ISO 16259:2025

International
Standard
ISO 16259
First edition
Ships and marine technology —
2025-09
Performance test procedures of
LNG BOG re-liquefaction system on
board a ship
Navires et technologie maritime — Modes opératoires d'essais
de performance d'un système de re-liquéfaction BOG GNL à bord
d'un navire
Reference number
© ISO 2025
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ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 2
5 Inspection and tests . 2
5.1 General .2
5.2 Test fluid .3
5.3 Design appraisal .3
5.4 Equipment safety function testing .3
5.5 Hydrostatic test .3
5.6 Instrument signal inspection .4
5.7 Cryogenic function test .4
5.8 Emergency shutdown test .4
5.9 Performance test at cryogenic condition .5
5.10 Visual inspection .5
6 Marking . . 5
7 Test reports . 6
Annex A (informative) Inspection and test procedure . 7
Annex B (informative) Example of document lists applicable to design appraisal .11
Annex C (informative) Examples of test report forms .12
Annex D (informative) Example of concept model for LNG re-liquefaction system . 17
Annex E (informative) Components of natural gas . 19
Bibliography .20

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
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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
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This document was prepared by Technical Committee ISO/TC 8, Ships and marine technology, Subcommittee
SC 25, Maritime GHG reduction.
Any feedback or questions on this document should be directed to the user’s national standards body. A
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iv
Introduction
The shipbuilding and marine industries have shown growing interest in the use of liquified natural gas (LNG)
to meet the increasingly strict international, regional and national regulations on gas emissions which are
harmful to the environment. One of the most important pieces of equipment used in the LNG vessel system
is the LNG boil-off gas (BOG) re-liquefaction system, since it can change natural gas to liquefied natural gas.
This allows the LNG ship to decrease fuel consumption and contribute to marine environmental protection.
The LNG fuelled vessel or bunkering vessel is expected to be designed and manufactured according to
[1] [2]
the requirements of internationally recognized standards such as IMO IGF and IGC Codes. However,
an internationally recognized standard on the performance testing of LNG BOG re-liquefaction systems
on board a ship has not yet been established. As the LNG BOG re-liquefaction system is tested according
to a case-by-case agreement between the owner and manufacturer, such an internationally recognized
standard would enable a test method which is commonly accepted by all stakeholders (e.g. ship owners,
classification societies, equipment manufactures and shipyards). For this reason, it is important to have an
objective document to test the performance of the LNG BOG re-liquefaction system on board a ship, which is
commonly acceptable to all stakeholders (e.g. ship owners, classification societies, equipment manufacturers
and shipyards).
In this regard, this document is designed to set a test procedure to evaluate the performance of the LNG BOG
re-liquefaction system on board a ship. This document aims to:
a) enable the performance of LNG BOG re-liquefaction system to be identified from an objective point of view;
b) provide useful requirements and guidelines to all stakeholders;
c) facilitate the development of more reliable LNG BOG re-liquefaction systems, with a view to contributing
to the growth of relevant industries and benefitting all stakeholders.
This document is intended to be applied in various situations, including factory acceptance tests or onboard
tests. The LNG BOG re-liquefaction system provider and ship owner are expected to agree on any additional
requirements that apply beyond the test requirements outlined in this document.
NOTE Relevant IACS requirements and legal requirements can apply.

v
International Standard ISO 16259:2025(en)
Ships and marine technology — Performance test procedures
of LNG BOG re-liquefaction system on board a ship
1 Scope
This document specifies requirements and procedures for the performance test to verify the mechanical
features of the liquified natural gas (LNG) boil-off gas (BOG) re-liquefaction system on board a ship. These
test procedures can be prepared for LNG BOG re-liquefaction systems of LNG ships such as LNG carriers,
LNG bunkering vessels and LNG fuelled vessels. Functions which are specific to manufacturers and any
additional tests or specifications required by clients are not covered in this document.
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
design pressure
pressure at which each component of a piping system is designed to operate
Note 1 to entry: The design pressure must not be less than the pressure at the most severe condition of coincidental
internal or external pressure and temperature (maximum or minimum) expected during service.
3.2
design temperature
maximum temperature at which each component is designed to operate
Note 1 to entry: The design temperature must not be less than the temperature of the component material at the most
severe condition of temperature and coincidental pressure expected during service.
3.3
minimum working temperature
lowest temperature at which each equipment is designed to operate
3.4
maximum allowable working pressure
MAWP
maximum pressure of a piping system determined, in general, by the design pressure (3.1) of the weakest
piping component in the system or by the relief valve setting
Note 1 to entry: The MAWP shall not exceed the design pressure of a piping system.

3.5
liquefied natural gas
LNG
natural gas that has been cooled at atmospheric pressure to very low temperatures and condensed into
liquid form
Note 1 to entry: It is characterized as a cryogenic liquid having a temperature typically around –162 °C under normal
atmospheric pressure.
[SOURCE: ISO 20519:2021, 3.12, modified — "at atmospheric pressure to very low temperatures" has been
added to the definition; "–161 °C" has been changed to "–162 °C" in the note to entry.]
3.6
liquefaction
transformation of a gas into a liquid
3.7
boil-off gas
BOG
evaporation of liquefied natural gas (LNG) (3.5)
Note 1 to entry: Although the tanks are insulated, a small amount of warming occurs, causing the LNG cargo to
evaporate as it reaches its boiling point. This natural evaporation, known as boil-off, is unavoidable and the generated
boil-off gas (BOG) must be removed to preserve the tank pressure.
3.8
re-liquefaction system
system that consists of joule-thomson valve, separator, re-condensor, compressor, etc. for the purpose of
liquefying boil-off gas (BOG) (3.7)
Note 1 to entry: During liquid natural gas (LNG) (3.5) filling in a cargo tank and under navigation condition of LNG fuel
tanks, boil off gas occurs, exchanging heat between the ambient temperature and fluid cooling temperature. This gas
increases inside of the cargo tank. In order to maintain stable storage pressure, BOG can be used and incinerated by
other equipment. Especially for LNG carriers, BOG can be re-liquefied through a re-liquefaction system such as partial
re-liquefaction cycle and methane re-liquefaction cycle for greater operational efficiency.
4 Abbreviated terms
For the purpose of this document, the following abbreviated terms apply.
ASME BPVC American Society of Mechanical Engineers Boiler and Pressure Vessel Code
IMO International Maritime Organization
IGC Code International Code of the Construction and Equipment of Ships Carrying Liquefied  Gases in Bulk
IGF Code International Code of Safety for Ships using Gases or Other Low-flashpoint Fuels
IACS International Association of Classification Societies
KO Knock out drum
5 Inspection and tests
5.1 General
This clause specifies the tests for the various models and capacities of the LNG BOG re-liquefaction system.
The tests are intended to show typical characteristics of the system.

Any additional tests other than those specified in this Clause should be carried out with the consent
of the provider and client, unless otherwise specified by either the IACS classification society or the flag
administration of a ship, according to applicable requirements such as rules or regulations.
Annex A gives an example of a general inspection and test procedure.
Different types of LNG BOG re-liquefaction systems can be subject to inspection and tests according to
Annex A. LNG BOG re-liquefaction systems are expected to meet the requirements therein under mutual
agreement between owner and LNG BOG re-liquefaction system provider.
A sample test report is included under Annex C.
Annex D provides examples of a concept model for the LNG BOG re-liquefaction system.
Annex E shows general characteristics of natural gas. Gas composition can change based on where and how
natural gas is extracted.
NOTE As the LNG BOG re-liquefaction system works with fuel gas supply system (FGSS), all examples in Annex D
cover FGSS.
5.2 Test fluid
As a principle, LNG shall be used as the medium to verify the performance of the LNG BOG re-liquefaction system.
Mediums other than LNG, such as liquefied nitrogen or other fluids colder than the liquefied temperature
of LNG, may be used in order to conduct the test as a reference to the performance of the LNG BOG re-
liquefaction system where it has been designed for the same colder temperatures if LNG is not practical.
If a fluid other than LNG is used, correction values should be applied as set forth in ISO 6976 based on the
actual testing result.
In order to demonstrate duration and strength, lower temperature fluid can be used (e.g. liquid nitrogen).
5.3 Design appraisal
Prior to the test, it is necessary to obtain the design approval from either the classification society or the
flag administration of a ship in accordance with applicable requirements. Changes on the design concept
can require a new type test. However, in case of minor changes in nonessential components, it is possible
that a new type test is not required, depending on the agreement with the provider and client. Unless
otherwise specified by the IACS classification society or the flag administration of a ship, the drawings may
be submitted to the classification society for approval according to Annex B.
5.4 Equipment safety function testing
It shall be confirmed that the alarm and safety devices such as emergency shutdown system (ESDS),
operating of valves, etc. operate properly at the ambient and minimum working temperature.
Test reports and certificates on accessory items, such as pressure safety valves (PSV) and gas detectors,
should be provided by the manufacturer. Quality representative data such as a failure mode effect criticality
analysis (FMECA) should be reviewed and confirmed before system assembly.
NOTE Minimum working temperature is the lowest temperature at which each equipment is designed to operate.
5.5 Hydrostatic test
The LNG BOG re-liquefaction system connecting components, such as casing, flanges, etc. shall be
hydrostatic tested.
The test pressure shall not be less than 1,5 times the design pressure. During testing, the pressure shall
be maintained for not less than 30 min to permit complete examination of the parts under pressure. The

results of the hydrostatic test shall be considered satisfactory if during a 30 min period the pr
...