ISO/TR 27929

ISO/DTR 27929
ISO/TC 265
Secretariat: SCC
Date: 2025-05-0708-11
Transportation of carbon dioxide by ship
Transport de dioxyde de carbone par bateau

ISO/DTR 27929:(en)
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ii
ISO/DTR 27929:(en)
Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 3
5 Regulatory regime for maritime and inland waterways for CO transportation . 4
5.1 General . 4
5.2 Maritime governance . 4
5.3 Technical safety regime for maritime transportation of liquid CO2 . 6
5.4 Greenhouse gas emissions . 7
5.5 Trading and cross-border transportation . 7
6 Ship transport of CO . 8
6.1 General . 8
6.2 CO cargo transport conditions . 8
6.3 Cargo tank design . 10
6.4 CCS ship transport concepts . 12
6.5 Multi-gas and dedicated carriers . 15
6.6 Ship design . 16
7 Properties of CO , CO streams and mixing of CO streams influencing the ship
2 2 2
transportation . 16
7.1 Thermodynamic properties of CO and CO composition . 16
2 2
7.2 CO impurities and trace components . 18
7.3 Flexibility and mixing of CO2 streams from different sources . 19
8 Ship operation . 20
8.1 Ship and terminal modes of operation . 20
8.2 Compatibility and interface . 21
8.3 Cargo operations . 22
8.4 Cargo management . 23
9 Technical gaps and development . 24
9.1 Applicability and precision of existing requirements . 24
9.2 Identification of additional relevant requirements such as practices onshore . 24
9.3 Qualification and process for new technology . 25
9.4 Gaps and need for development . 25
10 Safety and risks . 25
10.1 Health, safety and environment (HSE) . 25
10.2 Measures to mitigate risks . 26
10.3 Special risks with liquid CO as ship cargo . 26
11 Quantification and verification of CO2 cargo . 27
11.1 General . 27
11.2 Quantification and measurement . 27
11.3 Verification . 28
12 Summary status and development needs for CO ship transportation for CCS value chains29
Bibliography . 30

iii
ISO/DTR 27929:(en)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee has been
established has the right to be represented on that committee. International organizations, governmental and
non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely with the
International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types of
ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent rights
in respect thereof. As of the date of publication of this document, ISO had not received notice of (a) patent(s)
which may be required to implement this document. However, implementers are cautioned that this may not
represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 265, Carbon dioxide capture, transportation, and
geological storage.
This second edition cancels and replaces the first edition (ISO/TR 27929:2024), which has been technically
revised.
The main changes are as follows:
— Figure 4 has been corrected to represent the correct phase diagram for CO ;
— Figure 5 has been revised to be consistent in wording.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv
ISO/DTR 27929:(en)
Introduction
In a carbon dioxide capture and storage (CCS) value chain, the main means for transportation of CO from an
emitter to storage are by ships or by pipelines. Transportation of gas in liquid state is well established in the
shipping industry and has been done for decades. However, liquid CO2 is different from other gases carried by
ships and poses new challenges for both ship design and ship operation. Compatibility along the value chain
is an essential element in the development of CCS. It is important to have a common understanding of how
different aspects, such as cargo temperature and pressure, can influence the ship design and ship operation.
The purpose of this document is to support consistency and compatibility in the design of CCS value chains
and address important areas where future development and standardization can add value. This document
will discuss discusses CO ship types, ship logistics and interface-specific aspects related to the safe and
reliable design and operation of CO ships.
Transportation of liquified gas on ships is governed by the regulations, codes and conventions drawn up under
the International Maritime Organization (IMO) which is referred to under United Nations Convention on the
Laws of the Sea (UNCLOS). Ships carrying CO2 are regulated by the IMO International Code for the Construction
and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code), which serves as the main technical
regulation for CO carriers under the International Convention for the Safety of Life at Sea (SOLAS).
Ship transportation of CO2 is currently limited to commercial trade for small-scale use in industries such as
the food or beverage industries and is served by a handful of small ships. However, the evolving industry
around CCS will demand transportation volumes of a different magnitude and involve development of new
ship designs and ship logistics concepts. These are introducing a need for knowledge-sharing related to type
of transportation concepts, CCS value chain compatibility, technical and operational reliability and the safety
of CO carriers.
Quantification, verification and reporting along the different elements in the CCS value chain will become
important. This document briefly describes the limitations and challenges to them and how they can be done
onboard the ship.
In this document, CO means a captured CO stream, including potential impurities following the capture
2 2
process, if not otherwise explicitly referred to as pure CO2.
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ISO/DTR 27929:(en)
Transportation of carbon dioxide by ship
1 Scope
This document provides insights into the essential aspects of CO shipping and provides basic descriptions of
how the CO2 carrier and technology therein is technically integrated with the CCS value chain. It also includes
a description of specific challenges of transporting CO as cargo, how this differs from other gases transported
by ships today, and how this influences the shipship's design and operation. Finally, this document introduces
how CO ships are regulated within the existing international maritime regulatory framework.
This document’s main focus is on the technical aspects of CO shipping. Commercial, liability and financial
aspects are intentionally kept out of the report.this document. However, general reference to commercial
impact is made where relevant.
This document focuses on the ship transportation of CO2 between loading and offloading facilities where the
system boundaries are at the ship manifold equipment that connects the ship to the other components in the
value chain. In the document the basis for the description of ship operation is transportation between two
shore-based terminals. A high-level description of other relevant interfaces is given on a conceptual level as
this has an impact on the shipship's design. However, any further description of potential solutions upstream
and downstream from the CO2 carrier is outside the scope. This document also gives a high-level description
of the physical properties of CO streams at the conditions relevant for shipping and how relevant impurities
maycan impact the ship and ship operation.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
3.1 3.1
barge
floating unit carrying freight on canals, rivers and in ports, either under its own power or towed by another
3.2 3.2
cargo containment system
arrangement for containment of the cargo including, where fitted, a primary and secondary barrier, associated
insulation and any intervening spaces, and adjacent structure, if necessary, for the support of these elements
Note 1 to entry: SeeFor more details on the cargo containment system, see Reference [0].].
3.3 3.3
CO carrier
cargo ship or barge (3.1) constructed or adapted and used for the carriage of CO as cargo
ISO/DTR 27929:(en)
3.4 3.4
CO2 stream
stream consisting overwhelmingly of carbon dioxide
[SOURCE: ISO 27917:2017, 3.2.10], modified — Note 1 to entry has been deleted.]
3.5 3.5
dynamic positioning system
equipment and system that is used for keeping a vessel at a given position using the thruster and propulsion
of the vessel to compensate for the environmental loads, including waves, wind, current, etc.
3.6 3.6
export location
location where the ship loads the CO2 for transport to the import location (3.10)
3.7 3.7
flag state
jurisdiction under whose laws the ship is registered
3.8 3.8
heat ingress
transfer of heat from the surroundings into the cargo
3.9 3.9
heel
liquid cargo maintained at the bottom of the tank on the return voyage to maintain cargo tank temperature
3.10 3.10
import location
location where the ship offloads the CO2 that is transported from the export location (3.6)
3.11 3.11
inland waterway
natural or artificial navigable inland body of water, or system of interconnected bodies of water, used for
transport, such as lakes, rivers or canals
3.12 3.12
intermediate storage
storage of CO volumes before being loaded to a ship and storage after being offloaded from the ship
3.13 3.13
Formatted: English (United States)
multi-lobe
bi-lobe
tri-lobe
cargo tanks which consist of two (bi-lobe) or three (tri-lobe) lobes where lobes represent cylinder segments
partly merged and connected by a common bulkhead
3.14 3.14
muster area
location where the crew assemble in the event of an emergency
3.15 3.15
riser
flexible pipe that connects an offshore well to a ship or floating offshore unit
ISO/DTR 27929:(en)
3.16 3.16
territorial seassea
areasarea which extendextends up to 12 nautical miles from the baseline of a country’s coastal line
3.17 3.17
triple point
temperature and pressure at which three phases (gas, liquid and solid) of a substance coexist in
thermodynamic equilibrium
3.18 3.18
two-phase flow
simultaneous flow of gas and liquid
3.19 3.19
vapour return
connection between ship and terminal for vapour exchange to ensure pressure equilibrium between the shore
storage tanks and the ship cargo tanks
3.20 3.20
vapour-liquid equilibrium
state where a substance's liquid and vapour phases are in equilibrium
4 Abbreviations
4 Abbreviated terms
CO carbon dioxide
CCS carbon dioxide capture and storage
ESD emergency shut down
FPSO floating production storage and offloading
FSIU floating storage and injection unit
IGC Code International Code of the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk
IMO International Maritime Organization
LNG liquified natural gas
LPG liquefied petroleum gas
NIST National Institute of Standards and Technology
OCIMF Oil Companies International Maritime Forum
SIGTTO Society of International Gas Tanker and Terminal Operators
SOLAS International Convention for the Safety of Life at Sea
UNCLOS United Nations Convention on the Law of the Sea
IACS International Association of Classification Societies
ES-TRIN European Standard — Technical Requirements for Inland Navigation vessels
ISO/DTR 27929:(en)
5 Regulatory regime for maritime and inland waterways for CO transportation
5.1 General
International and national shipping are subject to an extensive and stringent set of laws and regulations which
are enforced by international, regional and national regulatory bodies. Considering the large number of
stakeholders and the significant environmental and economic impact the maritime industry has on the society,
regulations are developed to enable cooperation between stakeholders and to promote and improve safety,
security, and efficiency, and to prevent marine and atmospheric pollution by ships.
Marine transportation of liquefied gases, including CO in bulk by dedicated gas carriers, is well regulated with
proven and high safety standards developed by IMO and other governmental and industry organizations.
Considering the increased focus on CCS, it is however expected that laws and regulations for maritime
transportation of CO2 will be further developed.
GeneralA general description of the maritime governance scheme is explained in 5.2. It is followed by a
description of the main regulatory regime for CO carriers.
5.2 Maritime governance
International shipping involves vessels which operate across the oceans as well as territorial seas and
exclusive economic zones. Other vessels are limited to coastal and inland waterways transport within
territorial waters covered by the jurisdictions of a single state or multiple states. Maritime shipping is a mature
industry with well-established international governance institutions; however, the regulatory scheme can be
different depending on the area and type of operation. A generic overview of the maritime governance scheme
and stakeholders involved in maritime shipping is shown in Figure 1.
ISO/DTR 27929:(en)
Figure 1 — Governance and stakeholders in maritime shipping
The governance of the ocean emerges from the United Nations. UNCLOS lays down a comprehensive regime
of law and order in the world’s oceans and seas by, inter alia, defining the maritime geographical jurisdiction,
including the coastal states’ sovereignty over their respective maritime zones.
IMO is a specialized agency under the United Nations that develops conventions containing detailed
regulations to safety, security and environment, with the intention of establishing a global minimum standard
ISO/DTR 27929:(en)
for the shipping industry. Under IMO, more than 60 conventions, codes and regulations have been developed
which serve the basis for the implementation in the legislations of the individual member states.
Regional governmental organizations can develop additional regulations which apply for specific geographical
areas or member states. The European Union (EU) is an example of a regional governmental organization
which through, its regulations and directives, aim at ensuring common standards among the EU member
states.
The regulations developed by IMO or any other governmental organization are upon ratification implemented
into the national laws of the ratifying states. The flag states enforce the regulations for ships registered under
their flag. Port states exercise port state control on ships visiting their ports based on domestic laws, to ensure
the ship’s condition and equipment are in compliance with the provisions of international conventions and
that the ship is safely manned and operated pursuant to applicable international law. Flag and port states can
introduce additional regional or domestic regulations which apply within their jurisdiction.
Within the convention framework set by IMO and the regulations set by flag states, the classification societies
play an important role as independent governance actors. The major classification societies form the
International Association of Classification Societies (IACS), which works together with the industry and
maritime regulators to ensure that the legislative framework is supported and enhanced by the practical
implementation of classification rules. IACS has an observer role in IMO which allows them to provide support
and advise toadvice on the IMO process. The classification societies develop and maintain technical rules and
standards for the construction and operation of ships, and carry out classification, certification and verification
services, as well as surveys to ensure compliance with the standards. The classification is the basis for the
registration with the flag state and is required by IMO for international voyages. The classification standards
are generally internationally recognized and in compliance with international maritime regulations.
The classification societies can, on behalf of a flag state administration, undertake statutory certification to the
extent the society has been authorized to do so by the individual flag state administration. Statutory
certification includes among others approval, surveys, and the issuance of statutory certificates.
Other non-governmental organizations such as International Chamber of Shipping (ICS), International
Association of Independent Tanker Owners (INTERTANKO), Oil Companies International Maritime Forum
(OCIMF), Society of International Gas Tanker and Terminal Operators (SIGTTO), are also important
stakeholders in maritime shipping. These are industry organizations with the aim of sharing experiences,
addressing common problems and establishing a framework of standards, guidelines, and best practices for
the industry. Publications from these organizations often become industry standards and are important for
ensuring standardization particularly regarding operational compatibility and safety. Several of these
organizations have consultative status in IMO.
Considering the sovereignty of the territorial seas and internal waters as laid down in UNCLOS, the coastal
states are not bound by the framework issued by IMO and other organizations when forming the legislative
framework for ships operating within the territory of the state, unless the instruments are ratified by the
individual states. Hence, the regulatory framework which is basis for the national legislations can differ from
that of international shipping. Many states do however use the international legislative framework as a basis
for their national frameworks, potentially with modifications and adjustments as found relevant depending
on the type of ship and trade, the operational area, etc. regional (e.g. bi-lateral or multi-lateral) requirements
and agreements can apply to specific operational areas within the territories of two or more states. One
example is the regulations applicable for the inland waterways system in Europe, which is described in more
detail in 5.3.
5.3 Technical safety regime for maritime transportation of liquid CO
The carriage of liquid CO onboard ships for international trade is governed pursuant to the IMO framework
and by the provisions in the SOLAS, and is further detailed in mandatory codes, depending on the mode of
transport. The regulations distinguish between the carriage of the product in packaged form, e.g. as modular
ISO/DTR 27929:(en)
tank containers on cargo ships, and the carriage of the product in bulk on dedicated gas carriers as explained
in more detail in the following. Carriage of product in package form is regulated by the International Maritime
Dangerous Goods Code (IMDG) while the carriage of product in bulk on dedicated gas carriers is regulated by
the IGC Code.
The IGC Code is the governing international technical standard prescribing the design and construction
requirements of ships carrying liquid gases, including CO in bulk. The IGC Code requirements are targeted to
address the particular hazards related to different liquefied gases, including flammability, toxicity,
asphyxiation, etc., including a set of specific requirements for the carriage of CO .
Classification societies normally have specific class notations which cover design and construction
requirements for gas carriers. These requirements are normally based on the IGC Code, but are often more
detailed on the specific requirements to ensure practical implementation of the requirements given in the
Code and that the overall safety targets are met. Industry organizations such as SIGTTO have developed a
series of best practices, guidelines and standards targeting liquified gas carriers and terminals. Although these
are focusing on commonly transported products such as LNG and LPG, many of these can also be applicable
for CO transportation.
As explained in 5.2,, an individual state is not bound to the international standards and codes described above
for trades within the territory of the state, e.g. for inland waterways. Russia, Brazil, China, India, EU members,
and many other countries have well developed inland waterway systems which can be an attractive
alternative for CO transportation. It is expected that a regulatory framework will need to be developed for
the individual areas when and where this mode of transport becomes relevant, including cross border
agreements. Some countries have existing regulations for transport of dangerous goods on inland waterways
which can be relevant. It is, however, expected that IGC Code and other international standards will be used
as supplementary references for establishing the regulatory framework for domestic and cross-border CO
trades on inland waterways.
Vessels used for goods transport on inland waterways within the European Union are regulated by the EU
Directive 2016/1629 Technical Requirements for Inland Waterway Vessels, which is the mechanism for
incorporating technical standards (e.g. ES-TRIN) into EU law. Within this framework the European Agreement
concerning the International Carriage of Dangerous Goods by Inland Waterways contains the provisions for
the carriage of dangerous goods in packages and in bulk. Other countries, regions and territories can have
other applicable regulations.
5.4 Greenhouse gas emissions
Ships today use fossil fuels for propulsion, only with few exceptions, and in that sense ship transportation of
CO will have a CO footprint depending mainly on the type of fuel and the ship size. IMO has an ambition to
2 2
reduce the total emissions from the world fleet and in carbon intensity (CO emitted per cargo-carrying
capacity and nautical mile) by 2050. To meet the defined targets regulations are gradually being enforced.
Emissions from a ship’s machinery during operation are reported through schemes defined by both IMO and
EU (for vessels calling at European ports).
5.5 Trading and cross-border transportation
For cross-border transportation of CO2 where the CO2 is transported for the purpose of offshore storage, the
1972 Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter (London
Convention) and its 1996 London Protocol are important. The London Convention is one of the first
international conventions protecting the marine environment from human activities. Its objective is to
promote the effective control of all sources of marine dumping and take all practicable steps to prevent
pollution of the sea by the dumping of waste and other matter. The contracting parties (countries party to the
London Protocol) eventually recognized the need for a more precautionary and preventative approach and
undertook a full review of the Convention. This review resulted in the 1996 London Protocol.
ISO/DTR 27929:(en)
The London Protocol is a stand-alone agreement that supersedes the London Convention for the states that
are party to both instruments. This means that the London Protocol will prevail if there is a conflict between
the two instruments. The Protocol is more restrictive and adopts a general ban on all dumping activities, with
the exception of the wastes and other matter listed in Annex 1 of the London Protocol. The dumping of wastes
and other matter listed as an exception in the London Protocol Annex 1 requires a prior permit issued in line
with the London Protocol Annex 2 requirements. In 2006, the contracting parties to the London Protocol
adopted an amendment to Annex 1, adding CO to the list of exceptions, thereby creating a legal basis in
international environmental law to regulate CO storage in sub-seabed geological formations. This was
necessary as storage falls within the definition of dumping.
6Article 6 of the London Protocol prohibits the export of wastes and other matter across borders for dumping
at sea. This presented a barrier for cross-border CCS operations as it prohibits the export of CO for storage in
other countries where the intended storage site is offshore. In order to overcome this barrier, 6Article 6 was
amended in 2009 to allow for export of CO2 for offshore storage by adding a second paragraph exempting CO2
from the general ban. The new 6.2Article 6.2 sets out criteria for the export to occur, including the need for an
agreement or arrangement between the states involved identifying and allocating permitting responsibilities.
The amendment is not yet in force at the time of writing as an amendment requires two-thirds of the
contracting parties to have formally accepted it. After ten years with little progress of reaching the required
number, the contracting parties agreed to an interim solution of provisional application in 2019. This entails
that the contracting parties can provisionally apply the 2009 amendment and export CO across borders
provided that the parties have deposited a declaration of provisional application to the IMO (who acts as the
Secretariat for the London Convention and the Protocol).
While the London Protocol has implications for cross-border transportation and offshore storage for its
contracting parties, it can also impact non-contracting parties that wish to engage in such CCS operations. This
is the case where a contracting party seeks to export CO2 by pipeline or ship for offshore storage in a non-
contracting party. In this scenario, the contracting party has a duty to ensure that the non-contracting party
has a regulatory framework in place that is the same as, or provides better protection of the marine
environment than, the provisions contained in the Protocol. This is to ensure that the contracting parties do
not evade their obligations under the Protocol, and that the CO is always stored according to the Protocol’s
requirements in a safe and long-term manner. The Protocol can therefore indirectly apply to non-contracting
parties in these instances. In the case where a non-contracting party seeks to export CO in a contracting party,
no explicit obligation to enter a bilateral agreement exists from the perspective of the London protocol.
6 Ship transport of CO2
6.1 General
The main option for transporting CO2 by ships is in liquid state. CO2 is different from other liquefied gases
transported by ships as it cannot be liquid at atmospheric pressure regardless of temperature. CO therefore
needs to be pressurized to above the triple point pressure to exist in liquid form. The combination of
temperature and pressure required to maintain the CO in its liquid phase, as well as the high density, set high
requirements for the tank design, tank supporting structure and tank material terms of the strength and low
temperature properties.
The CO2 is conditioned to a liquid at the specified pressure and temperature before it can be transported on a
ship. This involves compressing, cooling and condensing the CO as well as removing any impurities to an
acceptable level.
6.2 CO cargo transport conditions
6.2.1 General
For liquid CO cargoes there are three different pressure and temperature regimes that are often referred to:
low pressure/low temperature, medium pressure/medium temperature and high pressure/ambient
ISO/DTR 27929:(en)
temperature. There is no clear definition of the boundaries between these three categories, however, a
designation in a CO transport context is proposed in Table 1. There is nothing preventing transporting CO
2 2
between the below proposed pressure and temperature ranges.
a
Table 1 — Pressure and temperature regimes for liquid CO cargo tank designs
Cargo designation Cargo vapour Equilibrium Density of liquid Density of vapour
a a a
pressure (operation) temperature CO2 CO2
1) 3 3
bara °C kg/m kg/m
) b
Low pressure 5,7 to 10 −54,3 to −40,1 1 170 to 1 117 15 to 26
Medium pressure 14 to 19 −30,5 to −21,2 1 078 to 1 037 36 to 50
High pressure 40 and above +5,3 and above 894 and lower 116 and higher
a
Applies for pure CO and properties taken from National Institute of Standards of Technology (NIST) database. Properties
willThe properties depend on the other components in the CO2 stream.
b 5 2
1 bar = 0,1 MPa = 10 Pa; 1 MPa = 1 N/mm .
Several factors through the CO transport chain influence the choice of pressure and temperature of the CO
2 2
from capture to storage. For shipping the chosen condition of the CO has a direct influence on the cargo tank
size and cargo system, tank material grades, insulation and boil-off management and hence the investment
cost and operation cost of the ships. The different pressure and temperature also result in different
requirements to liquefaction upstream of ship transport and handling downstream of ship transport and
hence the cost for the connecting elements of the value chain.
An alternative to transporting CO in liquid state is to transport it as solid which potentially couldcan allow
use of container ship or bulk carriers. See 6.2.5.
6.2.2 Low pressure
Low pressure is the cargo condition where the liquid cargo is transported close above the triple point but with
enough margin to avoid dry ice formation. The driver for transporting at low pressure is that it allows for
larger tank diameter and hence a large tank volume, hence lower mass of metal per amount of CO . The density
of CO2 is also higher at this condition compared to medium or high pressure. A direct consequence of low
pressure is that the temperature of the cargo is lower compared to cargo conditions with higher pressures.
A key challenge with a low-pressure cargo condition is the risk of dry ice formation in the cargo tank or cargo
handling system in case of a pressure reduction below the triple point pressure. The risk of dry ice formation
is also influenced by the impurities of the CO2.
6.2.3 Medium pressure
Medium pressure is the cargo condition used for the small CO carriers currently in operation for the food and
beverage industry. With medium pressure the margin to the triple point is large and there is limited risk for
dry ice formation during cargo operation.
Compared to low pressure transportation, a higher cargo pressure allows for higher design temperature for
the tank material; however, the maximum tank diameter and the cargo volume of each cargo tank is limited.

1) 5 2
1 bar = 0,1 MPa = 10 Pa; 1 MPa = 1 N/mm .
ISO/DTR 27929:(en)
6.2.4 High pressure
At near ambient temperature (assumed here as above 5 °C) the CO2 needs to be above approximately 40 bara
to be in liquid state. With this pressure the diameter of a pressure cargo tank is smaller than for medium and
low pressure, however the requirements to the cargo tank material properties becomes less challenging due
to the higher temperature.
The heat ingress into a high pressure tank system will beis less than for medium and low-pressure systems,
which can simplify the insulation requirement and cargo boil off management.
6.2.5 Density effects
The density for liquid CO increases with decreasing temperature, which increases the mass of liquid CO that
2 2
can be transported with the same tank volume. A space above the liquid level in the tank is occupied by CO -
vapour and allows the liquid CO to expand due to heat ingress.
To avoid large changes in temperature and pressure in the tanks on the return voyage, the pressure in the
cargo tanks is usually similar on the return voyage as on the loaded voyage. The pressure and temperature
are maintained by a small amount of liquid heel and a large volume of vapour (see 7).). As the density of vapour
increases with pressure, the mass of this returned CO is lowest for low pressure CO . The returned vapour
2 2
will eventually be re-liquefied at the export location.
When calculating the mass balance for liquid and gaseous CO volumes on the loaded and return voyage, the
net mass for CO transported in the same tank volume is higher for low pressure compared with medium
pressure. Correspondingly, the net mass of CO2 transported in a medium pressure system is higher than for
ana high pressure system for the same cargo volume.
6.2.6 Solid state CO (dry ice)
CO can also be transported in bulk in solid form, i.e. as dry ice. Dry ice will keep its state at atmospheric
pressure when the ambient temperature is lower than −78 °C. CO in solid form can in principle then be
transported onboard other ship types such as bulk carriers or container vessels designed for the low
temperature. The sublimated CO gas can be managed as boil off gas.
6.3 Cargo tank design
6.3.1 Cargo tank design considerations
The IGC Code defines different tank types which can be used for storage and transportation of liquefied gases
in bulk. Figure 2 shows the most common tank categories for tanks for liquefied gases as given by the IGC
Code. Independent tanks are self-supporting tanks which do not form part of the ship’s hull. Tanks are further
categorised into Types A, B and C based on shape, documentation approach and safety philosophy (see
Figure 2). Internal pressure limitation for Type A and B are relevant only for tank of prismatic shape. For
cylindrical or spherical shape higher internal pressures than 0,7 barg can be accepted. Membrane tanks are
non-self-supporting tanks with a gas and liquid tight membrane supported through insulation by the adjacent
hull structure.
As CO needs to be kept above minimum 5.,2 bara pressure to exist as liquid, the only available containment
system covered by the IGC Code is independent type C tanks. Type C tanks are a special type of pressure
vessels. These can either be of conventional cylindrical form or with multi-lobes (e.g. bi-lobe or tri-lobe).
Novel containment systems other than type C can be relevant for liquid CO2. This can be non-cylindrical
pressure vessel or pressure vessel designed to alternative design codes than the IGC Code. Concepts not
directly covered by tank types defined by the IGC Code need to go through a qualification process to ensure
that the safety targets of the IGC Code are met.
ISO/DTR 27929:(en)
The tank types, sizes and configurations offer different possibilities and limitations when it comes to utilizing
the ship cargo space. The high density of CO can yield different ship design constraints than for other liquefied
gas transportation.
Figure 2 — Gas tank categorization as given in IMO IGC Code
6.3.2 Tank material
The applicable materials for a liquid CO carrier cargo tank will be dependentdepends on the selected design
temperature. Due to the relatively high design pressure required for storage of liquid CO compared to other
gases carried by ships, materials which have a combination of high mechanical strength properties and a high
performance at low temperatures are required for medium and low-pressure concepts.
The IGC Code lists requirements for materials that are to be used in cargo tanks for liquefied gases. Materials
complying with these requirements can hence be used for CO transportation. Materials not meeting these
requirements can also be used if the material has successfully undergone a dedicated qualification process
which demonstrates the applicability for the intended use (see 8).).
The maximum diameter of a cargo tank for liquid CO is a function of the cargo design pressure, the material
properties and the maximum achievable tank wall thickness. In principle, high mechanical strength properties
of the tank material are essential to maximize cargo tank diameter. However, the combination of high strength
properties and toughness performance at the design temperature relevant for liquid CO is challenging,
particularly for medium and low pressure. To maximize the cargo tank diameter and size it is essential to use
materials with strength properties beyond what is typical covered by the IGC Code. Hence, a material
ISO/DTR 27929:(en)
qualification process of materials with high mechanical strength properties for relevant design temperature
is important for the development of large CO carriers for low and medium pressure.
Fatigue becomes more critical with use of high strength material
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