Physical Address
304 North Cardinal St.
Dorchester Center, MA 02124
ISO/TR 27929:2025 — Carbon Dioxide Capture, Transportation and Geological Storage — Transportation of CO2 by Ship
Guidelines for ship-based CO2 transportation including cargo containment, loading/unloading operations, and maritime safety
1. Introduction to Ship-Based CO2 Transportation
While pipeline transportation is the most common method for moving CO2 from capture facilities to storage sites, ship transportation offers important advantages in certain scenarios: long distances across oceans, access to offshore storage sites far from shore, flexible routing that can serve multiple sources and sinks, and potential for stepwise CCS deployment without requiring large upfront pipeline infrastructure investments. ISO/TR 27929:2025 addresses this transportation mode by providing comprehensive guidelines for ship-based CO2 transportation in the CCS chain.
Developed by ISO/TC 265, this Technical Report covers the full ship transportation chain including cargo containment systems, loading and unloading operations, ship design considerations, boil-off gas management, maritime safety, and integration with port facilities. It builds on experience from the established LPG and LNG shipping industries while addressing the unique properties of CO2, particularly its higher density, different phase behavior, and the potential for dry ice formation during pressure and temperature changes.
Ship transportation of CO2 is not a new concept — small-scale CO2 shipping has been practiced in the food and beverage industry for decades. However, the scale required for CCS applications (typically 1-2 MtCO2 per year per ship) demands new design approaches and operational procedures, which ISO/TR 27929 addresses.
2. CO2 Cargo Containment Systems
2.1 Operating Conditions and Cargo States
CO2 can be transported by ship in three states: low-temperature liquid (approximately -50°C at 0.7 MPa), medium-temperature liquid (-30 to -10°C at 1.5-3.0 MPa), or ambient-temperature dense-phase (near-ambient temperature at 4.0-7.0 MPa). Each state has distinct advantages and disadvantages. Low-temperature liquid minimizes pressure requirements but requires significant energy for refrigeration. Ambient-temperature dense-phase eliminates the need for onboard refrigeration but requires heavier pressure vessels. The standard provides guidance on selecting the optimal cargo state based on voyage distance, port facilities, upstream capture conditions, and downstream storage requirements.
| Cargo State | Temperature | Pressure | Density | Key Advantage | Key Challenge |
|---|---|---|---|---|---|
| Low-temperature liquid | -50°C | 0.7 MPa | ~1,150 kg/m³ | Low-pressure tanks | Refrigeration energy |
| Medium-temperature liquid | -20°C | 2.0 MPa | ~950 kg/m³ | Balance of pressure and temperature | Complex tank design |
| Dense-phase (ambient) | 10-20°C | 5.0 MPa | ~800 kg/m³ | No refrigeration needed | Heavy pressure vessels |
2.2 Tank Types and Materials
The standard reviews tank types suitable for CO2 service, adapted from the IMO International Gas Carrier (IGC) Code classification. Type C pressure vessels (cylindrical or spherical, designed for full containment pressure) are the most common choice for CO2 carriers due to their ability to handle the full range of potential operating pressures. Type C tanks for CO2 service require materials with adequate low-temperature toughness if operating in the liquid state range, or adequate strength at elevated pressure for dense-phase operation. The standard addresses material selection considerations including the need for impact testing at the minimum design temperature and resistance to CO2 corrosion in the presence of water.
A critical safety consideration for CO2 carriers is the risk of brittle fracture. CO2 at low temperature can embrittle carbon steel if not properly selected. The standard requires that tank materials have adequate Charpy V-notch impact energy at the minimum design temperature, following the requirements of the IGC Code and recognized classification society rules.
3. Loading and Unloading Operations
3.1 Cargo Transfer Systems
ISO/TR 27929 provides detailed guidance on cargo transfer procedures for both loading (capture facility to ship) and unloading (ship to storage facility or pipeline). Key considerations include: pre-cooling of cargo tanks and transfer lines, pressure management during transfer to prevent two-phase flow or dry ice formation, vapor return systems to balance pressure between the ship and shore, and emergency shutdown (ESD) systems. The standard emphasizes the importance of compatibility between ship and shore transfer systems, recommending standardized coupler sizes and communication protocols.
3.2 Boil-Off Gas Management
CO2 carriers, particularly those operating at low temperature, experience heat ingress that causes a portion of the cargo to vaporize (boil-off gas or BOG). Unlike LNG carriers where BOG is typically used as fuel, CO2 BOG cannot be vented to the atmosphere (it is a greenhouse gas) and is not suitable as ship fuel. The standard addresses BOG management strategies including: reliquefaction (recondensing the BOG and returning it to the cargo tanks), pressure build-up management (allowing the tank pressure to rise within design limits), and cold utilization (using the refrigeration capacity of the BOG for other shipboard systems). The standard provides guidance on selecting the appropriate BOG management strategy based on voyage duration, tank design, and environmental regulations.
From an engineering perspective, one of the most innovative aspects of ISO/TR 27929 is its guidance on integrating ship transportation with the wider CCS chain. The ship is not an isolated element — its operating conditions (pressure, temperature, cargo composition) must be compatible with both the liquefaction plant at the loading port and the receiving facility at the storage site. This systems integration perspective is essential for efficient CCS operations.
4. Maritime Safety and Regulatory Compliance
ISO/TR 27929 addresses maritime safety comprehensively, recognizing that CO2 presents unique hazards compared to other gas cargoes. CO2 is classified as a toxic and asphyxiant gas — high concentrations can cause rapid incapacitation and death. The standard provides guidance on cargo-area ventilation, gas detection systems, personal protective equipment (self-contained breathing apparatus), and emergency response procedures specific to CO2. Regulatory compliance with the IMO IGC Code and International Convention for the Prevention of Pollution from Ships (MARPOL) is addressed, noting that CO2 carriers must meet the same stringent safety standards as other gas carriers. The standard also addresses the evolving regulatory landscape, including potential future requirements under the IMO’s greenhouse gas reduction framework.
5. Port Facility Considerations
The standard addresses the shore-side infrastructure required for CO2 shipping operations. Port facilities must include: liquefaction plants or temporary storage (if CO2 is received as a gas from pipelines), intermediate storage tanks, loading arms and hoses rated for CO2 service, vapor return lines, and emergency response equipment. The standard provides guidance on the sizing of intermediate storage to ensure that ship loading operations are not disrupted by variations in CO2 supply from capture facilities. For ports that may serve multiple CCS projects, the standard recommends standardized interfaces to maximize operational flexibility.
6. Frequently Asked Questions
Q1: What is the typical capacity of a CO2 carrier designed for CCS service?
A: While ship sizes vary based on the specific project requirements, the standard references typical capacities in the range of 10,000-50,000 m³ for regional CCS operations, with larger vessels (up to 100,000 m³) considered for long-haul routes.
A: While ship sizes vary based on the specific project requirements, the standard references typical capacities in the range of 10,000-50,000 m³ for regional CCS operations, with larger vessels (up to 100,000 m³) considered for long-haul routes.
Q2: How does CO2 shipping compare economically with pipeline transportation?
A: Ship transportation typically has lower capital costs (no pipeline infrastructure) but higher operating costs (fuel, crew, port fees). The breakeven distance depends on factors including transport volume, distance, and pipeline route complexity. Generally, ships become competitive for distances exceeding 500-1000 km, particularly where offshore pipeline costs are high.
A: Ship transportation typically has lower capital costs (no pipeline infrastructure) but higher operating costs (fuel, crew, port fees). The breakeven distance depends on factors including transport volume, distance, and pipeline route complexity. Generally, ships become competitive for distances exceeding 500-1000 km, particularly where offshore pipeline costs are high.
Q3: Can existing LPG or LNG carriers be converted for CO2 service?
A: Conversion is technically possible but requires careful assessment. Key considerations include: different operating pressures and temperatures (CO2 typically requires higher pressure than LPG), different materials compatibility, and the need for reliquefaction systems specific to CO2. Dedicated new-build vessels are often preferred.
A: Conversion is technically possible but requires careful assessment. Key considerations include: different operating pressures and temperatures (CO2 typically requires higher pressure than LPG), different materials compatibility, and the need for reliquefaction systems specific to CO2. Dedicated new-build vessels are often preferred.
Q4: What happens to boil-off gas on a CO2 carrier?
A: Unlike LNG where BOG is burned as fuel, CO2 BOG cannot be vented (it is a GHG) and is not suitable as fuel. The standard recommends reliquefaction as the primary BOG management strategy, with the recondensed CO2 returned to the cargo tanks.
A: Unlike LNG where BOG is burned as fuel, CO2 BOG cannot be vented (it is a GHG) and is not suitable as fuel. The standard recommends reliquefaction as the primary BOG management strategy, with the recondensed CO2 returned to the cargo tanks.
