ISO 19970:2017

INTERNATIONAL ISO
STANDARD 19970
First edition
2017-06
Refrigerated hydrocarbon and non-
petroleum based liquefied gaseous
fuels — Metering of gas as fuel on
LNG carriers during cargo transfer
operations
Hydrocarbures réfrigérés et combustibles gazeux liquéfiés à base non
pétrolière — Mesurage du gaz comme carburant sur les transporteurs
de GNL pendant les opération de transfert de cargaison
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Design requirements . 3
4.1 General . 3
4.2 Flowmeter . 3
4.3 Indicating device . 3
5 Accuracy requirement . 3
6 Calibration and accuracy verification . 4
6.1 General . 4
6.2 Calibration at laboratory or factory . 4
6.3 Calibration and accuracy verification of indicating devices after installation . 4
6.4 Periodic accuracy verification of indicating devices . 4
7 Metering and calculation . 5
7.1 General . 5
7.2 Metering . 5
7.3 Calculation of heating value . 5
Annex A (informative) Typical installation of fuel gas flowmeters . 7
Annex B (informative) Application of the energy of BOG consumed by LNG carriers to
custody transfer measurement . 8
Bibliography . 9
Foreword
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The committee responsible for this document is ISO/TC 28, Petroleum products and related products
of synthetic or biological origin, Subcommittee SC 5, Measurement of refrigerated hydrocarbon and non-
petroleum based liquefied gaseous fuels.
iv © ISO 2017 – All rights reserved

Introduction
Concerns over the emission of pollutants from ship exhausts prompted IMO to enact MARPOL Annex
VI to the IMO Protocol of 1997 which sets limits on sulfur oxide and nitrogen oxide emissions. As for
sulfur, as from 1 January 2012, the annex specifies a global cap mass fraction of 3,5 % on the sulfur
content of fuel oil and special SOx emission control areas (SECAs) where the sulfur contents shall not
exceed a mass fraction of 1,0 %. In order to comply with these requirements, the ships in general need
to fit special facilities to limit SOx emissions unless they use low sulfur fuel oil.
However, liquefied natural gas (LNG) carrier is capable of utilizing BOG in ship’s and shore tanks, and
return gas during cargo transfer operation as fuel for their own power generation, if commercial value
of such energy consumed on board is accountable as an element of custody transfer measurement.
This document provides the procedures needed for metering gas and assessing its calorific value and
the requirements for metering devices.
Aspects of safety are not dealt with in this document. It is the responsibility of the user to ensure that
the system meets applicable safety regulations.
INTERNATIONAL STANDARD ISO 19970:2017(E)
Refrigerated hydrocarbon and non-petroleum based
liquefied gaseous fuels — Metering of gas as fuel on LNG
carriers during cargo transfer operations
1 Scope
This document specifies minimum requirements to quantify the boil-off gas (BOG) consumed on
liquefied natural gas (LNG) carriers for their own functions, notably for power generation, during cargo
transfer operations. BOG in this document refers to the low molecular gas returned from shore tanks to
ships’ tanks and the gas produced in ships’ cargo tanks.
This document provides requirements for the metering of BOG and the subsequent calorific value
calculations which can be taken into account when the energy transferred during cargo transfer is
determined. This document also introduces performance requirements and calibration of the elements
included in the BOG measurement system. A flowmeter of any type may be used to measure BOG as
long as it meets the performance requirements specified in this document.
This document, with some modification, can also be applied to the measurement of BOG consumed by
LNG carriers at sea.
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 terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1
boil off
process of evaporation of a liquid resulting from heat ingress or a drop in pressure
[SOURCE: ISO 10976:2015, 3.1.6]
3.2
boil-off gas
BOG
vapour (3.14) produced by boil off (3.1)
Note 1 to entry: BOG in this document refers to the low molecular gas returned from shore tanks to ships’ tanks
and the gas produced in ships’ cargo tanks during cargo transfer operation.
[SOURCE: ISO 10976:2015, 3.1.7 — Note 1 to entry has been added.]
3.3
closing custody transfer measurement
custody transfer measurement (3.4) implemented after loading or discharging cargo from the tank(s)
3.4
custody transfer measurement
measurement of liquid level, liquid and vapour temperature, vapour pressure and analysis of the
composition of LNG to be delivered to/from a tank, by which volumetric and other data are determined
to be a basis of payment of cost or assessment of duty
3.5
custody transfer measurement system
CTMS
system that processes inputs from an ATG system, thermometers, pressure gauges, etc., and provides
custody transfer measurement information on board, generating documents with regard to custody
transfer of LNG
[SOURCE: ISO 18132-1:2011, 2.1.4, modified]
3.6
flowmeter
flow measuring device which indicates the measured flowrate
3.7
gas dangerous space or zone
space or zone defined by the International Code for the Construction and Equipment of Ships Carrying
Liquefied Gases in Bulk (IGC Code)
3.8
indicating device
displaying device
set of components of a measuring instrument intended to indicate the measured value
3.9
intrinsic error
inherent error
error of a measuring device when it is tested against a reference standard under controlled conditions
as specified by the manufacturer
3.10
LNG carrier
LNGC
cargo ship specifically constructed and used for the carriage of LNG in bulk
3.11
maximum permissible error
MPE
extreme value of measurement error, with respect to a known reference quantity value, permitted by
specifications or regulations for a given measurement, measuring instrument, or measuring system
[SOURCE: ISO/IEC Guide 99:2007, 4.26, modified]
3.12
opening custody transfer measurement
custody transfer measurement (3.4) implemented before loading or discharging cargo from the tank(s)
3.13
uncertainty
non-negative parameter characterizing the dispersion of the quantity values being attributed to a
measurand, based on the information used
3.14
vapour
fluid in the gaseous state that is transferred to/from or contained within the cargo tank
2 © ISO 2017 – All rights reserved

3.15
verification
process of confirming the accuracy of an instrument by comparing to a source with known accuracy
4 Design requirements
4.1 General
Flowmeters shall be so constructed as not to leak g
...