Mastering LNG Measurement – Essential Practices and Operational Insights

Discover the intricacies of LNG measurement, from calculating energy transfer to operational procedures. Explore standardization, partial loading/unloading, and critical gassing-up and cooling down operations in this comprehensive guide.

Following the publication in 1985 by the NBS of its study “LNG Measurement – A User’s Manual for Custody Transfer“, the Executive Committee of the GIIGNL (Groupe International des Importateurs de Gaz Naturel Liquéfié) considered it would be useful to write a handbook, as simple and as practical as possible, aimed at organizations involved in the measurement of the energy transferred in the form of LNG in the context of a LNG purchase and sales agreement, whether this sale be FOB [Port of loading], DES or CIF. [Port of unloading].

During its session of October 1987, the General Assembly of GIIGNL decided that this practical handbook should be drawn up by a Study Group comprising companies of the GIIGNL and coordinated by Distrigas S. A (B).

The methods described in this handbook could serve to improve existing procedures. They could also be used in purchase and sales agreements for the GIIGNL members and serve as a reference in new import agreements.

This handbook is based on the measurement methods most used by GIIGNL members.

Detailed tests of the apparatus used can be found in “LNG Measurement Study” of NBS.

We wish to thank the companies:

• BG (UK);
• Distrigas Boston (USA);
• Enagas (E);
• Kansai Electric Power Co (JP);
• Snam (I);
• Tokyo Electric Power Co (JP);
• Tokyo Gas Co Ltd (JP);
• Ruhrgas (D);
• CMS Energy Trunkline LNG (USA);

for their cooperation in producing this handbook, and more particularly Gaz de France for drawing up Sections 6 and 7 of this handbook and Osaka Gas Co Ltd for coordinating the studies of the Japanese companies.

SECOND EDITION, OCTOBER 2001. Following the publication of the ISO 13398:1997 standard “LNG – Procedure for custody transfer on board ship“, the GIIGNL General Assembly requested the GIIGNL Study Group to revise the original edition (March 1991) of this GIIGNL LNG Custody Transfer Handbook, particularly taking into account this new ISO standard.

All 13 sections of the original edition have been reviewed and updated where appropriate. The following sections have been thoroughly revised:

• 2. General description of the measurement;
• 3. Volume measurement;
• 6. Sampling of LNG;
• 7. Gas analysis;

Moreover, a new section was added:

• 14. LNG Sales contract custody transfer checklist.

Worked out examples for LNG density and GCV have been rearranged in Appendices 1 and 2.

We wish to thank all companies and organizations and their delegates who together contributed to this second edition, viz. (in alphabetical order):

• Advantica Technologies Ltd. (UK);
• BG International (UK);
• CMS Energy Trunkline LNG Company (USA);
• Distrigas (B);
• Enagas (E);
• Gaz de France (F);
• Nigeria LNG (NI);
• NKKK (JP);
• Osaka Gas (JP);
• Rete Gas Italia (I);
• SIGTTO (UK);
• Tokyo Gas (JP).

Tractebel LNG North America (USA).

THIRD EDITION, MARCH 2010. Since the second edition, several new international standards and revisions of existing international standards related to the subject of this handbook, have been published or are forthcoming. Also, technologies and best current practices evolved in this past period. Therefore, the GIIGNL General Assembly requested the GIIGNL Technical Study Group to revise the second edition (October 2001) of the GIIGNL LNG Custody Transfer Handbook, with the upcoming new ISO standard ISO 10976 «Measurement of cargoes on board LNG carriers», which will replace and supersede the current ISO 13397 (1997) standard upon its publication.

Moreover, this third edition of the handbook has been updated and revised as appropriate to streamline it with new or revised international standards such as ISO, EN and other standards. These include:

• EN 437: Test Gases – test Pressures – Appliance Categories – Edition 2003;
• ISO 8943 Refrigerated light hydrocarbon fluids – Sampling of liquefied natural gas – Continuous and intermittent methods – Edition 2007;
• ISO 6974-6 Natural gas – Determination of composition with defined uncertainty by gas chromatography – Part 6: Determination of hydrogen, helium, oxygen, nitrogen, carbon dioxide and C1 to C8 hydrocarbons using three capillary columns – Edition 2002;
• ISO 16664: Gas analysis – Handling of calibration gases and gas mixtures – Guidelines – Edition 2004;
• ISO 4259: Petroleum products – Determination and application of precision data in relation to methods of test – Edition 2006;
• ISO/TR 24094: Analysis of natural gas – Validation methods for gaseous reference materials – Edition 2006;
• ISO 10723: Natural gas – Performance evaluation for on-line analytical systems – Edition 2002;
• ISO 6326-1: Natural gas – Determination of Sulphur compounds – Part 1: General introduction – Edition 2007;
• ISO 6327: Gas analysis – Determination of the water dew point of natural gas – Cooled surface condensation hygrometers – Edition 2007;
• ISO 19739: Natural gas – Determination of sulphur compounds using gas chromatography – Edition 2004;
• ISO 12213-1 -2 -3: Natural gas – Calculation of compression factor – Edition 2006;
• ISO 15112: Natural gas – energy determination – Edition 2007;
• ISO 18132-1: Refrigerated light hydrocarbon fluids – General requirements for automatic level gauges – Part 1: Gauges onboard ships carrying liquefied gases – Edition 2006;
• ISO 18132-2: Refrigerated light hydrocarbon fluids – General requirements for automatic level gauges – Part 2: Gauges in refrigerated-type shore tanks – Edition 2008;
• ISO/DIS 28460: Petroleum and natural gas industries – Installation and equipment for liquefied natural gas – Ship to shore interface and port operations – Edition 2009.

All 14 sections, enclosures and appendices of the second edition have been exhaustively reviewed and updated where appropriate.

• ActOn LNG Consulting – UK;
• Botas – Marmara Ereglisi – Turkey;
• BP – Sunbury-on-Thames – UK;
• Distrigas of Mass. – GDF Suez – Everett (Boston) USA;
• Dragon LNG – Milford Haven – UK;
• Elengy – GDF Suez – Paris – France;
• Enagas – Spain;
• Exxon Mobil – Houston – TX – USA;
• Fluxys LNG – Zeebrugge – Belgium;
• Gas Natural – Madrid – Spain;
• GL – Noble Denton – Loughborough – UK;
• Kogas – Seoul – South Korea;
• National Grid – Grain LNG – UK;
• Osaka Gas – Osaka – Japan;
• Shell Global Solutions – The Hague – The Netherlands;
• RasGas – Ras Laffan – Qatar;
• REN Atlantico – Sines – Portugal;
• Sempra LNG – San Diego – CA – USA;
• SGS – Belgium;
• SIGTTO – London – UK;
• Tokyo Gas – Tokyo – Japan;
• Total – Paris – France.

FOURTH EDITION, FEBRUARY 2015. Due to the rapidly changing market conditions, new (commercial) opportunities arise leading to new technical solutions and different operations (such as partial unloading, reloading at an LNG import terminal, ship-to-ship LNG transfer operations, development of the small scale LNG market not only with much smaller LNG ships and in much smaller quantities, but also with a different ship design and other cargo containment systems), the GIIGNL General Assembly requested the GIIGNL Technical Study Group to review and update the third edition (version 3.01 – March 2011) of the GIIGNL LNG Custody Transfer Handbook.

Furthermore, there is a continuous evolution in the LNG (sampling) technology and this fourth edition tries to incorporate and be in line with new or revised international standards.

All sections, enclosures and appendices have been thoroughly reviewed, amended and updated where appropriate. Especially Sections 6 and 7 have been adapted to streamline it with the current best practices. To make this handbook more readable, most of the examples have been replaced to the appendices. Moreover, all sections with regard to the uncertainty of the energy determination have been moved to Section 15 and the following sections have been added:

• Mastering LNG Measurement – Essential Practices and Operational Insights“Gassing-up and cooling down operations”;
• 15.8 Rounding of numbers;
• 16 Ship-to-ship LNG transfer operations;
• 17 Small LNG ship to shore transfer operations;
• 18 Reloading operations in regasification terminals.

With regard to these Mastering LNG Measurement – Essential Practices and Operational Insights“Gassing-up and cooling down operations”, 16, 17 and 18, the aim of this GIIGNL LNG Custody Transfer Handbook is to integrate the specific conditions for this special operations of gassing-up and cooling down, for ship-to-ship LNG transfer, for small LNG ship-to-shore transfer (or vice versa) and for reloading operations at regasification terminals, but not to integrate the small scale LNG transfer operations (such as bunkering or fuelling of ships and trucks, and filling of LNG trucks or containers).

We wish to thank all companies and organisations and their delegates who together contributed to this fourth edition, in alphabetical order:

• Botas – Marmara Ereglisi – Turkey;
• BG Group – Houston – Texas;
• Enagas – Spain;
• Fluxys LNG – Zeebrugge – Belgium;
• Gas Natural Fenosa – Spain;
• Gate Terminal – Rotterdam – The Netherlands;
• GDF Suez – Paris – France;
• National Grid – Grain LNG – UK;
• RasGas – Ras Laffan – Qatar;
• Shell Global Solutions – The Hague – The Netherlands;
• Tokyo Gas – Paris – France;
• Total – Paris – France.

FIFTH EDITION, FEBRUARY 2017. The fifth version comes as a quick follow-up and update of the fourth edition (February 2015). More than pointing at the differences and highlighting the points of attention when dealing with the relatively new operations of (un)loading small scale vessels and reloading operations at LNG import terminals, this fifth version provides answers and solutions for setting up (slightly) altered or new custody transfer procedures. It should be highlighted that the reason for these proposed changes is of truly technical nature and GIIGNL considers it as its duty to inform the LNG industry about this and its impact. Each stakeholder involved in the LNG custody transfer chain should determine whether or not a review of its contractual agreement(s) may be required.

We wish to express our appreciation and thanks to all companies and organisations and their delegates who together contributed to this fifth edition, in alphabetical order:

• Enagas – Spain;
• Engie – Paris – France;
• Fluxys LNG – Zeebrugge – Belgium;
• Gas Natural Fenosa – Spain;
• Gate Terminal – Rotterdam – The Netherlands;
• National Grid – Grain LNG – UK;
• Shell Global Solutions – The Netherlands & US;
• Statoil – Norway;
• Tokyo Gas – Paris – France.

SIXTH EDITION, MAY 2021. The sixth version comes as a quick follow-up and update of the fifth edition and includes, for the first time custody transfer for LNG truck loading: a new section has been added to integrate the specific conditions for truck loading, in particular the formula for calculating the LNG energy loaded and the instruments used for the measurements.

In addition, the sixth edition also contains the latest recommendations about the calibration and performance evaluation of chromatographs as well as some minor modifications.

We wish to express our appreciation and thanks to all companies and organisations and their delegates who together contributed to this sixth edition, in alphabetical order:

• Enagas – Spain;
• Elengy – Paris – France;
• Equinor – Norway;
• Fluxys LNG – Zeebrugge – Belgium;
• Gate Terminal – Rotterdam – The Netherlands;
• National Grid – Grain LNG – UK;
• Naturgy – Spain;
• Shell Global Solutions – The Netherlands & US.

General description of the measurement

Accuracy. The term «measurement accuracy» is defined in the most recent version of the International Vocabulary of Metrology (JCGM_200:2012) as «closeness of agreement between a measured quantity value and a true quantity value of a measurand». Measurement error is defined as measured quantity value minus a reference quantity value.

Uncertainty, combined standard uncertainty and expanded uncertainty. The terms «measurement uncertainty», «combined standard uncertainty» and «expanded uncertainty» (see Section 15) are used as defined in the JCGM 100:2008 document: «Evaluation of measurement data – Guide to the expression of uncertainty in measurement».

General formula for calculating the LNG Energy Transferred

The formula for calculating the LNG transferred depends on the contractual sales conditions. These can relate to several types of sale contract as defined by Incoterms 2010. In case of rules for sea and inland waterway transport, the most commonly used are an FOB sale, a CFR sale or a CIF sale.

In the case of a FOB (Free On Board) sale, the determination of the energy transferred and invoiced for will be made in the loading port. There is another sale contract similar to FOB, named FAS (Free Alongside Ship).

In the case of a CIF (Cost Insurance & Freight) or a CFR (Cost and Freight) sale, the energy transferred and invoiced for will be determined in the unloading port.

Other rules exist for any mode of transport which can hence also apply for maritime transport such as DAT (Delivered At Terminal) and DAP (Delivered At Place).

In FOB contracts, the buyer is responsible to provide and maintain the custody transfer measurement systems on board the vessel for volume, temperature and pressure determination and the seller is responsible to provide and maintain the custody transfer measurement systems at the loading terminal such as the sampling and gas analysis. For CIF and CFR (and DES according to Incoterms 2000) contracts the responsibility is reversed.

Both buyer and seller have the right to verify the accuracy of each system that is provided, maintained and operated by the other party.

The determination of the transferred energy usually happens in the presence of one or more surveyors, the ship’s cargo officer and a representative of the LNG terminal operator. A representative of the buyer can also be present.

An independent surveyor nominated and agreed between the parties involved shall witness or verify the verification/calibration certificates of all devices involved in the custody transfer, namely:

• On the (un)loading terminal: the gas chromatograph(s) and the reference gas certification(s);
• Onboard the vessel: level measurements, pressure measurements, etc.

Depending on the Terminal Rules of the delivery port, the calibration or verification may be done before and/or after the operations, or only periodically.

It is also recommended that the independent surveyor verifies the calibration certificates of the systems involved in the custody transfer on board the LNG vessel. The systems concerned are as follows:

• level measurements;
• pressure measurements;
• temperature measurements;
• trim and list measurement devices;
• gas meters (for engines, boilers and GCU);
• optional: Certificates of the calculation software.

The independent surveyor should have the same rights and obligations towards the parties involved, for which it is the mutual representative. The independent surveyor should not act as a consultant for any of the parties involved if there is a disagreement among the parties which it represents.

In general, the independent surveyor performs the transferred energy calculation according to the agreed terms and conditions and issues the final quantity report.

In all cases, the transferred energy can be calculated with the following formula:

where:

• E – the total net energy transferred from the loading facilities to the LNG carrier, from the LNG carrier to the unloading facilities or from one LNG carrier to another LNG carrier (ship-to-ship LNG transfer). In international LNG trading, the energy transferred is most frequently expressed in millions of British Thermal Units (10⁶BTU or MMBTU) although this is not a SI energy unit. Therefore, MMBTU is the preferred unit in this handbook. A conversion factor table for other commonly used energy units (such as MWh) can be found in ENCLOSURE 1: CONVERSION FACTOR TABLE FOR ENERGY UNITS (1), (2);
• V_LNG – the volume of LNG loaded or unloaded in m³;
• D_LNG – the density of LNG loaded or unloaded in kg/m³;
• GCV_LNG – the gross calorific value of the LNG loaded or unloaded in MMBTU/kg. The gross calorific value is generally used in international LNG trading rather than the net calorific value, see ENCLOSURE 4;
• E_{gas displaced} – the net energy of the displaced gas, also in MMBTU, which is either:
  • sent back by the LNG carrier to shore or to another LNG carrier when loading (volume of gas in cargo tanks displaced by same volume of loaded LNG);
  • or, gas received by the LNG carrier in its cargo tanks when unloading in replacement of the volume of discharged LNG;
  • or, the volume of LNG that has been replaced by gas (even without a vapor connection to shore or another vessel).
• E_{gas to ER} – if applicable, the energy of the gas consumed in the LNG carrier’s engine room (also including all gas burnt by the ship’s GCU (Gas Combustion Unit)) during the time between opening and closing custody transfer surveys, i. e., used by the vessel during the LNG transfer operation, which is:
  • + for an LNG loading transfer;
  • or – for an LNG unloading transfer.

For simplicity, the parties may also make a commercial decision to mutually agree a fixed gas quality/volume to estimate E_{gas displaced} and/or E_{gas to ER}.

General scheme of the Measurement Operations

The objective is to measure the quantity of energy loaded from production facilities into an LNG carrier, or unloaded from an LNG carrier to a receiving terminal. For ship-to-ship operations, the objective is to measure the quantity of energy transferred from one LNG carrier to another LNG carrier.

From the above formula, it can be inferred that five elements must be measured and/or calculated:

• LNG volume;
• LNG density;
• LNG gross calorific value;
• energy of the gas displaced during the transfer of LNG;
• energy of any gas consumed in the LNG carrier’s engine room during (un)loading operations.

A graphic overview of the measurement scheme is shown in the figure “Flowchart for determining the energy transferred”.

LNG volume

The standard method chosen for measuring the volume of LNG transferred is based on the LNG carrier’s instruments, mainly the use of level gauges and calibration tables.

For most of the vessels, gauging has become automated via the LNG carrier’s custody transfer measurement system. These systems are capable of drawing up reports of the volume of LNG on board at any time during (un)loading. This is achieved by converting the measured LNG levels in each cargo tank into the corresponding LNG volume in the cargo tank via the level-to-volume conversion tables and by applying correction factors for trim, list and temperature and then by totalling the volumes in all the individual cargo tanks. Further details are given in Volume measurement methods of LNG transferred“Automated systems”.

Usually a quantity of LNG, called a «heel», remains on board after unloading so as to keep the tanks cold. However, operators may sometimes prefer to strip out the cargo tanks partially in order to maximize the LNG delivery or totally before the LNG vessel is scheduled for dry-docking.

Determination of the volume transferred requires two sets of measurements, an initial one before starting loading or unloading and a final one at the end of the procedure. These are called the opening and closing custody transfer surveys (CTS) respectively. Two LNG volumes result and the difference between the larger volume and the smaller volume represents the volume of liquid transferred.

For an accurate volume measurement, it is recommended that LNG piping on the LNG carrier’s deck including manifolds be in an identical inventory condition during both custody transfer surveys (CTS). The piping should either be completely filled with LNG both during the opening custody transfer (i. e., before (un)loading) and the closing custody transfer (i. e., after (un)loading) or, provided that draining is possible before the closing CTS, alternatively be drained during both the opening and closing CTS. Where the piping is drained before or after the CTS measurement, it should be done for sufficient time to fully empty the piping.

As good practice it is recommended that the initial level gauging should be made prior to any cooling down operation, i. e., after the (un)loading arms have been connected but before any ship’s liquid and vapor manifold valves have been opened. Where the opening CTS is conducted prior to commencement of tank cool down, the CTS reading, where automated, may show some liquid in the tank(s). The system should have the capability of «zeroing» such readings for level and volume, since otherwise any liquid recorded at commencement will be deducted from the final CTS volume.

The final level gauging reading shall be made as soon as possible after completion of (un)loading with liquid and vapor arms (or flexible hoses) drained and inerted, and with liquid and vapor manifold valves closed.

The level gauge readings shall be determined by the arithmetic average of several successive readings at regular intervals. Further details are provided in Volume measurement methods of LNG transferred“Main liquid level gauging devices”, Volume measurement methods of LNG transferred“Timing of the level measurement” and Volume measurement methods of LNG transferred“Readings”.

In the event of failure of the primary level gauging device, an auxiliary device should be used.

Level corrections are to be made using correction tables provided for the LNG carrier as tank gauge tables for trim, list and also for temperature. Most LNG carriers are equipped with process control systems or stand-alone systems able to perform these corrections automatically. It is recommended to use the millimeter as the smallest unit of dimension, when applying a tank gauging table.

In some cases, the LNG carrier must be completely emptied after the unloading operation, e. g., before a long period of inactivity. In this case a special procedure explained in Volume measurement methods of LNG transferred“Complete unloading (tank stripping)” is followed for determination of the volume transferred.

Before loading operations, the LNG carrier may be in «ready-to-load» condition or otherwise, may require gassing-up and/or cooling down operations. In this case a special procedure explained in Mastering LNG Measurement – Essential Practices and Operational Insights“Gassing-up and cooling down operations” is followed for determination of the energy and volume transferred.

Unless parties explicitly agree otherwise (see below), gas flow is stopped and appropriate gas valve(s) to engine room shut and sealed during and between the opening and closing custody transfer surveys.

The possibility of using LNG and/or boil-off gas as fuel for the ship during transfer is considered in Mastering LNG Measurement – Essential Practices and Operational Insights“General formula for calculating the LNG energy transferred”, Mastering LNG Measurement – Essential Practices and Operational Insights“For the determination of the energy of «Gas to engine room»” and 12.2.

Since the calculation methods described in this handbook are based on volumes of LNG and LNG vapor before and after transfer, any use of LNG, regasified LNG and/or LNG vapor during the transfer should be fully accounted for by correction of V_LNG, according to the terms of the LNG purchase and sales agreement.

Note: In-line measurement of LNG quantity. Coriolis mass flow meters and ultrasonic flow meters are in use at some (un)loading terminals. However, at the time of writing, their use as part of a ship-shore custody transfer measurement system is not yet conventional. This is mainly due to their high cost, the inability of these flow meters to handle high flow rates and «proving» issues. For small scale LNG transfer operations these meters can be used as secondary (or even as primary system), if agreed upon by the parties in their commercial sales conditions, or just as (operational) verification for the parties involved. A further informative discussion can be found in Appendix 1.

LNG Density. The density of LNG is determined by calculation from the measured composition of the LNG transferred and the temperature of the LNG measured in the LNG carrier’s tanks.

LNG Gross calorific value. The composition of the LNG is used to calculate the gross calorific value.

Energy of the gas displaced by the transfer of LNG. This energy is calculated according to the composition and volume of the gas displaced, and the pressure and temperature of the gas inside the tanks of the LNG carrier before loading or after unloading. The calculation procedure is explained in Section 12.1.

Instruments used

For the determination of the LNG volume. For the determination of the LNG volume the following are required:

• the LNG carrier’s calibration tables, including the main gauge tables for each tank and different correction tables accounting for list and trim variances (if any) for the main and secondary gauging systems, tank contraction tables for Moss-type and SPB-type cargo containment systems, and possibly, other correction tables according to the type of level measuring devices;
• the equipment for measuring the level of LNG in the LNG carrier’s tanks. Each cargo tank usually has two level gauge systems installed, one designated as «main» or «primary» and the other as «secondary». Capacitance and microwave (radar) level gauging systems are widely used as primary CTS systems onboard LNG tankers, backed up by a secondary CTS system generally consisting of a float gauging system;
• recently a laser system (called LIDAR) was introduced in the industry but at the time of writing its use is restricted to very few LNG carriers. A further informative discussion can be found in Appendix 2. Some very old LNG carriers still in operation are fitted with nitrogen bubbler systems, which rely on good knowledge of the LNG density to give accurate readings;
• temperature probes distributed over the height of the LNG carrier’s tanks;
• other measuring devices required for applying the correction factors.

Note: Automated systems. The calculation to determine LNG volume may be automated by processing the level, temperature and pressure measurements, taking into account the above-mentioned calibration and correction tables to produce a report meeting CTS requirements. LNG carriers may be fitted with certified custody transfer measurement systems for this purpose. See Section 3.2.7.

For the determination of LNG density and gross calorific value

The determination of the density and the gross calorific value of the LNG transferred is made on the basis of the average composition of the LNG obtained by:

• continuous or discontinuous sampling of LNG in the LNG transfer line(s) between the ship and the terminal;
• gas chromatographic analysis;

followed by:

• a calculation based on the average composition of the LNG, its average temperature and the coefficients given by the National Bureau of Standards for the density;
• a calculation based on the average composition of LNG and characteristics of elementary components (GCV, molar volume, molar weight) given by reference tables or standards for the gross calorific value.

At a loading terminal or during Cargo Total Weight Calculation of Liquefied Gas on the LNG and LPG Carriersship-to-ship transfers, LNG sampling and analysis are made in the LNG transfer line(s) prior to possible flashing (vaporization) in the ship’s cargo tanks. If flashing occurs in the ship’s cargo tanks, then this causes a minor change in LNG composition since the most volatile components (typically nitrogen and methane) are preferentially vaporized and returned to shore via the vapor return line. Therefore, this effect should be avoided if possible, or otherwise minimized, e. g., by ensuring that the tank pressure in the ship’s cargo tanks is sufficiently higher than the saturated vapor pressure of the LNG being loaded.

Note (for information only): A novel LNG analysis method. A measurement device that analyses LNG composition directly in the LNG transfer line(s) and hence eliminates the need for an LNG sampling device, vaporizer and gas analyzer, is being tested in a small number of pilot applications (see Section Raman spectroscopy).

For the determination of the energy of displaced gas

The energy of the displaced gas can be determined from:

• sampling of the gas displaced;
• a gas chromatographic analysis of this sample gas, enabling the GCV to be calculated;
• pressure and temperature measurements within the LNG carrier’s tanks.

However, for the determination of the energy displaced, some parameters such as pressure, gas composition and temperature can be estimated from experience and taken as constant for both custody transfer surveys before and after (un)loading.

For instance, the displaced gas may be assumed to be a fixed mixture of nitrogen and methane, or pure methane. This assumption will hardly increase the overall uncertainty.

For the determination of the energy of «Gas to engine room»

Parties may explicitly agree to allow gas consumption in the LNG carrier’s engine room (also including the gas burnt by the ship’s GCU) during the time between the opening and closing custody transfer surveys (CTS’s). This could be to ensure low air-emission operation in the engine room whilst at berth and so may favor the use of boil-off gas perhaps complemented by regasified LNG rather than fuel oil in the engine room. This practice may enable the LNG carrier operator to comply with MARPOL Annex VI.

For (re)loading operations or ship-to-ship LNG transfer operations, this could also be done on the LNG carrier(s) in order to handle the boil-off gas (flash gas) produced during such operations, hence reducing the vapor returned to shore or to the vessel being unloaded.

It is recommended in this case that the LNG carrier has proper measuring equipment on board and procedures accepted by both parties to accurately measure the gas energy consumption in the engine room between the opening and closing custody transfer surveys (CTS’s), and that this on-board gas energy consumption is taken into account as «Gas to Engine Room» as per the general formula above. However, for simplicity the parties may make a commercial decision to mutually agree to a fixed gas quantity/volume.

Periodic instruments recalibration. It is recommended that, unless it is specified by the fiscal authorities or by the Classification Society, Buyer and Seller agree on the periodicity of recalibration intervals, e. g., at each dry-docking.

Standardization

International standards exist for the classical methods and techniques used for LNG Custody Transfer such as ISO 6976 for calculation of the GCV of gas.

On the other hand, a number of existing LNG supply, shipping and purchase agreements specify GPA 2261 for gas chromatography and HM 21 or GPA 2145 and GPA 2172 for the calculation of the GCV of the gas. Buyer and Seller may approve one of these editions, usually the most recent edition.

As far as methods and techniques dealing with static measurement procedures for LNG are concerned, it should be noted that ISO has issued numerous international standards, (see ENCLOSURE 2: LNG AND NATURAL GAS CUSTODY TRANSFER METHODS). The recommendations included in these documents and future international standards are appropriate to be considered for new agreements.

Partial loading or unloading of LNG carriers

In recent years there has been a worldwide increase in short term and spot cargo LNG trading, involving two new operating trends in LNG shipping:

• more and more LNG shippers are using LNG carriers as floating LNG storage;
• several LNG shippers have considered, and some have carried out, partial unloading and/or partial loading of one or several cargo tanks of LNG carriers.

When performing such operations, due attention should be given to:

• safe ship/shore operating practices and procedures;
• proper LNG ship/shore custody transfer procedures.

Please refer to Appendix 3 for recommended safe practices for partial (un)loading.

Gassing-up and cooling down operations

Gassing-up operations. When an LNG vessel is delivered or after dry dock, the cargo tanks are often filled with inert (exhaust) gas. As inert exhaust gas contains carbon dioxide which will freeze during loading, it must be replaced with warm LNG vapor prior to cooling down the tanks in preparation of loading.
This process is called gassing-up and is normally preceded by a process of drying and inerting.

Drying can be done with hot air (or nitrogen) and inerting is performed to remove the oxygen out of the cargo tanks and replace the air/oxygen by inert gases (exhaust gases of the ship or nitrogen). In case drying and inerting are performed with nitrogen, both steps can be combined in one operation. The reason behind the preliminary operations of drying and inerting is not to directly replace air by natural gas due to safety reasons (hence avoiding an explosive atmosphere during the operation).

Once inerted, the shore will supply LNG which is sent to the main vaporizer of the ship to produce vapor warmer than the dew point temperature in the cargo tank. This vapor is then injected at the top of the cargo tank to displace the inert gas. The increase of pressure and the difference in density forces the inert gases out of the cargo tanks. The exhaust gas is generally directed to the ship’s (forward) vent mast, to a vent on shore or burnt in the terminal’s flare (at the beginning it is fully inert and gradually becomes a mixture of nitrogen and inert gases (which percentage is decreasing) and natural gas (which percentage is increasing). This process continues until the exhaust gas is measured to have approx. 98-99 % of methane and the CO₂-content is less than a certain low threshold (e. g., 0,1 %). Once this is accomplished, the vessel is ready to receive cold vapor and start the cooling down process.

The quantity of LNG required for this operation depends on the size and construction of the vessel’s cargo tanks. The gas quality considered for gassing up is usually the same as the quality for cooling and for loading, but some loading/reloading facilities can use different gas qualities for each operation. The energy required for gassing-up the cargo tanks is stated in the terminal rules or in the contract between the parties, but the most commonly used technique is to use the certified gassing-up tables of the LNG vessel.

The certified gassing-up tables apply a volume of natural gas which is between 1,7 and 2 times (each LNG vessel has a coefficient) the volume of each cargo tank, which is the theoretical volume of gas to be supplied to each cargo tank for the purpose of gassing-up this cargo tank.

Each vessel will have a gassing-up table for each tank on board. These are provided by the Liquefied Natural Gas Tank Protectiontank manufacturer and confirmed by an independent surveyor during construction at the shipyard. Each table is designed specifically for that particular type of containment system. For example, a 145 000 m³ size membrane vessel will require approx. 420 m³ of LNG or 2 850 MWh for a complete gassing-up operation. This gassing-up operation will take approx. 20 hours to complete depending upon the LNG supply and the vaporization rate on the vessel. The gassing-up tables should be reviewed and agreed upon by all parties at the preliminary meeting prior to commencing any operations.

There are some terminals which correct the theoretical volume taking into account the measurement conditions when these are different than the ones in the certified gassing-up tables.

Read also: Usage of Natural Gas Compressors in the Gas Production Operations

Other alternatives to measure the energy for gassing-up (especially for old LNG vessels which do not have certified gassing-up tables) are the following ones:

• a) the procedure stated in the LNG vessel Operations Manual;
• b) to apply twice the theoretical volume of each cargo tank to obtain the theoretical energy for gassing up;
• c) to measure the delivered LNG (in cubic meters) during the gassing-up operation by means of an LNG mass or flow meter;
• d) to measure the difference in level in the shore LNG tanks. In this case, it is necessary to take into account any other filling (e. g., cold circulation) or emptying (e. g., send out for regasification purposes) operations of the tanks.

Cooling down operations

Cooling down operations are performed to slowly reduce the temperature of the cargo tanks close to that of the LNG to be loaded in order to avoid any structural damages by thermal shock or stress to the tank construction. The target is to reach a certain reference temperature according to the performance criteria stated in the operations manual or cool down tables of the LNG vessel.

Cooling down operations are generally performed on an LNG ship of which the cargo tanks are under (warm) natural gas (e. g., after a long ballast voyage during which the cargo tanks have been warmed up or (immediately) after gassing-up operations). However, it is also possible to cool down an LNG ship of which the cargo tanks have been fully dried and inerted with nitrogen, hence avoiding gassing-up operations.

LNG is supplied by the LNG terminal and it is vaporized and injected at a controlled rate into the ship’s cargo tanks through spray nozzles at the top of the cargo tanks to avoid thermal shock in the tanks and its devices (such as pumps, pump columns, probes…). Once the LNG is vaporized, there is an increase of pressure in the cargo tanks and the gas return is sent to the terminal’s vapor return system or to the terminal’s flare to keep the pressure in the cargo tanks under control. The LNG vessel can also help reducing its cargo tank pressure by burning gas as fuel (in the engines or in the gas combustion unit).

The required temperature needed before loading can be started is identified in the Operations Manual or the cool down tables provided by the tank manufacturer. Each cool down table is specific to that tank type, is issued by the manufacturer during construction and is verified by an independent surveyor prior to the vessel being delivered. The cool down tables identify the quantity and length of time required to complete the operation prior to loading. The gas quality considered for cooling down is usually the same than the quality for loading (and gassing-up if there is any), but some loading/reloading facilities can consider different gas qualities for each operation. The energy required for the cooling down operations of the cargo tanks is stated in the terminal rules or in the contract between the parties, but it is usual to apply the use of the cool down tables or one of the following alternatives. Cooling down tables.

The most common method is the use of the cool down tables. Based on the size of the cargo tank, the manufacturer calculates how many cubic meters of LNG or what energy content are required to lower the temperature inside the tank one degree Celsius. Then based on the vapor temperature inside the tank at the beginning of the operation, the quantity is calculated accordingly. The time it takes to complete a cooling down operation depends on the temperature prior to starting the operation. The cooling down tables should be reviewed and agreed upon by all parties at the preliminary meeting prior to commencing any operation.

Cooling down operations are generally faster than gassing-up operations. It takes approx. 10 to 12 hours for a membrane type LNG vessel and approx. 20 hours for a Moss type vessel.

• a) Membrane type LNG vessels.

In this type of vessel, the reference temperature is the average temperature (vapor phase) of the pump tower in each tank excluding the first top or two top sensors (depending on terminal rules/contract between the parties).

The reference temperature for loading a membrane type LNG vessel shall be approximately -130 °C.

It is usual to use the certified cooling down tables. These tables give the energy required to cool down each tank from its arrival temperature to -130 °C.

The certified cooling down tables have been made up supposing a certain gas type in order to obtain the energy. Once cooling down energy has been obtained, it is possible to obtain an LNG cubic meter equivalent for this cooling down operation.

The tables are divided into two sections:

• warm conditions. These are used in case the average temperature in the cargo tanks is higher than -40 °C. The table normally spans the range between +40 °C and -130 °C. In this case the operation can take about 10 to 12 hours;
• cold conditions. These are used in case the average temperature in the cargo tanks is equal to or lower than -40 °C. The table normally spans the range between -40 °C and -130 °C. In this case the operation can take about 6 to 8 hours.

• b) Moss type LNG vessels.

In this type of vessel, the temperature reference is the equatorial temperature of the cargo tanks. The reference temperature for loading a Moss type LNG vessel normally spans the range between -110 °C and -130 °C depending on the operations manual of the LNG vessel.

In this case, the certified cooling down tables could give:

• the LNG volume needed to reduce the equator temperature of the tank by one degree Celsius. The total LNG quantity required for cooling down the cargo tanks will be calculated by multiplying the difference between the initial equator temperature and temperature reference by the value of cubic meters that are needed to lower the equator temperature by one degree Celsius;
• the LNG volume and energy needed to reach the reference temperature (as the equatorial temperature).

Nozzle pressure. The cooling down operations are performed by injecting LNG through special spray nozzles into the cargo tanks. The flow is fully dependent on the applied pressure. The number of nozzles, the average pressure, and the duration of the spraying for each tank can be used to determine the volume of LNG used for the cooling down operations.

In case the cooling down operations are performed on an LNG ship of which the cargo tanks are under nitrogen, the cool down tables may be used as well, however this should be agreed upon by all parties.