management reduce specific energy demand. Nevertheless, the parasitic consumption remains substantial and must be transparently presented in every project assessment.
Not every vessel and not every fleet is a sensible candidate for OCCS. A structured decision framework helps deploy resources purposefully and filter out unrealistic projects early.
Step 1: Space analysis. Is there at least 200 m³ of contiguous area for the process plant and at least 100 m³ for the CO2 tank? If not, OCCS is very likely not economically viable.
Step 2: Weight reserve. Does the vessel have sufficient reserve deadweight to carry the plant plus a full CO2 tank without unacceptably reducing cargo capacity? Rule of thumb: at least 3 % reserve deadweight should be available.
Step 3: Route profile. Do at least 60 % of destination ports have CO2 offtake facilities or are expected to receive them within three years? Without a functioning offtake chain, OCCS is operationally unviable.
Step 4: Economics. Does the expected saving on EU ETS costs and FuelEU penalties exceed the annuity of the investment plus running costs? Only if this calculation is positive does a deeper feasibility study warrant the effort.
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