ISO 10976:2012

INTERNATIONAL ISO
STANDARD 10976
First edition
2012-07-01
Refrigerated light hydrocarbon fluids —
Measurement of cargoes on board LNG
carriers
Hydrocarbures légers réfrigérés — Mesurage des cargaisons à bord
des navires méthaniers
Reference number
©
ISO 2012
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Published in Switzerland
ii © ISO 2012 – All rights reserved

Contents Page
Foreword .iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms, definitions and abbreviated terms . 1
3.1 Terms and definitions . 1
3.2 Abbreviated terms . 5
4 General operating safety precautions and regulatory requirements . 6
4.1 General . 6
4.2 Electrical equipment classification . 7
4.3 Electromagnetic disturbance . 7
4.4 Maintenance . 7
4.5 Service conditions . 7
4.6 Compatibility . 7
4.7 Personnel protection . 7
4.8 Procedures . 7
5 Measurement systems and equipment . 7
5.1 General . 7
5.2 Measurement equipment performance . 8
5.3 Calibration and certification of measurement equipment . 8
5.4 Verification of measurement equipment between dry dockings . 9
5.5 Inspection of measurement equipment during transfer operations . 9
5.6 Static measurement systems and equipment . 9
5.7 Dynamic measurement systems and equipment .19
6 Measurement procedures .19
6.1 General .19
6.2 Static measurement .20
6.3 Gas-up and cool-down quantification.25
6.4 Dynamic measurement .25
7 Cargo calculations .25
7.1 General .25
7.2 LNG volume determination .26
7.3 LNG density determination .26
Annex A (informative) LNGC design and marine operations .27
Annex B (informative) Additional considerations for measurement on board an LNGC .34
Annex C (informative) Examples of tank capacity tables for a spherical tank .38
Annex D (informative) Calculation examples .44
Annex E (informative) Sampling .53
Annex F (informative) Marine measurement witnessing checklists .57
Bibliography .61
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.
International Standards are drafted in accordance with the rules given in the ISO/IEC Directives, Part 2.
The main task of technical committees is to prepare International Standards. Draft International Standards
adopted by the technical committees are circulated to the member bodies for voting. Publication as an
International Standard requires approval by at least 75 % of the member bodies casting a vote.
Attention is drawn to the possibility that some of the elements of this document may be the subject of patent
rights. ISO shall not be held responsible for identifying any or all such patent rights.
ISO 10976 was prepared by Technical Committee ISO/TC 28, Petroleum products and lubricants, Subcommittee
SC 5, Measurement of refrigerated hydrocarbon and non-petroleum based liquefied gaseous fuels.
This first edition of ISO 10976 cancels and replaces ISO 13398:1997, which has been technically revised.
iv © ISO 2012 – All rights reserved

Introduction
This International Standard provides accepted methods for measuring quantities on liquefied natural gas (LNG)
carriers for those involved in the LNG trade on ships and onshore. It includes recommended methods for
measuring, reporting and documenting quantities on board these vessels.
This International Standard is intended to establish uniform practices for the measurement of the quantity of
cargo on board LNG carriers from which the energy is computed. It details the commonly used current methods
of cargo measurement, but is not intended to preclude the use or development of any other technologies or
methods or the revision of the methods presented. It is intended that the reader review, in detail, the latest
editions of the publications, standards and documents referenced in this International Standard in order to gain
a better understanding of the methods described.
This International Standard is not intended to supersede any safety or operating practices recommended by
organizations, such as the International Maritime Organization (IMO), the International Chamber of Shipping
(ICS), the Oil Companies lnternational Marine Forum (OCIMF), the International Group of LNG Importers
(GIIGNL) and the Society of International Gas Tanker and Terminal Operators (SIGTTO), or individual operating
companies. This International Standard is not intended to supersede any other safety or environmental
considerations, local regulations or the specific provisions of any contract.
The International System of units (SI) is used throughout this standard as the primary units of measure since
this system is commonly used in the industry for these types of cargoes. However, as some LNG carrier’s
tanks are calibrated in US customary units and some sales and purchase agreements (SPA) are made in US
customary units, both SI and US customary equivalents are shown. Proper unit conversion is intended to be
applied, documented and agreed upon among all parties involved in the LNG custody transfer.
INTERNATIONAL STANDARD ISO 10976:2012(E)
Refrigerated light hydrocarbon fluids — Measurement of
cargoes on board LNG carriers
1 Scope
This International Standard establishes all of the steps needed to properly measure and account for the quantities
of cargoes on liquefied natural gas (LNG) carriers. This includes, but is not limited to, the measurement of liquid
volume, vapour volume, temperature and pressure, and accounting for the total quantity of the cargo on board.
This International Standard describes the use of common measurement systems used on board LNG carriers,
the aim of which is to improve the general knowledge and processes in the measurement of LNG for all parties
concerned. This International Standard provides general requirements for those involved in the LNG trade on
ships and onshore.
2 Normative references
The following referenced documents are indispensable for the application of this document. For dated
references, only the edition cited applies. For undated references, the latest edition of the referenced document
(including any amendments) applies.
ISO 8310, Refrigerated light hydrocarbon fluids — Measurement of temperature in tanks containing liquefied
gases — Resistance thermometers and thermocouples
ISO 8943, Refrigerated light hydrocarbon fluids — Sampling of liquefied natural gas — Continuous and
intermittent methods
ISO 18132-1, Refrigerated hydrocarbon and non-petroleum based liquefied gaseous fuels — General
requirements for automatic tank gauges — Part 1: Automatic tank gauges for liquefied natural gas on board
marine carriers and floating storage
IEC 60533, Electrical and electronic installations in ships — Electromagnetic compatibility
EN 1160, Installations and equipment for liquefied natural gas — General characteristics of liquefied natural gas
API Standard 2217A, Guidelines for Work in Inert Confined Spaces in the Petroleum and Petrochemical Industries
IACS Unified Requirements E10
ICS Tanker Safety Guide — Liquefied Gas
ICS/OCIMF/IAPH International Safety Guide for Oil Tankers and Terminals (ISGOTT)
IMO International Code for the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk (IGC Code)
NOTE Earlier versions of the gas codes can apply to older ships (see the note to 3.1.13).
SIGTTO Liquefied Gas Handling Principles on Ships and in Terminals
SIGTTO Liquefied Gas Fire Hazard Management
3 Terms, definitions and abbreviated terms
3.1 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
3.1.1
absolute pressure
total of the gauge pressure plus the pressure of the surrounding atmosphere
3.1.2
aerating
introduction of fresh air with an acceptable dew point into the tank
to purge inert gases and to increase the oxygen content to approximately 21 % of volume so as to ensure a
breathable atmosphere
3.1.3
approved equipment
equipment of a design approved by a recognized authority, such as a governmental agency, classification
society or other accredited agency which certifies the particular equipment as safe for use in a specified
hazardous atmosphere
3.1.4
automatic tank gauge
ATG
instrument that automatically measures and displays liquid levels or ullages in one or more tanks, either
continuously, periodically or on demand
3.1.5
automatic tank thermometer
ATT
instrument that automatically measures and displays the temperature of the contents in a tank, continuously,
periodically or on demand
3.1.6
boil off
process of evaporation of a liquid resulting from heat ingress or a drop in pressure
3.1.7
boil-off gas
vapour produced by boil off
3.1.8
cool down
process of reducing the temperature of equipment, such as piping, transfer arms and tanks associated with
custody transfer cargo movements, to required operating temperatures
3.1.9
constant pressure/floating piston sample container
CP/FP sample container
sample container, usually used for intermittent sampling, usually used for intermittent sampling, capable of
maintaining constant pressure during the sampling of gas from the process line into the gas cylinder
NOTE Adapted from ISO 8943:2007, definition 3.4.
3.1.10
continuous sampling
sampling from gasified LNG with constant flow rate
[ISO 8943:2007, definition 3.5]
3.1.11
drying
process of reducing the moisture in the ship tank by displacement or dilution with an inert gas or by the use of
a drying system
2 © ISO 2012 – All rights reserved

3.1.12
filling limit
filling ratio
quantity to which a tank may be safely filled, taking into account the possible expansion (and change in density)
of the liquid
NOTE Filling limit (i.e. volume) and filling ratio are expressed as a percentage of the total capacity of a tank.
3.1.13
gas codes
regulations on the construction of ships carrying liquefied gases developed by the International Maritime Organization
NOTE These include the IMO International Code for the Construction and Equipment of Ships Carrying Liquefied
Gases in Bulk (IGC Code) (generally applies to ships built after 17 July 1986), the IMO Code for Construction and Equipment
of Ships Carrying Liquefied Gases in Bulk (GC Code) (generally applies to ships built on or after 31 December 1976 but
prior to 17 July 1986) and the IMO Code for Existing Ships Carrying Liquefied Gases in Bulk (generally applies to ships
delivered before 31 December 1976), as applicable to each vessel.
3.1.14
gas sample container
sample container, usually used for continuous sampling and used for the retention of the gas sample and for
its transfer to an analysing instrument
[ISO 8943:2007, definition 3.6]
3.1.15
gassing up
process of replacing an inert atmosphere in a cargo tank with the vapour from shore or from another cargo
tank to a suitable level to allow cooling down and subsequent loading to achieve a specified environment with
at least a defined methane (CH ), carbon dioxide (CO ) and oxygen (O ) content
4 2 2
3.1.16
heel
amount of cargo retained in a cargo tank prior to loading or after discharge
3.1.17
inerting
introduction of inert gas into a tank with the object of attaining the inert condition
3.1.18
intermittent sampling
sampling from gasified LNG with predetermined intervals or with predetermined flow amount intervals
[ISO 8943:2007, definition 3.9]
3.1.19
letter of protest
letter issued by any participant in a custody transfer citing any condition with which issue is taken and which
serves as a written record that a particular action or finding was observed/questioned at the time of occurrence
3.1.20
LNG carrier
cargo ship specifically constructed and used for the carriage of LNG in bulk
3.1.21
LNG sample vaporizer
apparatus to completely gasify the LNG sample collected from the LNG transfer line
[ISO 8943:2007, definition 3.11]
3.1.22
multiple-spot ATT
multiple-point ATT
ATT consisting of multiple spot temperature element sensors to measure the temperature(s) at selected liquid level(s)
NOTE 1 The readout equipment for a multiple-point averaging ATT averages the readings from the submerged
temperature elements sensors to compute the average temperature of the liquid in the tank, and can also display the
temperature profile in the tank.
NOTE 2 Adapted from ISO 4266-5:2002, definition 3.4.
3.1.23
notice of apparent discrepancy
notice issued by any participant in a custody transfer citing any discrepancy in cargo quantities and which
serves as a written record that such a discrepancy was found
3.1.24
offline analysis
procedure of analysis implemented on the representative sample gas that is once charged into a gas sample
container or a CP/FP sample container
[ISO 8943:2007, definition 3.13]
3.1.25
online analysis
procedure of analysis implemented using analytical equipment that is directly connected through pipelines or
other means to the sampling device
[ISO 8943:2007, definition 3.14]
3.1.26
online gas chromatograph
gas chromatograph that is directly connected to the pipelines or sampling device to implement online analysis
[ISO 8943:2007, definition 3.15]
3.1.27
seal water
water used in the water seal type gas sample holder to preclude contact of the gas sample with the atmosphere
[ISO 8943:2007, definition 3.19]
3.1.28
tank capacity table
numeric tables that relate the liquid level in a tank to the volume contained in that tank
3.1.29
vapour
fluid in the gaseous state that is transferred to/from or contained within the cargo tank
3.1.30
vapour pressure
pressure at which a liquid and its vapour are in equilibrium at a given temperature
3.1.31
verification
process of confirming the accuracy of an instrument by comparing to a source with known accuracy
3.1.32
warming up
process of warming the cargo tanks from cargo carriage temperature to required temperature
4 © ISO 2012 – All rights reserved

3.1.33
waterless-type gas sample holder
holder without seal water (typically using an expandable/contractible, transformable rubber membrane) and
used for collecting gasified LNG
[ISO 8943:2007, definition 3.22]
3.1.34
water-seal-type gas sample holder
holder with seal water used for collecting gasified LNG
[ISO 8943:2007, definition 3.23]
3.2 Abbreviated terms
API American Petroleum Institute
ATG Automatic tank gauge
ATT Automatic tank thermometer
BOG Boil-off gas
CTMS Custody transfer measurement system
EMC Electromagnetic compatibility
FSRU Floating storage and re-gasification unit
GCU Gas combustion unit
GIIGNL Groupe International des Importateurs de Gaz Naturel Liquéfié
GNG Gaseous natural gas
GPA Gas Processors Association
IACS International Association of Classification Societies
IAPH International Association of Ports and Harbors
ICS International Chamber of Shipping
IEC International Electrotechnical Commission
IGC Code International Gas Carrier Code
IMO International Maritime Organization
ISGOTT International Safety Guide for Oil Tankers and Terminals
ISO International Organization for Standardization
LNG Liquefied natural gas
LNGC Liquefied natural gas carrier
MPMS Manual of Petroleum Measurement Standards
MSDS Material safety data sheet
OBQ On board quantity
OCIMF Oil Companies International Marine Forum
ROB Quantity remaining on board
SI International System of Units (Système International d’Unités)
SIGTTO Society of International Gas Tanker and Terminal Operators Limited
SPA Sales and purchase agreement
VEF Vessel experience factor
4 General operating safety precautions and regulatory requirements
4.1 General
Clause 4 applies to all types of measurement on board LNG carriers. However, while these precautions
represent safe operating practices, they should not be considered complete or comprehensive. In addition to
those listed in this International Standard, reference should be made to all safety precautions contained in any
relevant governmental, local or company operating guidelines.
IMPORTANT  Anyone working with the vessel’s measurement equipment shall be, at all times, under
the direction and supervision of the Master of the vessel or its designated representative and be
properly trained in its use.
Personnel involved in the handling of liquefied natural gas should be familiar with its physical and chemical
characteristics, including potential for fire, explosion, cryogenic burns (frostbite) and reactivity, as well as the
appropriate emergency procedures. These procedures should comply with the individual company’s safe
operating practices, in addition to local, state and federal regulations, including those covering the use of proper
protective clothing and equipment. Personnel should be alert in order to avoid potential sources of ignition.
SIGTTO publications Liquefied Gas Fire Hazard Management and Liquefied Gas Handling Principles on
Ships and in Terminals should be consulted to ensure familiarity with the characteristics and hazards of LNG,
all fire protection and fire fighting equipment on board LNG carriers along with the appropriate fire hazard
management plan.
API Standard 2217A and any applicable regulations should be consulted where entering into confined spaces.
Information regarding particular material safety and conditions should be obtained from the employer,
manufacturer or supplier of that material or the material safety data sheet (MSDS).
LNG is carried and handled at extremely low temperatures. The very nature of liquids at very low temperatures
is a hazard, added to which LNG itself has properties that shall be taken into account at all times. Any party
involved in handling operations shall read and act on information contained within the appropriate MSDS and
supporting documents.
Nothing contained in this International Standard is intended to supersede any regulatory requirements or
recommended operating practices issued by the vessel’s flag administration, classification societies or
organizations, such as IMO, SIGTTO or OCIMF, or individual operating companies. This International Standard
is not intended to conflict with any safety or environmental considerations, local conditions or the specific
provisions of any contract.
Accordingly, the latest editions of relevant IMO, SIGTTO, API and OCIMF publications, and, in particular, the
latest editions of the ICS Tanker Safety Guide — Liquefied Gas, the OCIMF/ICS/IAPH International Safety
Guide for Oil Tankers and Terminals (ISGOTT) and SIGTTO Liquefied Gas Fire Hazard Management should
be consulted for applicable safety precautions.
Any changes to measurement systems require the approval of the vessel’s flag administration and/or
classification society and require external verification of accuracy by a competent metrological authority for
LNG custody transfer measurement purposes.
All described equipment shall meet minimum requirements as detailed by the vessel’s flag administration and
classification society.
6 © ISO 2012 – All rights reserved

4.2 Electrical equipment classification
All measurement equipment used shall be approved equipment (see 3.1.3), which is certified intrinsically
safe or otherwise approved for its intended use, including appropriate grounding. Also, all measurement
equipment shall be designed and installed to meet applicable national and international marine safety codes
and regulations.
4.3 Electromagnetic disturbance
All custody transfer measurement systems (CTMS) shall be designed for electromagnetic compatibility (EMC),
complying with user requirements and other proper standards. This means that the equipment shall neither
interfere with nor be affected by interference from other equipment. Requirements and tests shall be in
accordance with IACS Unified Requirements E10 and IEC 60533.
4.4 Maintenance
All measurement equipment shall be maintained in safe operating condition and in compliance with the
manufacturers’ instructions.
4.5 Service conditions
All measurement equipment shall be capable of withstanding the vibration, pressure, temperature, humidity
and other environmental operating conditions likely to be encountered in the LNG carrier’s service.
4.6 Compatibility
All measurement equipment shall be constructed with appropriate materials suitable for use in LNG service in
accordance with the appropriate gas codes (see the note to 3.1.13) or EN 1160, and other applicable regulations.
4.7 Personnel protection
All personnel involved in LNG cargo activities should wear the appropriate personnel protective equipment
required for the operation and be trained in its proper use. They should also be trained regarding the inherent
hazards of LNG, as required by the ICS Tanker Safety Guide — Liquefied Gas and the LNG material safety
data sheet (MSDS).
4.8 Procedures
An adequate work procedure shall be established and available as guidance for safe work by the ship and
terminal personnel.
5 Measurement systems and equipment
5.1 General
Determination of cargo quantities on board an LNG carrier by the static measurement method requires
measurement of the liquid level (which is the liquid/vapour interface) as well as the pressure of the vapour
and average liquid and vapour temperature of each cargo tank. The volume of the liquid cargo is calculated
using the tank capacity table with any necessary corrections made. The custody transfer measurement system
(CTMS) includes the following:
a) cargo tank capacity tables;
b) inclinometers and/or draft gauges;
c) automatic tank gauges (see 3.1.4);
d) multiple-spot ATTs (see 3.1.22);
e) pressure sensors;
f) a CTMS computer.
NOTE As LNG quantities are generally transferred in units of energy, an automatic sampler system, typically located
onshore, provides a representative sample of the cargo, which is analysed for the determination of cargo quality, including
density by compositional analysis using a gas chromatograph.
To determine the quantities of cargoes on board LNG carriers, the amount of liquid in each tank shall be
determined. The factors needed to accomplish this include a calibrated tank as well as liquid level, pressure,
temperature and trim/list measurement equipment. The tank gauging systems used shall be of the closed type.
The most commonly used equipment is described in this clause. Certified systems other than those described
in this International Standard may be used for custody transfer measurement if the accuracies of each can be
ascertained and if the SPA permits their use.
5.2 Measurement equipment performance
The performance criteria of the primary and secondary equipment used to determine measured variables
are established in International Standards, governmental regulations, SPAs, manufacturers’ instructions
and calibration certificates, and are limited by the uncertainty of the instrument. In the absence of specified
tolerances, the maximum permissible error from certification shall meet the tolerances described in Table 1.
Table 1 — LNG measurement equipment performance criteria
Tolerance Display resolution
a
Level ±5,0 mm 1 mm
Pressure ±0,3 kPa 0,1 kPa
Temperature
≤ –145 °C ±0,2 °C 0,1 °C
> –145 °C ±1,5 °C 0,1 °C
Draft reading ±50 mm 10 mm
List (inclinometer) ±0,05° 0,01°
a
Some existing ATGs are not able to meet this verification tolerance, in which case a verification
tolerance of ± 7,5 mm may be applied.
5.3 Calibration and certification of measurement equipment
All specified measurement equipment used on board an LNG carrier shall be certified prior to initial use.
Subsequently, measurement equipment and systems shall be re-calibrated and re-certified on a periodic basis,
subject to SPA or national requirements. Measurement equipment shall be re-certified where modification or
repairs are carried out and which affect the accuracy of the measurement data.
The components of the CTMS and the accuracy of the quantity calculation of the CTMS shall be certified by a
recognized inspection body.
Calibration and re-calibration shall be performed by a qualified technician and witnessed by an independent
inspector. Upon successful calibration, the results shall be certified by the party witnessing the calibration and
a certificate of calibration issued.
Manufacturers of the measurement equipment and systems may participate in the calibration, which often
require setting, maintenance or replacement prior to final calibration of the equipment and the related
measurement system. For measurement equipment and systems, the calibration work should be witnessed by
the parties or their appointed independent inspector, who should be responsible for incorporating the results
in the certificate issued.
Calibration shall cover the local and remote readout, and data transmission to ensure the equipment, which
may consist of components of the measurement subsystem(s), delivers the specified accuracy.
8 © ISO 2012 – All rights reserved

5.4 Verification of measurement equipment between dry dockings
In addition to calibration during each dry docking, all measurement devices used in custody transfer shall be
checked before use at each loading or discharge to ensure they are in good working condition.
The comparison of the primary and secondary measurement device within a tank should be performed as one
means of verification. The results of this comparison should be recorded and tracked by the vessel operator.
One method of evaluating the results is through the use of a control chart. For control charts, see B.3.
Other devices may be verified while the ship is in service. For example, pressure gauges may be verified
against a reference standard device. Trim/list gauges, such as inclinometers or draft gauges (if used for level
corrections) may be verified/calibrated at even keel by comparison to manual draft measurements or other
equivalent procedure.
Where equipment is suspect or has failed, secondary devices shall be used in its place until the equipment
is repaired or verified to be in good working order. For example, in situ temperature verification/calibration at
cryogenic conditions is not practicable; therefore, temperature sensors which have been shown to be faulty
when verified during normal operation shall be replaced as soon as practicable.
Where the measurement equipment can be verified against a known value, the results of this verification
should be recorded and tracked. If the primary measurement system is found to be out of calibration, use of the
secondary measurement system should be considered in accordance with contractual agreement.
5.5 Inspection of measurement equipment during transfer operations
Prior to and during a custody transfer, the involved parties or an appointed independent inspector should
inspect the measurement equipment described in 5.1 to ensure that it is fully functional, and should also identify
any deficiencies. The ship’s records should be reviewed to determine whether the calibration certificates are
valid and current.
Exceptions and malfunction of measurement equipment, if any, prior to and during a custody transfer should
be immediately reported to the LNG carrier operator and the involved parties.
Upon specific request by the involved parties, on board testing, checks or verification may be carried out on the
measurement devices in question, and the results should be documented.
5.6 Static measurement systems and equipment
5.6.1 General
Static measurement systems and equipment are those individual systems and equipment which are used to
measure cargo in the tank. They include the following components (see 5.6.2 to 5.6.9).
5.6.2 Tank capacity tables
5.6.2.1 General
An independent company usually performs the calibration and generates the tank capacity tables during the
building of the LNG carrier. They take into account the configuration of the tank, its contraction according to the
temperature of the liquid, and the volume occupied by various devices, e.g. cargo pumps.
Tank capacity tables are divided into:
a) main gauge tables correlating liquid level and volume under reference conditions;
b) correction tables or methods, taking into account actual conditions of the LNG carrier and its measuring
instruments.
The tank capacity tables and related information, including measurements carried out and observations made
by the party performing the tank calibration and traceability of the equipment used, may be contained in a tank
calibration report. Additional discussion is provided below in 5.6.2.2 to 5.6.2.5.
Accuracy in determining cargo tank quantities is directly related to the accuracy of the LNG carrier’s capacity
tables. Therefore, the LNG carrier’s cargo tanks shall be measured and tank capacity tables developed and
maintained in accordance with API, ISO or other internationally recognized standard or regulatory requirements.
For each LNG carrier, there is a tank capacity table applicable to each custody transfer automatic tank gauging
device (ATG) for each tank. For a typical tank equipped with a primary and secondary ATG, this may be
presented as two separate capacity tables, each with its own set of correction tables or as a single capacity
table based on the primary level device location, with separate correction tables for each ATG and an offset
correction for the secondary level device to account for any differences in gauge reference height.
Each set of tank capacity tables and related correction tables or methods shall
— be certified as meeting the standard used,
— state the volumetric uncertainty of the capacity,
— identify the calibration method within the tank capacity tables or in the tank calibration report,
— include examples illustrating their intended use,
— be documented in English, with any additional languages optional, and
— be made available in printed form.
An example of a tank capacity table for a spherical tank is given in Annex C. The same principles generally
apply to those vessels with prismatic tanks.
Each set of tables shall include corrections for trim, list, thermal effects and any measurement equipment
adjustments as necessary to accurately adjust the quantities observed in the tank to the tank conditions at
the time of measurement. In addition, for each tank, the tank capacity tables shall include certified values for
any measured level used for verification of the tank gauging system. Tank tables shall indicate the location
of the primary and secondary level gauge (i.e. the gauge reference points). One or more examples shall be
included in the tank calibration report or tank capacity table indicating the correct use and interpretation of any
correction tables.
Such tables shall be made available to personnel performing the measurements as needed. If such tables are not
made available or cannot be verified, a letter of protest noting the situation shall be filed at the time of measurement.
NOTE Tank calibrations reports typically state the tank’s volumetric uncertainty at ambient temperature to be ±0,2 %
3 3
or better, which translates to a maximum uncertainty for a tank of 26 000 m of ±52 m LNG.
5.6.2.2 Tank capacity tables resolution
Tank capacity tables shall be capable of being read to a resolution of 1 mm throughout the range of levels
commonly encountered during opening and closing gauges. In practice, this is usually achieved by tank
capacity tables in any one of three formats:
a) tables showing volumes for each centimetre of gauge height, with volumes for each millimetre corresponding
to the normal ranges during opening and closing of gauges (i.e. near the top and bottom of the tank);
b) tables showing volumes for each centimetre of gauge height with the incremental volume for each row;
c) tables showing volumes for each millimetre of gauge height throughout the total volume of the tank.
See Table C.1 for an example of a section of a spherical tank capacity table.
10 © ISO 2012 – All rights reserved

5.6.2.3 List and trim correction tables
The main gauge tables are established for an LNG carrier with zero list and trim. Therefore, it is necessary to
correct the gauge height reading to take into account a list or a trim which is not zero. This correction differs
depending on the position of the gauging device relative to the tank; therefore, unique corrections are required
for each different ATG.
These corrections can be positive or negative. So the real height is equal to the algebraic sum of the height
reading, the correction for list and the correction for trim. These tables are made up in degrees for the list and
in metres for the trim, with fixed steps of variation. For intermediate values, the correction is calculated by
interpolation.
See Table C.2 for a sample section of a trim correction table; see Table C.3 for an example of a section of a
list correction table.
5.6.2.4 Tank thermal correction tables
Thermal correction tables shall be provided for self-supporting tanks and may be required for other tank designs.
The corrections are related to the volume variations resulting from the contraction of the tanks according to
the temperature of the liquid and gaseous phases. See Table C.5 for a sample section of a thermal correction
table for the tank shell.
5.6.2.5 Level gauging device thermal correction tables
Thermal correction tables may be provided for LNG carriers with level gauging devices of certain types. Such
tables attempt to correct the level gauge reading for the effect of temperature, based on the difference between
the reference conditions during calibration versus the operating temperature. Corrections may be applied
automatically or may have to be applied manually.
For example, the corrections may take into account the shrinkage of the float tape or wire according to the
temperature of the gaseous phase and the height of the liquid and the movement of the reference gauge height.
See Tables C.4 and C.6 for examples of sections of thermal correction tables for a radar-type level gauge and
float-type level gauge, respectively.
5.6.2.6 Density correction tables
Density correction tables may be provided for float-type level gauges to compensate for the float buoyancy as
it varies with LNG density. See Table C.7 for an example.
5.6.3 Trim and list measurement
5.6.3.1 General
Tank capacity tables are based on the ship being on an even keel. Trim and list shall be determined by
— taking the draft fore and aft (either manually or by measurement), and/or
— measuring the list of the LNG carrier.
The impact of trim and list varies with the tank type. On an LNG carrier with spherical tanks, due to the
centralized location of the level gauge on the tank, trim and list have a minor impact on the uncertainty of the
measured quantities. However, for a membrane tank LNG carrier, the trim correction is affected by the large
distance from the tank centre to the typical position of the level gauge near the aft tank bulkhead.
5.6.3.2 Trim and list by inclinometer
Where inclinometers are used in LNG carrier service, they are predominantly two-axis type and are used to
measure trim and list, although they may also be used to measure either individually.
Inclinometers measure trim and/or list based on gravitational principles. The most common methods are
capacitance based; otherwise, they make use of electrolytic technology, where a liquid in a precisely designed
and closed chamber is moving. Other types exist, but only those with servo-assisted technology and an inertial
mass/optical sensor within a servo feedback loop give sufficiently accurate and stable measurements. These
are electronic instruments which can communicate with the CTMS, preferably using digital signals.
Verification tolerances for inclinometers are provided in Table 1, but it should be noted that this tolerance
represents the combined influence of inclinometer uncertainty and the possible contributions from structural
bending differences between the inclinometer location and the individual tank locations for the state of load of
the LNG carrier.
5.6.3.3 Trim and list by draft measurement
An alternative to inclinometers is draft (alternative spelling: draught) measurement. The draft may be measured
manually or automatically, with an electro-pneumatic draft measurement system (with digital communication)
being common.
B.4 outlines the process for taking draft readings of the vessel to determine trim and list.
5.6.4 Tank gassing-up tables or means of determination
After lay-up or dry dock, the LNG carrier cargo tanks are filled with nitrogen or other inert gas. If the cargo
tanks contain nitrogen, the cool-down process may begin without purging. In order to be in a condition to
receive cargo, inert gas may need to be purged with LNG vapour prior to cool down to eliminate high boiling
temperature gases, such as carbon dioxide.
LNG carriers usually have gassing-up tables or equations/formulae which are used for determining the quantity
of LNG required to gas up the cargo tank(s). These tables give an estimation of the LNG quantity used to gas up
the cargo tanks by applying a displacement ratio depending of the type of the cargo tanks (usually between 1,4
and 1,8 for prismatic tanks, and between 1,1 and 1,4 for Moss tanks). Gassing-up tables are usually provided by
the tank manufacturer or shipbuilder and should be certified by the class society or an independent company.
Some shore terminals rely on meters as means to measure such quantities.
5.6.5 Tank cool-down tables or means of determination
5.6.5.1 General
LNG carriers have cool-down tables or formulae, which are used for determining the quantity of LNG required
to cool a tank down to a specified temperature. Cool-down tables are usually provided by the tank manufacturer
or shipbuilder and should be certified by the class society or an independent company. Other methods, such
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