Technology qualification of low-pressure CO₂ ship transportation

Background
Carbon capture and storage (CCS) will be a key technology to meet the goals of the Paris and Glasgow agreements. Although the technologies and the industry are very much still emerging, a possible challenge is connecting capture sources to facilities for use or storage sites, especially where pipelines are not an option due to combination of transport distance and volumes. As a result, it is expected that CO₂ ship transport technology will be needed if large quantities are to be safely transported at costs that are commercially viable.

Shipping of liquified CO₂ already takes place, although at a limited scale and exclusively for commercial trade (food and beverage, cleaning, chemical). Liquid CO₂ is currently transported on ships as a semi-refrigerated under pressure, similarly as for other gases (e.g., propane, butane, etc.). At this so-called medium-pressure condition, carbon dioxide is transported in liquid state at pressure in the range of 15 to 18 barg and a temperature of approximately -21 to -26 °C. A medium-pressure value chain has limited risk due to its technical maturity and hence it was chosen for the Northern Lights. The current ship maximum cargo capacities for medium-pressure condition are in the range of 7500 m3.

For a large-scale value chain, a development towards larger transport volumes per ship is foreseen, this is also expected to be an enabler for direct injection in offshore cargo transfer. This is expected to benefit from reduced tank pressure, denoted low-pressure. At low-pressure, the carbon dioxide is foreseen to be transported at tank pressure in range of 6 to 7 barg and corresponding liquid temperatures in range of -46 to -49 °C. The reduced pressure allows for larger cargo tank diameter and benefit from increased product (liquid) mass density, hence increased cargo capacity per ship.

Considering both investment and operating cost, increased ship cargo capacities is foreseen to enable a reduction in overall transport cost.
Besides the increased ship tank size and cargo volumes, a low-pressure condition (and temperature) will also have technical and commercial implications for the entire transport value chain with regards to conditioning of product for transport, loading and offloading operations and intermediate storage of product.

The potential challenge with respect to the lower pressure is the proximity to the triple point for CO₂. At the triple point, the CO₂ can be present in either solid (dry ice), liquid and gas state. The solid state is perceived as a challenge with regards to operability within the value chain. Currently, there is no industry experience with large scale low-pressure CO₂ ship transport.

Objective
DNV AS on the behalf of the consortium composed by Equinor Energy AS, Gassco AS, TotalEnergies EP Norge AS and Shell is leading a Technology Qualification Programme for low-pressure CO₂ ship transportation concept.
CETO (CO₂ Efficient Transport via Ocean) aims to reduce the uncertainties related to design, construction, and operation of a low-pressure ship transport chain and to enhance solutions for ship transport of CO₂ compared to the current practice and commercial scale.

Activities
A series of engineering/desktop studies and experimental activities have been defined and are being carried out to de-risk low-pressure value chain and prove the technical feasibility. These activities include:
• Conceptual design of a conditioning and liquefaction plant and experimental demonstration of the liquefaction at low-pressure
• Basic ship concept design of an appropriate and efficient dedicated CO₂ carrier and cargo handling system to accommodate and handle large volume of CO₂ ( ̴ 30 000 m3)
• Design of the cargo containment system and suitable material selection to accommodate the large cargo weight, ensure constructability and operation for design temperature at -55 °C.
• Investigate, by executing experimental activities, the operability of a low-pressure system in proximity to the triple point and determine a safe envelope for the operation as well as demonstrate ability to deal with eventual dry ice formation.
• Simulation tools for assessing of cargo handling and operations, interaction with the loading and offloading onshore facilitates and benchmarking of process simulation tools.
• Experimental and modelling work within CO₂ thermodynamics to reduce the uncertainty in software design tools.

What has the project achieved?
The execution of the qualification activities provides valuable technical experience related to the uncertainties and technical risks for a low-pressure value chain. At the current stage of the project, not all the results are consolidated, and it is therefore premature to conclude on the technical feasibility of the value chain.

Through the engineering activities, it was shown that a low-pressure liquefaction plant can be designed to operate at the design temperature. Suitable materials for the plant and for the storage tank have been also identified as well as technical solutions to deal with the different impurities and dehydration. On the other hand, the study highlighted that a significant storage volume is necessary to sustain the required export. This aspect shall be addressed in relation to the increased transported volume in the context of CCS.

The design activities related to the concept of the LCO₂ carrier have shown that the ship can be designed in compliance with current rules. However, during the design iteration, meeting the draft limitation of 10 m was shown to be a limiting factor when designing larger vessels. The wide breath and corresponding high accelerations have a direct influence on the design of the containment system.

The tasks related to the tank design and material selection for the containment system confirmed that fatigue is an aspect that need to be addressed in a design phase and it may limit the size of the containment system. Extra high tensile steel (P690) was found to be an attractive option with respect to cost and achievable size for the containment system. On the other hand, the qualification activities indicate that it was challenging to document a sufficient material performance at the design temperature of -55°C Alternatively, nickel alloy could be selected for the containment system with implication to the tank size, design temperature and cost. Additional investigations are recommended to identify the optimal material for the containment system.

The experimental activities related to liquefaction (NCCS) and cargo handling indicated that it is possible to produce and handle liquid CO₂ at low pressure condition. The testing campaign resembling the ship operation showed that the liquid product could be transferred from ship tank to onshore tank and vice versa without any dry ice formation for a pressure in the range of 6 barg. Tests at lower pressure were also carried out and were partially successful as it was possible to transfer the product only from the ship tank to the onshore tank. It shall be noted that these results are obtained for the current CO₂ specification and test rig which features some differences from full scale cargo handling system.

The work executed under the process simulation and thermodynamic provided an understanding of the level of accuracy of thermodynamic predictions and capabilities of commercial calculation tools. Journal papers and technical articles extracted from the executed work have been published. As per date these are:

• GHGT16 Proceedings, CETO: Technology Qualification of Low-Pressure CO₂ Ship Transport; Gabriele Notaro, Jed Belgaroui, Knut Maråk, Roe Tverrå, Steve Burthom, Erik Mathias Sørhaug. 16th Greenhouse Gas Control Technologies Conference, Lyon 23-27 October 2022
• Michael Drescher, Adil Fahmi, Didier Jamois, Christophe Proust, Esteban Marques-Riquelme, Jed Belgaroui, Leyla Teberikler, Alexandre Laruelle. “Blowdown of CO₂ vessels at low and medium pressure conditions: Experiments and simulations” 0957-5820/© 2023 Institution of Chemical Engineers. Published by Elsevier Ltd.
• Rod Burgass, Antonin Chapoy Dehydration requirements for CO₂ and impure CO₂ for ship transport, Fluid Phase Equilibria. Volume 572, September 2023, 113830
• GHGT16 Proceedings, Poster presentation, BLOWDOWN OF CO₂ VESSELS AT LOW AND MEDIUM PRESSURE CONDITIONS: EXPERIMENTS AND SIMULATIONS; Michael Drescher, Adil Fahmi , Didier Jamois , Christophe Proust , Esteban Marques-Riquelme, Jed Belgaroui, Leyla Teberikler, Alexandre Laruelle. 16th Greenhouse Gas Control Technologies Conference, Lyon 23-27 October 2022

Future plans
The partners are currently discussing further activities related to the material qualification.
The study indicated that as per date, the optimal material for the design and construction of the containment system for the carriage of liquid CO₂ at low pressure has not been yet identified.
Such material shall combine high strength, low-temperature performance, be competitive with respect to cost and fabrication of the containment system (i.e., welding technology, welding consumable, and eventual post weld heat treatment). Cooperation with suppliers and manufacturers is expected to reduce the risks related to the materials selection and tank manufacturing.