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ISO 27971:2025 — CO2 Transport by Ship: Vessel Design and Operational Requirements
Standardized Framework for Marine CO2 Transport in CCS Value Chains
1. Scope and Applicability
ISO 27971:2025 provides requirements and guidelines for the transport of CO2 streams by ship, covering vessel design, loading and unloading operations, cargo containment systems, monitoring, and emergency response. Marine CO2 transport is an essential component of CCS value chains where storage sites are located offshore or where pipeline transport is economically or geographically infeasible. The standard addresses both dedicated CO2 carriers and multipurpose chemical tankers adapted for CO2 service, with cargo capacities typically ranging from 2,000 to 50,000 m3.
CO2 is transported by ship in one of three conditions: fully refrigerated (-50 C, 0.7-1.0 MPa), semi-refrigerated (-30 to -40 C, 1.5-2.5 MPa), or pressurized ambient temperature (15-20 C, 5.0-6.0 MPa). Each condition has specific advantages for different transport distances and scales. Fully refrigerated transport is most common for large-scale CCS shipping operations.
Marine CO2 transport bridges the spatial gap between emission sources and storage sites. For distances exceeding 500 km, ship transport is often more economical than pipeline transport, with costs of $20-40 per tonne of CO2 compared to $30-60 per tonne for offshore pipelines.
2. Cargo Containment and Material Selection
The standard specifies cargo containment system requirements including insulation performance, boil-off gas management, and pressure control. For fully refrigerated CO2 transport (-50 C), the cargo tanks must be constructed from materials capable of maintaining ductility at low temperatures, typically 5% nickel steel or 9% nickel steel with suitable impact testing at -55 C. Tank types include independent prismatic tanks (Type A), independent cylindrical tanks (Type C), or membrane containment systems adapted from LNG carrier technology.
| Transport Condition | Temperature | Pressure | Density (kg/m3) | Typical Tank Material |
|---|---|---|---|---|
| Fully refrigerated | -50 C | 0.7-1.0 MPa | 1150-1200 | 5% or 9% Nickel steel |
| Semi-refrigerated | -30 to -40 C | 1.5-2.5 MPa | 1050-1150 | 3.5% Nickel or 5083 Al |
| Pressurized ambient | 15-20 C | 5.0-6.0 MPa | 750-850 | Carbon steel (high-tensile) |
Material selection for CO2 cargo tanks requires special attention to low-temperature brittle fracture prevention. Unlike LNG (-163 C), CO2 is transported at intermediate temperatures (-50 C) where standard carbon steel loses impact resistance but LNG-grade materials (9% Ni, stainless steel) may be unnecessarily expensive.
3. Loading and Unloading Operations
ISO 27971 provides detailed procedures for marine CO2 transfer operations including pre-transfer inspections, connection testing, inerting and purging sequences, cooldown procedures, transfer rate control, and vapor return management. The loading rate is typically limited by boil-off gas handling capacity and tank cooling rate to prevent excessive thermal stress. Unloading operations require pressure management to maintain the cargo above the triple point (-56.6 C, 0.52 MPa) to prevent dry ice formation.
Risk of dry ice formation is a unique challenge for CO2 ship transport. During unloading, if pressure drops below the triple point, solid CO2 (dry ice) can form, blocking piping and valves. The standard specifies minimum pressure and temperature margins to avoid this condition, typically maintaining a 0.2 MPa safety margin above the triple point pressure.
Dry ice formation during CO2 unloading is a serious operational hazard. Solid CO2 can completely block unloading lines within seconds, requiring emergency shut-down and thermal defrosting procedures that can delay operations by 12-24 hours.
4. Engineering Design and Operational Considerations
Key engineering considerations for CO2 shipping include boil-off gas management (typically 0.2-0.5% of cargo volume per day), reliquefaction system design, port infrastructure requirements, and compatibility with CO2 loading terminals. The standard recommends that CO2 carriers be designed with reliquefaction capacity of at least 1.5 times the expected maximum boil-off rate to maintain cargo condition during extended voyages and potential delays.
For integration with CCS value chains, the standard addresses CO2 quality specifications for ship transport. The most critical quality parameter is water content (must be below 50 ppmv to prevent carbonic acid corrosion and below 30 ppmv to prevent hydrate formation at operating conditions). Non-condensable gases (N2, O2, Ar) affect the triple point and vapor pressure of the cargo and must be limited to below 5 mol% to maintain manageable operating conditions.
Modern CO2 carriers with reliquefaction systems achieve boil-off rates below 0.25% per day. The reliquefaction system is typically designed with redundancy, as loss of containment pressure control is the most common cause of cargo loss during marine transport.
5. Frequently Asked Questions
Q: What ship sizes are typical for CO2 transport?
Current CO2 carriers range from 2,000 m3 (small-scale, regional) to 50,000 m3 (large-scale, international). The most common size for emerging CCS projects is 10,000-25,000 m3, carrying 10,000-25,000 tonnes per voyage.
Current CO2 carriers range from 2,000 m3 (small-scale, regional) to 50,000 m3 (large-scale, international). The most common size for emerging CCS projects is 10,000-25,000 m3, carrying 10,000-25,000 tonnes per voyage.
Q: How does CO2 shipping compare economically with pipeline transport?
For distances under 500 km, pipelines are generally cheaper. Above 500 km, ships become competitive. For transoceanic transport (>2000 km), shipping is the only viable option.
For distances under 500 km, pipelines are generally cheaper. Above 500 km, ships become competitive. For transoceanic transport (>2000 km), shipping is the only viable option.
Q: What is the typical boil-off rate for CO2 carriers?
For well-insulated tanks, boil-off is 0.2-0.5% of cargo volume per day. A 20,000 m3 carrier on a 10-day voyage would lose approximately 400-1,000 m3 of cargo to boil-off.
For well-insulated tanks, boil-off is 0.2-0.5% of cargo volume per day. A 20,000 m3 carrier on a 10-day voyage would lose approximately 400-1,000 m3 of cargo to boil-off.
Q: Can existing LPG or chemical tankers be converted to CO2 service?
Yes, but conversion requires assessment of tank materials, insulation capacity, pressure ratings, and piping compatibility. The reliquefaction system must be specifically designed for CO2. Conversion costs 30-50% of newbuilding cost.
Yes, but conversion requires assessment of tank materials, insulation capacity, pressure ratings, and piping compatibility. The reliquefaction system must be specifically designed for CO2. Conversion costs 30-50% of newbuilding cost.
