Maritime decarbonisation: turns out it is rocket science

New vessel designs will be needed to enable the shipping industry to transition to a cleaner energy value chain, writes Georgios Plevrakis, VP, Global Sustainability at ABS

While most observers agree that a ‘net-zero’ shipping strategy will be challenging to achieve, there are pathways that could lead to that goal. The problems associated with a net-zero fleet are complex, requiring more engaged collaboration across sectors.

Various external factors such as diverse emission levels, economic impacts, public perception and politics will influence the outcome. Improving the energy and operational efficiency of vessels alone will not result in net-zero emissions in the future; unless a carbon offset mechanism is established, using ‘net-zero’ fuels will be essential.

The ‘net-zero’ approach may allow the industry as it transitions, to use fossil-based resources to produce fuels such as hydrogen and ammonia, methanol or synthetic methane provided the emissions are captured and stored. This will be critical to producing marine bunker fuels at volume because ensuring access to ‘green’ energy is the industry’s holy grail.

Net-zero emissions can be achieved when the amount of GHGs released into the atmosphere equals the amount absorbed by sinks. Net-zero quantification is not easy and requires a life-cycle approach from well to wake to ensure that all the emissions are considered.

The energy transition that needs to occur as we strive to reach the decarbonization targets will be based on two value chains, Hydrogen and Carbon. By 2050, shipping will annually require 46 million tons (Mt) of green Hydrogen, or 1.4-5.6 terawatts (TW) in terms of renewable energy.

Carbon capture is a vital part of to the transition to net zero. It provides solutions for current energy assets, as well a pathway for rapidly scaling up low-emission hydrogen production. However, a limiting factor might still prove to be the capacity of infrastructure used for carbon capture utilisation and storage (CCUS) in mid-and long-term.

For the latest in the series of ABS sustainability publications – the Zero Carbon Outlook – ABS worked with Herbert Engineering Corp (HEC) to develop conceptual ship designs for capture and storage of carbon aboard conventional fossil fuelled cargo ships.

The design philosophy choice made for these LCO2 carriers will be using conventional fossil fuel propulsion and auxiliary plants and will have an appropriately sized CCS system. The high specific gravity cargo (liquid CO2 weighs approximately 1 Mt/m3) and the weight of the tank imposes large residual buoyancy in addition to net cargo tank volume. 

The LCO2 ships have two main cargo tanks occupying the ship’s mid-body and a smaller CCS tank at the bow. The CCS tank is a vertical cylinder to accommodate it in the finer bow sections. In addition, the CCS liquefaction plant is separate from the cargo and CCS refrigeration plant to simplify and minimise plant power requirements. 

Since the market is still nascent, the ships are designed to carry alternative cargo which is easier for refrigerated LPG (liquefied petroleum gas) but very expensive for Ammonia, since it would require doubling tank thickness and a complex cargo handling and piping system.

ABS and HEC also collaborated on the development of designs for liquid hydrogen (LH2) carriers, still in the early prototype stages, focusing on two size categories, of 25,000 m3 and 80,000 m3, both using double wall spherical tanks carrying LH2 at ambient temperatures and -253 degrees C.

This design incorporates advanced technologies similar to those employed by NASA for its hydrogen storage extension program at the Kennedy Space Center. However, to reflect the feasibility of current technology to realise such design, the ship’s main and auxiliary engines are powered by LNG, which is stored in membrane tanks at the bow. The main engine is sized to meet the maximum propulsion power capacity of 6.7 MW, plus 3.2 MW for auxiliaries.

The LH2 storage system features NASA’s IRAS (Integrated Refrigeration and Storage) to achieve zero boil-off, on the knowledge that, even at current LNG prices, venting of hydrogen cargo would not only be dangerous but also significantly more expensive than the methane needed by the cargo refrigeration system and associated CAPEX. Cargo refrigeration is provided by a helium plant needing around 1.0 MW of electrical power.

The propulsion power is provided to twin high performance propellers matched to rudder bulbs, having assumed that the remaining auxiliaries’ power would be in the range of 0.85 MW while sailing. The main engine and fuel tank capacity are sized to provide enough power to the vessel to sail at 16.2 knots with a 20% sea margin for 15.5 days, covering 6,000 nautical miles with the full auxiliary load of 1.85 MW.

Image: © ABS in partnership with Herbert Engineering

Lesley Bankes-Hughes