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 10976:2012(E)
©
ISO 2012

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ISO 10976:2012(E)
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© ISO 2012
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ISO 10976:2012(E)
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
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ISO 10976:2012(E)
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.
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ISO 10976:2012(E)
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.
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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.
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ISO 10976:2012(E)
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
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ISO 10976:2012(E)
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]
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ISO 10976:2012(E)
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
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ISO 10976:2012(E)
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
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ISO 10976:2012(E)
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.
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ISO 10976:2012(E)
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);
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ISO 10976:2012(E)
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 me
...