LNG Custody Transfer Handbook_2E_2001

FINAL DRAFT SECOND EDITION

GIIGNL - DS TML/Z - CG - 2001/10/09

G.I.I.G.N.L. 2001


GIIGNL - DS TML/Z - CG - 2001/10/09

DISCLAIMER

This new edition of the LNG Custody Transfer Handbook reflects best current practice at the time of publication.

The purpose of this handbook is to serve as a reference manual, but it is neither a Standard nor a Specification.

G.I.I.G.N.L.*, and any of its members, disclaim any direct or indirect liability as to information contained in thisdocument for any industrial, commercial or other use whatsoever.

* G.I.I.G.N.L.: Groupe International des Importateurs de Gaz Naturel Liquéfié – Paris

(International Group of Liquefied Natural Gas Importers – Paris (France))


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LNG CUSTODY TRANSFER HANDBOOK

1. INTRODUCTION 3

2. GENERAL DESCRIPTION OF THEMEASUREMENT 3

2.1. GENERAL FORMULA FOR CALCULATINGTHE LNG ENERGY TRANSFERRED 3

2.2. GENERAL SCHEME OF THEMEASUREMENT OPERATIONS 4

2.2.1. Volume 4

2.2.2. Density 4

2.2.3. Gross calorific value 4

2.2.4. Energy of the gas displaced by thetransfer of LNG 4

2.2.5. Flowchart for determining the energytransferred 4

2.3. INSTRUMENTS USED 5

2.3.1. For the determination of the LNG volume 5

2.3.2. For the determination of LNG density andgross calorific value 5

2.3.3. For the energy of displaced gas 5

2.3.4. Periodic instruments recalibration 6

2.4. STANDARDISATION 6

3. VOLUME MEASUREMENT 6

3.1. GAUGE TABLES 6

3.1.1. Use of gauge tables 6

3.1.2. Correction tables 6

3.1.2.1 Correction according to thecondition of the LNG carrier 6

3.1.2.2 Corrections according to thetemperatures in the liquid andgaseous phases 12

3.1.3. Approval by authorities 12

3.1.4. Inaccuracy of the table 12

3.2. INSTRUMENTS AND METHODS FORMEASURING THE LEVEL OF LIQUID INTHE LNG CARRIER'S TANKS 12

3.2.1. Main liquid level gauging devices 12

3.2.1.1 Capacitance gauge 12

3.2.1.2 Float gauge 12

3.2.1.3 Microwave gauge 16

3.2.2. Timing of the level measurement 16

3.2.2.1 In an FOB agreement 16

3.2.2.2 In a CIF or DES agreement 16

3.2.3. Readings 16

3.2.3.1 Reading of the level with floatgauges 16

3.2.3.2 Reading of the level withcapacitance and microwave gauges 16

3.2.4. Correction of readings 16

3.2.4.1 Float gauge 16

3.2.4.2 Capacitance and microwavegauge 16

3.2.5. Use of spare level gauge 17

3.2.6. Complete unloading (tank stripping) 17

3.3. CALCULATION OF THE VOLUME OF LNGTRANSFERRED 17

3.4. INACCURACY OF THE VOLUMEMEASUREMENT 17

3.4.1. Cargo Liquid Lines 23

4. TEMPERATURE MEASUREMENT 23

4.1. LIQUID TEMPERATURE 23

4.1.1. Device 23

4.1.2. Testing and accuracy 25

4.2. VAPOUR TEMPERATURE 25

5. VAPOUR PRESSURE MEASUREMENT 25

6. SAMPLING OF LNG 25

6.1. LNG QUALITY 25

6.2. SAMPLING PRINCIPLES 25

6.3. SAMPLING POINT 26

6.4. SAMPLING PROBES 26

6.5. PIPING ARRANGEMENT BETWEENSAMPLING PROBE AND VAPORISER 29

6.6. LNG VAPORISER AND CONTROLDEVICES 29

6.6.1. Main devices 29

6.6.2. Description of vaporising devices 29

6.6.3. Auxiliary vaporisation control devices 29

6.6.4. Operating parameters 31

6.7. COMPRESSOR FOR TRANSFERRINGGAS SAMPLE 31

6.7.1. GAS SAMPLE HOLDER 31

6.8. GAS SAMPLE CONDITIONING 32

6.8.1. Gas sample bottles 32

6.8.2. Direct piping to gas analyser 32

6.9. EXAMPLES OF GENERALARRANGEMENT OF SAMPLING DEVICES 36

6.10. PERFORMANCES OF THE DEVICES 36

6.11. SAMPLING PROCEDURE 37

6.11.1. Sampling period 37

6.11.2. Sampling frequency 37

6.11.3. Purging 37

6.11.4. Sampling parameters 37


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6.11.5. Utilisation of gas sample bottles 37

6.12. SPOT SAMPLING DEVICE 38

7. GAS ANALYSIS 38

7.1. TYPE OF GAS CHROMATOGRAPH 38

7.1.1. General arrangement 38

7.1.2. Columns 38

7.1.3. Detectors 40

7.1.4. Carrier gas 40

7.1.5. Quality of the separation of components 40

7.2. INTEGRATOR AND DATA PROCESSING 41

7.2.1. Integrator system 41

7.2.2. Data processing 41

7.3. CALIBRATION 41

7.3.1. Calibration procedure 41

7.3.2. Calibration gas/working standard 41

7.4. QUANTITATIVE ANALYSIS 42

7.4.1. Response factors 42

7.5. ENVIRONMENT FOR A GASCHROMATOGRAPHIC SYSTEM 42

7.6. ANALYSIS OF REGASIFIED LNG ANDRETAINED SAMPLES 42

7.7. INACCURACY OF GAS ANALYSIS 43

7.8. IMPURITIES 43

7.8.1. Carbon dioxide 43

7.8.2. Sulphur 43

7.8.2.1 Total sulphur 43

7.8.2.2 Sulphur components 43

7.8.3. Mercury 43

8. DENSITY 44

8.1. GENERAL 44

8.2. DENSITY CALCULATION METHODS 44

8.3. REVISED KLOSEK-Mc KINLEY METHOD 44

8.3.1. Limits of the method 44

8.3.2. Formula 46

8.3.3. Charts available for calculation 46

8.3.4. Example of LNG density calculation 46

8.3.5. Rounding off 46

9. GROSS CALORIFIC VALUE 46

9.1. GENERAL 46

9.2. METHOD OF DETERMINATION OF THEGROSS CALORIFIC VALUE 47

9.2.1. Determination with the help ofcalorimeters 47

9.2.2. Determination of GCV by calculation 47

9.2.2.1 Examples of formula 47

9.2.2.2 Examples of charts of basicphysical constants 48

9.2.2.3 Example of calculation 48

10. ANALYSIS REPORT 49

10.1. IDENTIFICATION 49

10.2. BASIC DATA 49

10.3. RESULTS 49

11. ENERGY OF GAS DISPLACED DURINGLOADING OR UNLOADING OPERATION 49

11.1. ENERGY OF GAS DISPLACED FROMTHE TANKS OF THE LNG CARRIER 49

11.2. ENERGY OF GAS CONSUMED AS FUELBY THE LNG CARRIER 50

12. ENERGY TRANSFER MEASUREMENT 50

13. OVERALL INACCURACY OF THE ENERGYTRANSFER MEASUREMENT 50

13.1. VOLUME 50

13.2. DENSITY 50

13.3. GROSS CALORIFIC VALUE 50

13.4. GAS DISPLACED 52

13.5. TOTAL INACCURACY IN THEDETERMINATION OF THE ENERGYTRANSFERRED 52

14. LNG SALES CONTRACT CUSTODYTRANSFER CHECKLIST 52

ENCLOSURE 1: Conversion factor table 54

ENCLOSURE 2: ISO Standards 55

ENCLOSURE 3: Other relevant standards & 58references

List of figures – List of tables 59

References 60

Appendices 61


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1. INTRODUCTIONFollowing the publication in 1985 by the N.B.S. of itsstudy "LNG Measurement - A User's Manual forCustody Transfer" [8], the Executive Committee ofthe G.I.I.G.N.L. (Groupe International desImportateurs de Gaz Naturel Liquéfié) considered itwould be useful to write a handbook, as simple andas practical as possible, aimed at organisationsinvolved with the measurement of the energytransferred in the form of LNG in the context of aLNG purchase and sales agreement, whether thissale be F.O.B. or C.I.F.

During its session of October 1987, the GeneralAssembly of G.I.I.G.N.L. decided that this practicalhandbook should be drawn up by a Study Groupcomprising companies of the G.I.I.G.N.L. and co-ordinated by Distrigas S.A (B).

The methods described in this handbook could serveto improve existing procedures. They could also beused in purchase and sales agreements for theG.I.I.G.N.L. members and serve as a reference innew import agreements.

This handbook is based on the measurementmethods most used by G.I.I.G.N.L. members.

The apparatus used is accepted as is, and detailedtests of this apparatus can be found in "LNGMeasurement Study" of N.B.S. [8].

We wish to thank the companies – BG (UK) -Distrigas Boston (USA) - Enagas (E) - KansaiElectric Power Co (JP) - Snam (I) - Tokyo ElectricPower Co (JP) - Tokyo Gas Co Ltd (JP) - Ruhrgas(D) – CMS Energy Trunkline LNG (USA) for their co-operation in producing this handbook, and moreparticularly Gaz de France for drawing up Chapters6 and 7 of this manual and Osaka Gas Co Ltd forco-ordinating the studies of the Japanesecompanies.

SECOND EDITION OCTOBER 2001Following the publication of the ISO 13398:1997standard "LNG - Procedure for custody transfer onboard ship", the G.I.I.G.N.L. General Assemblyrequested the G.I.I.G.N.L. Study Group to revise theoriginal edition (March 1991) of this G.I.I.G.N.L. LNGCustody Transfer Handbook, particularly taking intoaccount this new ISO standard.

All 13 sections of the original edition have beenreviewed and updated where appropriate. Thefollowing sections have been thoroughly revised:2. General description of the measurement3. Volume measurement6. Sampling of LNG7. Gas analysis

Moreover, a new section was added:14. LNG Sales contract custody transfer checklist.Worked out examples for LNG density and GCVhave been rearranged in Appendices 1 and 2.

We wish to thank all companies and organisationsand their delegates who together contributed to thissecond 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)

2. GENERAL DESCRIPTIONOF THE MEASUREMENT

2.1. GENERAL FORMULA FORCALCULATING THE LNGENERGY TRANSFERRED

The formula for calculating the LNG transferreddepends on the contractual sales conditions. Thesecan relate to an FOB sale, a CIF sale or a DES sale(Incoterms 2000).

In the case of an FOB sale, the determination of theenergy transferred and invoiced for will be made inthe loading port (FOB: Free On Board).In the case of a CIF or a DES sale, the energytransferred and invoiced for will be determined in theunloading port (CIF: Cost Insurance Freight – DES:Delivery Ex Ship).

In all cases, the formula can be summarised asfollows:

( ) displacedgasLNGLNGLNG EGCVDVE −= ..

E = the energy transferred from theloading facilities to the LNG carrier orfrom the LNG carrier to the unloadingfacilities.In international LNG trading, theenergy transferred is most frequentlyexpressed in millions of BritishThermal Units (106 BTU or MMBTU);although tis is not a SI energy unit,this unit will be adopted in thishandbook. A conversion factor tablefor other commonly used energyunits can be found in Enclosure 1.

LNGV = the volume of LNG loaded orunloaded in m3.

LNGD = the density of LNG loaded orunloaded in kg/m3.

LNGGCV = the gross calorific value of the LNGloaded or unloaded in MMBTU/kg.


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displacedgasE = the energy of the gas in gaseousform, also in MMBTU, which iseither:- sent back onshore by the LNG

carrier when loading. In mostcases, this energy is returnedfree of charge to the loadingfacilities: Egas displaced is then nil;

- or received by the LNG carrierwhen unloading in replacementof the LNG transferred. In thiscase it is generally taken intoaccount.

2.2. GENERAL SCHEME OF THEMEASUREMENT OPERATIONS

The objective is to measure the quantity of energyloaded from production facilities into an LNG carrier,or unloaded from an LNG carrier to a receivingterminal.

From the above formula, it can be inferred that 4elements must be measured and calculated:- LNG volume,- LNG density,- LNG gross calorific value,- energy of the gas displaced during the transfer

of LNG.

For a graphic overview please refer to the‘measurement flowchart’ on page 5.

2.2.1. Volume

The method chosen of measuring the volume isbased on the LNG carrier's instruments, mainly theuse of level gauges and calibration tables.

Usually a quantity of LNG, called 'heel', remains onboard after unloading so as to keep the tanks cold.Determination of the volume transferred requires twomeasurements, one before and one after loading orunloading; so the result will be two LNG volumes.The difference between the larger volume and thesmaller volume will represent the volume of liquidtransferred.

For an accurate custody transfer it is recommendedthat LNG manifolds on ship’s deck be in an identicalinventory condition during both custody transfersurveys (CTS) : either completely filled with LNGboth during the opening custody transfer (i.e. before(un)loading) and the closing custody transfer (i.e.after (un)loading), or otherwise be drained duringboth the opening and closing CTS (custody transfersurvey).

In some cases the LNG carrier must be completelyemptied, e.g. before a long period of inactivity. In thiscase a special procedure explained in section 3.2.6is followed for determination of the volumetransferred.

The use of in-line flowmeters has not beenconsidered since, at time of writing, the accuracy ofthis apparatus has not been acceptable to theindustry.

2.2.2. Density

The density of LNG is determined by calculation fromthe composition of the LNG transferred and thetemperature of the LNG from measurements in theLNG carrier's tanks.

2.2.3. Gross calorific value

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

2.2.4. Energy of the gas displaced by thetransfer of LNG

This energy is calculated according to thecomposition and volume of the gas displaced, andthe pressure and temperature of the gas inside thetanks of the LNG carrier before loading , resp. afterunloading.

The calculation procedure is explained in Section 11.

2.2.5. Flowchart for determining the energytransferred


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FLOWCHART FOR DETERMINING THE ENERGY TRANSFERRED

LNG CARRIER

Level gauge Pressure gauge Thermal probes Laboratory analyses

Sampling ofLNG

Sampling gasdisplaced

Correction factorsTrim-list

Temperature

Gauge table

Volume before loading or discharging = V1Volume after loading or discharging = V2Difference V loaded = V2 - V1

V unloaded = V1 - V2

Volume of gasdisplaced

Density of LNG GCV of gasdisplaced GCV of LNG

Energy of gasdispaced

E = VLNG x D LNG x GCV LNG - E gas displaced

Correctionfactor

2.3. INSTRUMENTS USED2.3.1. For the determination of the LNG volume

For the determination of the LNG volume the followingare required:

- the calibration tables, including the main gaugetables for each tank and the correction tables forlist, trim, tank contraction and possible additionalfactors according to the type of level measuringdevices,

- the equipment for measuring the level of LNG inthe LNG carrier's tanks which are either floatgauges and/or capacitance type level gaugesand/or microwave (radar) type level gauges.Each cargo tank usually has two level gaugesystems installed, one designated as 'main' or'primary' and the other as 'secondary',

- temperature probes distributed over the height ofthe LNG carrier's tanks,

- all measuring devices required by the correctionfactors.

2.3.2. For the determination of LNG density andgross calorific value

The determination of the density and the grosscalorific value of the LNG transferred will be made onthe basis of the average composition of the LNGobtained by :

- continuous or discontinuous sampling of LNG,

- gas chromatographic analysis,

- a calculation based on the average compositionof the LNG, its average temperature and thecoefficients given by the National Bureau ofStandards for the density [10],

- a calculation based on the average compositionof LNG and characteristics of elementarycomponents (GCV, molar volume, molarweight) given by reference tables or standardsfor the gross calorific value.

2.3.3. For the energy of displaced gas

The energy of the displaced gas can be determinedfrom:

- sampling of the gas displaced,

- a gas chromatographic analysis of thissampling enabling the GCV to be calculated,

- pressure and temperature measurementswithin the LNG carrier's tanks.

However, for the determination of the energydisplaced, some parameters such as pressure, gascomposition and temperature can be estimated fromexperience and taken as constant for both custodytransfer surveys before and after (un)loading.

For instance, the displaced gas may be assumed tobe pure methane. This assumption will hardlyincrease the overall inaccuracy.


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2.3.4. Periodic instruments recalibration

It is recommended that, unless it is specified by thefiscal authorities or by the Classification Society,Buyer and Seller agree on the periodicity ofrecalibration intervals, e.g. at each drydocking.

2.4. STANDARDISATIONInternational standards exist for the classical methodsand techniques used for LNG Custody Transfer suchas ISO 6568 for gas chromatography and ISO 6976for calculation of the GCV of gas.

On the other hand many LNG shipping projects,especially existing ones, specify GPA 2261-72 for gaschromatography and IP 251/76 or GPA 2145-86 forthe calculation of the GCV of gas. Buyer and Sellermay approve these editions, or a more recent edition.As far as methods and techniques dealing with staticmeasurement procedures for LNG are concerned, itmust be noted that international standards have beenissued in recent years by ISO (see Enclosure 2 page55). The recommendations included in thesedocuments and future international standards mightbe considered for new applications.

3. VOLUME MEASUREMENT3.1. GAUGE TABLES3.1.1. Use of gauge tables

The gauge tables are numerical tables which, relatethe height of the liquid in an LNG carrier's tank, to thevolume contained in that tank. The height may needto be corrected taking into consideration variousfactors.

An independent surveyor usually produces the gaugetables during the building of the LNG carrier.They take into account the configuration of the tank,its contraction according to the temperature of theliquid, and the volume occupied by various devices,e.g. cargo pumps.

The calibration tables are usually divided into:

- main gauge tables: height/volume correlation inideal conditions,

- correction tables taking into account actualconditions of the LNG carrier and its measuringinstruments.

For each LNG carrier there is one main gauge tableper tank. Generally the volumes are given for heightsvarying cm by cm, the volume for intermediate heightsin mm being calculated by interpolation. An exampleof a gauge table is given in Table 1 (see page 7). Theexamples used in this section are taken from a vesselwith prismatic cargo tanks. The same principlesgenerally apply to those vessels with spherical cargotanks.

These tables are established for levelmeasurements using the main level-measuringdevice installed in each tank.

To avoid these interpolations, which can be asource of accuracy, the most used parts of thegauge tables - i.e. heights between 10 and 60 cmand heights corresponding to a volume between95% and 98% of the total volume of the tank -sometimes are developed and the volumes will becalculated mm by mm. This then reduces thedetermination of the volume to a mere reading in atable (see Table 2 page 8, Table 3 page 9).

Various methods exist for establishing the gaugetables. The main methods are:

- macrometrology with tapes,

- laser measuring system,

- photogrammetric measuring system.

It is not the purpose of this handbook to describethe different methods [1 - 8].

For details of calibration procedures for tanks,reference can be made to existing ISO standards(see Enclosure 2 page 55).

3.1.2. Correction tables

The gauge tables are completed with correctiontables established according to:

- the condition of the LNG carrier (trim/list),

- the temperature in the tank that influencescontraction or expansion of the tank,

- the temperature in the gaseous phase, and/orthe density of the LNG, influencing the levelmeasuring devices.

It should be noted that LNG vessels normally havetwo level measurement devices in each cargo tank(and often of two different types) and that correctiontables are specific to a level gauge. Using thecorrection tables for the wrong gauge can result insignificant inaccuracies.

3.1.2.1 Correction according to the conditionof the LNG carrier

The gauge tables are established for an LNG carrierwith zero list and trim. Therefore, it will benecessary to correct the height reading to take intoaccount a list or a trim which is not zero.

Correction tables are made up according to:

- the position of the gauge in the tanks,

- the list of the LNG carrier (see figure 1, page10),

- the trim of the LNG carrier (see figure 2, page10).


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TABLE 1

EXAMPLE OF GAUGE TABLES WITH ZERO LIST AND TRIM

TANK N° 5

HEIGHTS VOLUMES HEIGHTS VOLUMES HEIGHTS VOLUMES HEIGHTS VOLUMES HEIGHTS VOLUMES

m m3 m m3 m m3 m m3 m m3

0.00 5.51 0.25 233.02 0.50 464.94 0.75 701.27 1.00 942.01

0.01 14.52 0.26 242.21 0.51 474.31 0.76 710.82 1.01 951.73

0.02 23.54 0.27 251.41 0.52 483.69 0.77 720.37 1.02 961.46

0.03 32.57 0.28 260.62 0.53 493.07 0.78 729.93 1.03 971.20

0.04 41.61 0.29 269.83 0.54 502.46 0.79 739.50 1.04 980.94

0.05 50.66 0.30 279.05 0.55 511.86 0.80 749.07 1.05 990.69

0.06 59.71 0.31 288.28 0.56 521.26 0.81 758.65 1.06 1000.45

0.07 68.77 0.32 297.52 0.57 530.67 0.82 768.24 1.07 1010.21

0.08 77.83 0.33 306.76 0.58 540.09 0.83 777.83 1.08 1019.98

0.09 86.90 0.34 316.01 0.59 549.52 0.84 787.43 1.09 1029.76

0.10 95.98 0.35 325.26 0.60 558.95 0.85 797.04 1.10 1039.54

0.11 105.07 0.36 334.52 0.61 568.39 0.86 806.66 1.11 1049.33

0.12 114.16 0.37 343.79 0.62 577.83 0.87 816.28 1.12 1059.13

0.13 123.26 0.38 353.07 0.63 587.29 0.88 825.91 1.13 1068.94

0.14 132.37 0.39 362.36 0.64 596.75 0.89 835.54 1.14 1078.75

0.15 141.49 0.40 371.65 0.65 606.21 0.90 845.19 1.15 1088.57

0.16 150.61 0.41 380.94 0.66 615.69 0.91 854.84 1.16 1098.40

0.17 159.74 0.42 390.25 0.67 625.17 0.92 864.50 1.17 1108.23

0.18 168.87 0.43 399.56 0.68 634.66 0.93 874.16 1.18 1118.07

0.19 178.02 0.44 408.88 0.69 644.15 0.94 883.83 1.19 1127.92

0.20 187.17 0.45 418.21 0.70 653.66 0.95 893.51 1.20 1137.78

0.21 196.32 0.46 427.54 0.71 663.17 0.96 903.20 1.21 1147.64

0.22 205.49 0.47 436.88 0.72 672.68 0.97 912.89 1.22 1157.51

0.23 241.66 0.48 446.23 0.73 682.21 0.98 922.59 1.23 1167.39

0.24 223.84 0.49 455.58 0.74 691.74 0.99 932.30 1.24 1177.27

Total volume: 25577.680 cubic metres.


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TABLE 2EXAMPLE OF BOTTOM FINE GAUGE TABLE

HEIGHT +0 MM + 1 MM + 2 MM + 3 MM + 4 MM + 5 MM + 6 MM + 7 MM + 8 MM + 9 MM0.10 41.340 41.750 42.160 42.570 42.980 43.390 43.800 44.210 44.620 45.0300.11 45.440 45.851 46.262 46.673 47.084 47.495 47.906 48.317 48.728 49.1390.12 49.550 49.962 50.374 50.786 51.198 51.610 52.022 52.434 52.846 53.2580.13 53.670 51.082 54.494 54.906 55.318 55.730 56.142 56.554 56.966 57.3780.14 57.790 58.202 58.614 59.026 59.438 59.850 60.262 60.674 61.086 61.4980.15 61.910 62.323 62.736 63.149 63.562 63.975 64.388 64.801 65.214 65.6270.16 66.040 66.454 66.868 67.282 67.696 68.110 68.524 68.938 69.352 69.7660.17 70.180 70.594 71.008 71.422 71.836 72.250 72.664 73.078 73.492 73.9060.18 74.320 71.734 75.148 75.562 75.976 76.390 76.804 77.218 77.632 78.0460.19 78.460 78.875 79.290 79.705 80.120 80.535 80.950 81.365 81.780 82.1950.20 82.610 83.026 83.442 83.858 84.274 84.690 85.106 85.522 85.938 86.3540.21 86.770 87.186 87.602 88.018 88.434 88.850 89.266 89.682 90.098 90.5140.22 90.930 91.347 91.764 92.181 92.598 93.015 93.432 93.849 94.266 94.6830.23 95.100 95.517 95.934 96.351 96.768 98.185 97.602 98.019 98.436 98.8530.24 99.270 99.688 100.106 100.524 100.942 101.360 101.778 102.196 102.614 103.0320.25 103.450 103.868 104.286 104.704 105.122 105.540 105.958 106.376 106.794 107.2120.26 107.630 108.049 108.468 108.887 109.306 109.725 110.144 110.563 110.982 111.4010.27 111.820 112.239 112.658 113.077 113.496 113.915 114.334 114.753 115.172 115.5910.28 116.010 116.430 116.850 117.270 117.690 118.110 118.530 118.950 119.370 119.7900.29 120.210 120.630 121.050 121.470 121.890 122.310 122.730 123.150 123.570 123.9900.30 124.410 124.831 125.252 125.673 126.094 126.515 126.936 127.357 127.778 128.1990.31 128.620 129.042 129.464 129.886 130.308 130.730 131.152 131.574 131.996 132.4180.32 132.840 133.262 133.684 134.106 134.528 134.950 135.372 135.794 136.216 136.6380.33 137.060 137.482 137.904 138.326 138.748 139.170 139.592 140.014 140.436 140.8580.34 141.280 141.703 142.126 142.549 142.972 143.395 143.818 144.241 144.664 145.0870.35 145.510 145.934 146.358 146.782 147.206 147.630 148.054 148.478 148.902 149.3260.36 149.750 150.174 150.598 151.022 151.446 151.870 152.294 152.718 153.142 153.5660.37 153.990 154.414 154.838 155.262 155.686 156.110 156.534 156.958 157.382 157.8060.38 158.230 158.655 159.080 159.505 159.930 160.355 160.780 161.205 161.630 162.0550.39 162.480 162.906 163.332 163.758 164.184 164.610 165.036 165.462 165.888 166.3140.40 166.740 167.166 167.592 168.018 168.444 168.870 169.296 169.722 170.148 170.5740.41 171.000 171.427 171.854 172.281 172.708 173.135 173.562 173.989 174.416 174.8430.42 175.27 175.697 176.124 176.551 176.978 177.405 177.832 178.259 178.686 179.1130.43 179.540 179.968 180.396 180.824 181.252 181.680 182.108 182.536 182.964 183.3920.44 183.820 184.248 184.676 185.104 185.532 185.960 186.388 186.816 187.244 187.6720.45 188.100 188.529 188.958 189.387 189.816 190.245 190.674 191.103 191.532 191.9610.46 192.390 192.819 193.248 193.677 194.106 194.535 194.964 195.393 195.822 196.2510.47 196.680 197.110 197.540 197.970 198.400 198.830 199.260 199.690 200.120 200.5500.48 200.980 201.410 201.840 202.270 202.700 203.130 203.560 203.990 204.420 204.8500.49 205.280 205.711 206.142 206.573 207.004 207.435 207.866 208.297 208.728 209.1590.50 209.590 210.022 210.454 210.886 211.318 211.750 212.182 212.614 213.046 213.4780.51 213.910 214.341 214.772 215.203 215.634 216.065 216.496 216.927 217.358 217.7890.52 218.220 218.653 219.086 219.519 219.952 220.385 220.818 221.251 221.684 222.1170.53 222.550 222.983 223.416 223.849 224.282 224.715 225.148 225.581 226.014 226.4470.54 226.880 227.313 227.746 228.179 228.612 229.045 229.478 229.911 230.344 230.7770.55 231.210 231.644 232.078 232.512 232.946 233.380 233.814 234.248 234.682 235.1160.56 235.550 235.985 236.420 236.855 237.290 237.725 238.160 238.595 239.030 239.4650.57 239.900 240.335 240.770 241.205 241.640 242.075 242.510 242.945 243.380 243.8150.58 244.250 244.686 245.122 245.558 245.994 246.43 246.866 247.302 247.738 248.1740.59 248.610 249.046 249.482 249.918 250.354 250.790 251.226 251.662 252.098 252.534

Heights are in metres, volumes in cubic metres.


GIIGNL - DS TML/Z - CG - 2001/10/09 9

TABLE 3

EXAMPLE OF TOP FINE GAUGE TABLE

HEIGHT +0 MM + 1 MM + 2 MM + 3 MM + 4 MM + 5 MM + 6 MM + 7 MM + 8 MM + 9 MM21.75 26021.430 26022.439 26023.448 26024.457 26025.466 26026.475 26027.484 26028.493 26029.502 26030.51121.76 26031.520 26032.527 26033.534 26034.541 26035.548 26036.555 26037.562 26038.569 26039.576 26040.58321.77 26041.590 26042.598 26043.606 26044.614 26045.622 26046.630 26047.638 26048.646 26049.654 26050.66221.78 26051.670 26052.675 26053.680 26054.685 26055.690 26056.695 26057.700 26058.705 26059.710 26060.71521.79 26061.720 26062.726 26063.732 26064.738 26065.744 26066.750 26067.756 26068.762 26069.768 26070.77421.80 26071.780 26072.786 26073.792 26074.798 26075.804 26076.810 26077.816 26078.822 26079.828 26080.834

21.81 26081.840 26082.843 26083.846 26084.849 26085.852 26086.855 26087.858 26088.861 26089.864 26090.86721.82 26091.870 26092.873 26093.876 26094.879 26095.882 29096.885 26097.888 26098.891 26099.894 26100.89721.83 26101.900 26102.903 26103.906 26104.909 26105.912 26106.915 26107.918 26108.921 26109.924 26110.92721.84 26111.930 26112.932 26113.934 26114.936 26115.938 26116.940 26117.942 26118.944 26119.946 26120.94821.85 26121.950 26122.951 26123.952 26124.953 26125.954 26126.955 26127.956 26128.957 26129.958 26130.95921.86 26131.960 26132.961 26133.962 26134.963 26135.964 26136.965 26137.966 26138.967 26139.968 26140.96921.87 26141.970 26142.969 26143.968 26144.967 26145.966 26146.965 26147.964 26148.963 26149.962 26150.96121.88 26151.960 26152.959 26153.958 26154.957 26155.956 26156.955 26157.954 26158.953 26159.952 26160.95121.89 26161.950 26162.950 26163.950 26164.950 26165.950 26166.950 26167.950 26168.950 26169.950 26170.95021.90 26171.950 26172.947 26173.944 26174.941 26175.938 26176.935 26177.932 26178.929 26179.926 26180.92321.91 26181.920 26182.918 26183.916 26184.914 26185.912 26186.910 26187.908 26188.906 26189.904 26190.90221.92 26191.900 26192.896 26193.892 16194.888 26195.884 26196.880 26197.876 26198.872 26199.868 26200.864

21.93 26201.860 26202.855 26203.850 26204.845 26205.840 26206.835 26207.830 26208.825 26209.820 26210.81521.94 26211.810 26212.806 26213.802 26214.798 26215.794 26216.790 26217.786 26218.782 26219.778 26220.77421.95 26221.770 26222.763 26223.756 26224.749 26225.742 26226.735 26227.728 26228.721 26229.714 26230.70721.96 26231.700 26232.693 26233.686 26234.679 26235.672 26236.665 26237.658 26238.651 26239.644 26240.63721.97 26241.630 26242.624 26243.618 26244.613 26245.606 26246.600 26247.594 26248.588 26249.582 26250.57621.98 26251.570 26252.562 26253.554 26254.546 26255.538 26256.530 26257.522 26258.514 26259.506 26260.49821.99 26261.490 26262.480 26263.470 26264.460 26265.450 26266.440 26267.430 26268.420 26269.410 26270.40022.00 23271.390 26272.383 26273.376 26274.369 26275.362 26276.355 26277.348 26278.341 26279.334 26280.32722.01 26281.320 26282.309 26283.298 26284.287 26285.276 26286.265 26287.254 26288.243 26289.232 26290.22122.02 26291.210 26292.200 26293.190 26294.180 26295.170 26296.160 26297.150 26298.140 26299.130 26300.12022.03 26301.110 26302.098 26303.086 26304.074 26305.062 26306.050 26307.038 26308.026 26309.014 26310.00222.04 26310.990 26311.978 26312.966 26313.954 26314.942 26315.930 26316.918 26317.906 26318.894 26319.882

22.05 26320.870 26321.858 26322.846 26323.834 26324.822 26325.810 26326.798 26327.786 26328.774 26329.76222.06 26330.750 26331.736 26332.722 26333.708 26334.694 26335.680 26336.666 26337.652 26338.638 26339.62422.07 26340.610 26341.595 26342.580 26343.565 26344.550 26345.535 26346.520 26347.505 26348.490 26349.47522.08 26350.460 26351.446 26352.432 26353.418 26354.404 26355.390 26356.376 26357.362 26358.348 26359.33422.09 26360.320 26361.304 26362.288 26363.272 26364.256 26365.240 26366.224 26367.208 26368.192 26369.17622.10 26370.160 26371.144 26372.128 26373.112 26374.096 26375.080 26376.064 26377.048 26378.032 26379.01622.11 26380.000 26380.984 26381.968 26382.952 26383.936 26384.920 26385.904 26386.888 26387.872 26388.85622.12 26389.840 26390.821 26391.802 26392.783 26393.764 26394.745 26395.726 26396.707 26397.688 26398.66922.13 26399.650 26400.631 26401.612 26402.593 26403.574 26404.555 26405.536 26406.517 26407.498 26408.47922.14 26409.460 26410.442 26411.424 26412.406 26413.388 26414.370 26415.352 26416.334 26417.316 26418.29822.15 26419.280 26420.260 26421.240 26422.220 26423.200 26424.180 26425.160 26426.140 26427.120 26428.10022.16 26429.080 26430.060 26431.040 26432.020 26433.000 26433.980 26434.960 26435.940 26436.920 26437.900

22.17 26438.880 26439.858 26440.836 26441.814 26442.792 26443.770 26444.748 26445.726 26446.704 26447.68222.18 26448.660 26449.638 26450.616 26451.594 26452.572 26453.550 26454.528 26455.506 26456.484 26457.46222.19 26458.440 26459.418 26460.396 26461.374 26462.352 26463.330 26464.308 26465.286 26466.264 26467.24222.20 26468.220 26469.196 26470.172 26471.148 26472.124 26473.100 26474.076 26475.052 26476.028 26477.00422.21 26477.980 26478.955 26479.930 26480.905 26481.880 26482.855 26483.830 26484.805 26485.780 26486.75522.22 26487.730 26488.707 26489.684 26490.661 26491.630 26492.615 26493.592 26494.569 26495.546 26496.52322.23 26497.500 26498.473 26499.446 26500.419 26501.392 26502.365 26503.338 26504.311 26505.284 26506.257

Heights are in metres, volumes in cubic metres.


GIIGNL - DS TML/Z - CG - 2001/10/09 10

FIGURE 1

List represented by the angle a in degrees to port. In this illustrative case, the correction will be negative.

FIGURE 2

Trim expressed in metres or fractions of a metre, according to the position in the tank.The correction is negative or positive according to whether the bow of the LNG carrier is settled down in the sea orotherwise.In this illustrative case the correction will be negative.


GIIGNL - DS TML/Z - CG - 2001/10/09 11

TABLE 4

EXAMPLE OF VOLUME CORRECTIONS ACCORDING TO TANK SERVICETEMPERATURES FOR SELF-SUPPORTING TANKS

Vt = K. V-160°C

T°C K T°C K

-165.0 0.99980 -160.8 0.99997-164.9 0.99980 -160.7 0.99997-164.8 0.99981 -160.6 0.99998-164.7 0.99981 -160.5 0.99998-164.6 0.99981 -160.4 0.99998-164.5 0.99982 -160.3 0.99999-164.4 0.99982 -160.2 0.99999-164.3 0.99983 -160.1 1.00000-164.2 0.99983 -160.0 1.00000-164.1 0.99983 -159.9 1.00000-164.0 0.99984 -159.8 1.00001-163.9 0.99984 -159.7 1.00001-163.8 0.99985 -159.6 1.00002-163.7 0.99985 -159.5 1.00002-163.6 0.99985 -159.4 1.00002-163.5 0.99986 -159.3 1.00003-163.4 0.99986 -159.2 1.00003-163.3 0.99987 -159.1 1.00004-163.2 0.99987 -159.0 1.00004-163.1 0.99987 -158.9 1.00005-163.0 0.99988 -158.8 1.00005-162.9 0.99988 -158.7 1.00005-162.8 0.99989 -158.6 1.00006-162.7 0.99989 -158.5 1.00006-162.6 0.99989 -158.4 1.00007-162.5 0.99990 -158.3 1.00007-162.4 0.99990 -158.2 1.00008-162.3 0.99991 -158.1 1.00008-162.2 0.99991 -158.0 1.00008-162.1 0.99991 -157.9 1.00009-162.0 0.99992 -157.8 1.00009-161.9 0.99992 -157.7 1.00010-161.8 0.99993 -157.6 1.00010-161.7 0.99993 -157.5 1.00010-161.6 0.99993 -157.4 1.00011-161.5 0.99994 -157.3 1.00011-161.4 0.99994 -157.2 1.00012-161.3 0.99995 -157.1 1.00012-161.2 0.99995 -157.0 1.00013-161.1 0.99995 -156.9 1.00013-161.0 0.99996 -156.8 1.00013-160.9 0.99996 -156.7 1.00014


GIIGNL - DS TML/Z - CG - 2001/10/09 12

These corrections can be positive or negative. Sothe real height will be equal to the algebraic sum ofthe height reading, the correction for list and thecorrection for trim. These tables are made up indegrees for the list and in metres for the trim, withfixed steps of variation. For intermediate values, thecorrection will be calculated by interpolation.

In practice, these corrections are not usedfrequently since the LNG carrier's cargo officer willusually manage the vessel's ballast to obtain zerolist and trim.

3.1.2.2 Corrections according to thetemperatures in the liquid andgaseous phases

The corrections are related to the volume variationsresulting from the contraction of the tanks and theirinsulation according to the temperature of the liquidand gaseous phases.

This phenomenon is significant for LNG carriers withself-supporting tanks. Table 4 gives an example ofthese tables (see page 11).

3.1.3. Approval by authorities

The gauge tables may be approved by either theauthorities of the countries concerned with the LNGsale and purchase or by independent swornmeasurers.

This approval may be valid for a limited duration,generally 10 to 12 years, provided there are nomodifications to the tanks. For the EuropeanCommunity, this approval corresponds to aCommunity Directive.

When an LNG carrier is put into operation, a list ofall works on the tanks must be supplied, and thetanks must be inspected for any modifications whichmight affect the volume.

In the case of any distortion or modification to atank, the gauge table must be adjusted accordingly.

3.1.4. Inaccuracy of the table

The inaccuracy generally guaranteed by thecontractor's calibrations is ± 0.2% at ambienttemperature. The study carried out by the NationalBureau of Standards [8] on the calibration of thetanks of LNG carriers shows that the real inaccuracyis far better, and is about ± 0.05% to ± 0.1%.

Also according to this study, the systematicinaccuracy due to the effect of shrinkage of thetanks when they have been cooled down should notbe more than 0.07%. Therefore for a tank of 26,000m3, the maximum guaranteed inaccuracy would be± 52 m3 LNG, and even less by half if we considerthe work of the NBS.

3.2. INSTRUMENTS AND METHODSFOR MEASURING THE LEVELOF LIQUID IN THE LNGCARRIER'S TANKS

3.2.1. Main liquid level gauging devices

The main types of gauges are:

- electrical capacitance gauge (cfr. ISO 8309)

- float gauge (cfr. ISO 10574)

- microwave type gauge (cfr. ISO 13689)

Any of these gauges can serve as the main and usualinstruments for measuring the height of the liquid.Usually (but not always) two of these three types areinstalled. One of these should be agreed as the main(primary) level gauging device by Buyer and Seller.The gauge not used will be considered as theauxiliary (secondary) gauge.A few LNG shipping projects do not specify the typeand only specify the required accuracy (ex. ± 7.5 mmor better).On old ships other devices may be found,such as nitrogen bubbling devices, but the accuracyof these is generally lower.

3.2.1.1 Capacitance gauge

The electrical capacitance gauge (see figure 3, page13) consists of two concentric aluminium tubes. Theinner tube is supported by the outer tube by means ofconcentric insulators placed regularly spaced intervalsalong the whole length of the tubes. The resultingassembly forms a series of cylindrical capacitors,having the same total height as the cargo tank of theLNG carrier.

The LNG, according to its level, will fill the spacebetween the concentric tubes. The liquid affects thedieletric characteristics such that, by measuring thechange in capacitance, the height of the LNG in theannular space, and hence the level in the tank, can bedetermined. The contraction of the aluminium tube atlow temperature may be taken into account to correctthe level measurement.

The accuracy of the measurement resulting from thecalibration of the dimensions and the linearity of thecapacitor and of the electronics should be, for thegauges as a whole, ± 7.5 mm [1].

3.2.1.2 Float gauge

Measurements are made with a float hanging on atape or a ribbon (see figure 4, page 14). According tothe level of the liquid, the float is displaced, and thetape or the ribbon on which it hangs is unrolled orrolled up on a drum whose cycles of rotation arerecorded. This enables the position of the probe, andthus the level of liquid in the tank, to be known.

With float gauges, it is necessary to take into accountthe shrinkage of the ribbon, according to thetemperature of the gaseous phase and the height ofthe liquid, and the density of the LNG, which willinfluence the float buoyancy. The correction tables willtabulate the corrections for these effects.


GIIGNL - DS TML/Z - CG - 2001/10/09 13

FIGURE 3

1. Outer aluminium tube.2. Inner aluminium tube.3. Concentric electrical insulator.4. Isolation of inner tube sections by a gap or dielectric plug.5. Isolation from the tank bottom.6. Bolting together the sections of the outer tube making a single electrical conductor.7. Transfer line of the signals from the outer tube and each centre of the inner tube to a control junction box

outside the cargo tank.8. LNG cargo tank.


GIIGNL - DS TML/Z - CG - 2001/10/09 14

FIGURE 4


GIIGNL - DS TML/Z - CG - 2001/10/09 15

FIGURE 5

MICROWAVE TYPE LEVEL GAUGE


GIIGNL - DS TML/Z - CG - 2001/10/09 16

The corrections for temperature are required only inthe case of a stainless steel ribbon. In the case of aninvar ribbon, the shrinkage is much less and isgenerally considered as negligible.

The precision of this type of gauge, designed formarine application, is in the range of ± 4 mm to±8 mm.

3.2.1.3 Microwave gauge

The microwave gauge works on the same principleas a ship's radar (see figure 5, page 15). Atransmitter is mounted on the top of the cargo tankand emits radar waves vertically down towards thesurface of the liquid. The signal is reflected from thesurface, received by the transmitter's antenna andsent back to the control panel. The signal is thenprocessed to determine the distance of the liquidsurface from the transmitter and hence ullage.

Since all the level detection components aremounted external to the cargo tank, the microwavesystem allows for the possibility of changing thegauge in service.

The precision of this type of gauge is claimed to bebetter than ± 7.5 mm.

3.2.2. Timing of the level measurement

3.2.2.1 In an FOB agreement

The level readings will be made just before startingto load, when the loading arms have been connectedand before starting to cool them down.

This level reading will enable the determination of thequantity of LNG remaining on board as cooling liquid,also called "heel".

The second level reading will be made 15 to 30minutes after the end of loading, when the surface ofthe liquid is nearly stabilised.

3.2.2.2 In a CIF or DES agreement

In the case of a CIF or DES agreement, proceed asabove mutatis mutandis. It is also a good idea to wait15 to 30 minutes after the end of unloading so thatthe surface of the remaining liquid is nearlystabilised.

3.2.3. Readings

It is good practice that all readings are witnessed byboth parties. The readings of the levels of liquid inthe tanks are taken after the readings of the list (portor starboard), and the trim (bow or stern) of the LNGcarrier. The temperatures of the liquid and thegaseous phase are also measured (see § 4.1. and §4.2.). For some vessels, the atmospheric pressure isalso read.

3.2.3.1 Reading of the level with float gauges

A test could be carried out before the reading bycomparing the height indicated by the gauge in itsstowed position with that given by the last calibration

check in this position. If this test is satisfactory thelevel readings can be taken. It should be noted thatthe surface of the liquid is not motionless: the liquidmay be in an effervescent state and is subject to themovements of the ship.It is advisable to take several readings, from 2 to 6according to the amplitude of the movement of thefloat, the height recorded being the average of themaximum and minimum readings.

If the level indication is unusual, the float may bestuck; it is suggested that it is raised and loweredagain in an attempt to obtain the expected reading.

It must be noted that float gauges are always stowedin their fixed upper position when sailing so that theribbon or the tape does not break due to liquidmovements.

The float is released before the first reading whichcan be carried out either locally, by reading the levelindication on the gauge head or, if equipped with atransmitter, by reading it on a digital display in thecargo control room.

3.2.3.2 Reading of the level with capacitanceand microwave gauges

The reading is taken in the cargo control room of theLNG carrier. On older capacitance gauges, the levelis read for each tank several times, up to 5 times atregular intervals, and the arithmetic average value isthen calculated.

For modern systems, a computer processes all theinformation, including averaging of the level readingsover time, temperature and pressure, and draws oncomputer based gauging tables to produce a printeddocument containing all the ship-generatedinformation required for the custody transfer,however not always including gas displaced.

3.2.4. Correction of readings

3.2.4.1 Float gauge

The readings made on the measurement appliancesshould be corrected according to:

- list,

- trim,

- density of LNG, affecting float buoyancy,

- coefficient of contraction of the material and theinsulation of the tanks; this coefficient isapplicable in the case of self-supporting tanks(table 4, see page 11),

- temperature of the gaseous phase if the ribbonis not made of invar.

The corrections are made by using tables.

3.2.4.2 Capacitance and microwave gauge

In this case, only the corrections for list and trim andthe contraction of the tanks are taken intoconsideration. For accurate level measurement thecontraction of the capacitance gauge at lowtemperature may need to be considered. For some


GIIGNL - DS TML/Z - CG - 2001/10/09 17

microwave level gauges a temperaturecompensation of the microwave guide pipe isnecessary.

Modern computer based systems usually can accepttrim and list data either manually or from externalsensors and automatically apply the corrections.

3.2.5. Use of spare level gauge

When the main or primary level gauge cannot beused, the secondary level gauge is used to measurethe level of LNG. If the calibration tables of the LNGcarrier are available only for the main level gauge, aconversion table is required in order to take intoaccount the respective locations of main andsecondary gauges, or the statistical differencesbetween the two level gauge measurements, and toevaluate the corresponding corrections which mustbe applied to level measurement before using thecalibration tables.

3.2.6. Complete unloading (tank stripping)

For the complete unloading of an LNG carrier withprismatic tanks, the condition of the LNG carrier willbe with a maximum trim in order to concentrate theLNG in the part of the tank occupied by the gaugeand the pumps.

A procedure may consist of unloading until theminimum measurable height indicated in thecalibration table.

The remaining immeasurable LNG may be unloadedusing stripping pumps by agreement with all parties.Afterwards the remaining quantity can be vaporisedby warming up to a certain temperature e.g. -80°C.At that temperature the LNG carrier is considered tobe empty of LNG. These stripping and warming upoperations require several additional hours ofunloading time.

The energy of this remaining LNG transferred eitherin the liquid or in gaseous form can also bedetermined by mutual agreement by all parties.

The technical possibilities of the receiving terminalmust also be taken into account.

3.3. CALCULATION OF THEVOLUME OF LNGTRANSFERRED

This calculation is illustrated by an example given intables 5 (see page 18) and 6 (see page 19) showingthe results of the volume determination before andafter loading the LNG cargo, with the followinghypotheses:

- a "Gaz Transport"-type LNG carrier with 5 invarmembrane prismatic tanks,

- in each tank, one float gauge with a stainlesssteel ribbon.

The example as illustrated is representative of anLNG carrier with an older level gauging system. A

similar procedure would be adopted for thesecondary gauging system of a modern levelgauging system. For the primary system on amodern instrumented LNG carrier, the procedure iscarried out automatically by computer usingessentially the same methods, however not alwaystaking into account the gas displaced.

3.4. INACCURACY OF THE VOLUMEMEASUREMENT

Table 11 gives an example of the overall inaccuracyin volume determination as a result of the respectiveinaccuracy of each measurement and of the gaugetable.

This example is based on loading of a tank of a "GazTransport"-type membrane LNG carrier with a totalcapacity of 26,770 cubic metres.

Upon arrival at the loading port, the heel represents1.5% of the total capacity of the tank which is thenfilled to 98%.


GIIGNL - DS TML/Z - CG - 2001/10/09 18

TABLE 5

VOLUME DETERMINATION BEFORE LOADING LNG

LNG CARRIERVOYAGE No

CARGO DENSITY: 451.61 kg/m3

TRIM: +80 cm (AFT)LIST: -0.50 m (PORT)BERTH:

CARGO ON BOARD AT ARRIVALSURVEY DATE:SURVEY TIME GMT:LOCAL TIME:REMARKS:

CorrectionsVapourtemp.

°C(1)

Levelgauge

readingmm

Shrink.mm(2)

Densitymm(3)

Listmm(4)

Trimmm(5)

Overallmm(6)

Correctedheightmm(7)

LiquidVolume

m3

(8)

TANK 1 -117.22 575 +42 +2 +4 -30 +18 593 249.918

TANK 2 -124.32 435 +44 +2 +4 -51 -1 434 425.150

TANK 3 -125.16 458 +44 +2 +4 -56 -6 452 509.560

TANK 4 -124.63 505 +44 +2 +4 -56 -6 499 563.084

TANK 5 -122.46 543 +44 +2 +4 -54 -4 539 502.460

TOTAL O/B 2,250.172

Notes:

(1) For measurement of temperature in thegaseous phase, see section 4.

(2) Correction for ribbon shrinkage of the float levelgauge due to the cryogenic temperature in thegaseous phase according to table 7 (see page20).

(3) Correction for LNG density, established fromthe density calculated on the basis of the LNGcomposition, according to table 8 (see page21).

(4) Correction for the list corresponding to the liquidheight according to the correction tables, anexample of which is shown in table 9 (see page21) for tank no 1. In this case the position of thegauge is at the starboard side of the ship'scentre line.

(5) Correction for the trim corresponding to theliquid height according to the correction tables,an example of which is shown in table 10 (seepage 22) for tank no 1 (an interpolation wasmade in this case between the correctionvalues for a 50 cm and a 100 cm trim). In thiscase the position of the gauge is on thestern-side of the sideline of the tank.

(6) Algebraic sum of corrections (2), (3), (4) and(5).

(7) Corrected height resulting from the algebraicsum of the five previous columns.

(8) Determination of the liquid volume given in thecalibration tables from the corrected height.These tables are established for heights varyingmm by mm (see example in table 1, see page7) on the basis of certified tables indicatingvolumes for heights varying cm by cm. Anexample is given in table 2 (see page 8) for theempty tank no 1 and in table 3 (see page 9) forthe full tank no 2.


GIIGNL - DS TML/Z - CG - 2001/10/09 19

TABLE 6

VOLUME DETERMINATION AFTER LOADING LNG

LNG CARRIERVOYAGE No

CARGO DENSITY: 450.90 kg/m3

TRIM: +50 cm (AFT)LIST: 0.00 mBERTH:

CARGO ON BOARD AT DEPARTURESURVEY DATE:SURVEY TIME GMT:LOCAL TIME:REMARKS:

CorrectionsVapourtemp.

°C(1)

Levelgauge

readingmm

Shrink.mm(2)

Densitymm(3)

Listmm(4)

Trimmm(5)

Overallmm(6)

Correctedheightmm(7)

LiquidVolume

m3

(8)

TANK 1 -152.00 22,040 1 2 0 -19 -16 22,024 12,833.320

TANK 2 -152.00 22,123 1 2 0 -32 -29 22,094 26,364.256

TANK 3 -152.00 22,138 1 2 0 -35 -32 22,106 29,914.104

TANK 4 -156.00 22,165 1 2 0 -35 -32 22,133 29,962.222

TANK 5 -156.00 22,150 1 2 0 -34 -31 22,119 25,210.599

% Cargo on board: 98

TOTAL O/B dep.TOTAL O/B arr.TOTAL loaded

124,284.5012,250.172

122,034.329

Notes (1) to (8): see page 18.


GIIGNL - DS TML/Z - CG - 2001/10/09 20

TABLE 7EXAMPLE OF GAUGE CORRECTIONS FOR LOW TEMPERATURES

VAPOUR TEMPMETRES

-165C°mm

-160C°mm

-155C°mm

-150C°mm

-145C°mm

-140C°mm

-135C°mm

-130C°mm

-125C°mm

-120C°mm

-115C°mm

-110C°mm

0.250 56 55 54 52 51 49 48 46 45 43 42 400.500 56 54 53 52 50 49 47 46 44 43 42 400.750 55 54 52 51 50 48 47 45 44 43 41 401.000 55 53 52 50 49 48 46 45 43 42 41 391.250 54 53 51 50 49 47 46 44 43 42 40 391.500 53 52 51 49 48 47 45 44 42 41 40 381.750 53 51 50 49 47 46 45 43 42 41 39 382.000 52 51 49 48 47 46 44 43 41 40 39 372.250 51 50 49 48 46 45 44 42 41 40 38 372.500 51 50 48 47 46 44 43 42 41 39 38 362.750 50 49 48 46 45 44 43 41 40 39 37 363.000 50 48 47 46 45 43 42 41 40 38 37 363.250 49 48 47 45 44 43 42 40 39 38 36 353.500 48 47 46 45 44 42 41 40 39 37 36 353.750 48 47 45 44 43 42 41 39 38 37 36 344.000 47 46 45 44 42 41 40 39 38 36 35 344.250 47 45 44 43 42 41 39 38 37 36 35 334.500 46 45 44 42 41 40 39 38 37 35 34 334.750 45 44 43 42 41 40 38 37 36 35 34 325.000 45 44 42 41 40 39 38 37 36 34 33 325.250 44 43 42 41 40 38 37 36 35 34 33 325.500 43 42 41 40 39 38 37 36 35 33 32 315.750 43 42 41 40 38 37 36 35 34 33 32 316.000 42 41 40 39 38 37 36 35 34 32 31 306.250 42 41 39 38 37 36 35 34 33 32 31 306.500 41 40 39 38 37 36 35 34 33 32 30 296.750 40 39 38 37 36 35 34 33 32 31 30 297.000 40 39 38 37 36 35 34 33 32 31 30 287.250 39 38 37 36 35 34 33 32 31 30 29 287.500 38 37 37 36 35 34 33 32 31 30 29 287.750 38 37 36 35 34 33 32 31 30 29 28 278.000 37 36 35 34 33 33 32 31 30 29 28 278.250 37 36 35 34 33 32 31 30 29 28 27 268.500 36 35 34 33 32 31 31 30 29 28 27 268.750 35 34 34 33 32 31 30 29 28 27 26 259.000 35 34 33 32 31 30 29 29 28 27 26 259.250 34 33 32 32 31 30 29 28 27 26 25 249.500 33 33 32 31 30 29 28 28 27 26 25 249.750 33 32 31 30 30 29 28 27 26 25 24 24

10.000 32 31 31 30 29 28 27 27 26 25 24 2310.250 32 31 30 29 28 28 27 26 25 24 24 2310.500 31 30 29 29 28 27 26 26 25 24 23 2210.750 30 30 29 28 27 27 26 25 24 23 23 2211.000 30 29 28 28 27 26 25 24 24 23 22 2111.250 29 28 28 27 26 25 25 24 23 22 22 2111.500 29 28 27 26 26 25 24 23 23 22 21 2011.750 28 27 27 26 25 24 24 23 22 22 21 2012.000 27 27 26 25 25 24 23 22 22 21 20 2012.250 27 26 25 25 24 23 23 22 21 21 20 1912.500 26 25 25 24 23 23 22 21 21 20 19 19


GIIGNL - DS TML/Z - CG - 2001/10/09 21

TABLE 8

FLOAT GAUGESEXAMPLE OF FLOAT BUOYANCY / DIP CORRECTIONS

The float gauges have been adjusted to read correctly with the float floating in LNG of a density of 470 kg/m3.

The following table gives corrections to apply to the float gauge readings when floating in liquids of densities otherthan 470 kg/m3.

DENSITY(kg/m3)

CORRECTIONS(mm)

450 - 452 +2453 - 462 +1463 - 472 0473 - 482 -1483 - 493 -2494 - 505 -3506 - 516 -4517 - 529 -5530 - 542 -6543 - 550 -7

TABLE 9

EXAMPLE OF LIST CORRECTION TABLE

LIST

HEIGHTS -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.5 1.0 1.5 2.0 2.5 3.0

0.51 0.034 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0060.52 0.034 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0070.53 0.033 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0070.57 0.033 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0070.58 0.033 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0080.81 0.033 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0080.82 0.033 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0070.88 0.033 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0070.89 0.034 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.01 0.034 0.026 0.019 0.013 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.02 0.034 0.026 0.019 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.05 0.034 0.026 0.019 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.06 0.034 0.026 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.36 0.034 0.026 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.37 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.43 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.008 -0.0071.44 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.007 -0.0071.47 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.008 -0.007 -0.0071.48 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.007 -0.007 -0.0071.59 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.007 -0.007 -0.0071.60 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.007 -0.007 -0.0061.68 0.034 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.007 -0.007 -0.0061.69 0.035 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.007 -0.007 -0.0062.29 0.035 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.007 -0.007 -0.007 -0.0062.30 0.035 0.027 0.020 0.014 0.008 0.004 -0.003 -0.005 -0.006 -0.007 -0.007 -0.006

The list angles are in degrees, negative values to port, positive values to starboard.Heights and corrections are in metres.


GIIGNL - DS TML/Z - CG - 2001/10/09 22

TABLE 10

EXAMPLE OF TRIM CORRECTION TABLE

TRIM

HEIGHTS -2.00 -1.50 -1.00 -0.50 0.50 1.00 1.50

0.55 0.077 0.057 0.038 0.019 -0.019 -0.038 -0.057

0.56 0.077 0.057 0.038 0.019 -0.019 -0.038 -0.057

0.59 0.077 0.057 0.038 0.019 -0.019 -0.038 -0.057

0.60 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.61 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.62 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.67 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.68 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.72 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.73 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.78 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.79 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.80 0.077 0.058 0.038 0.019 -0.019 -0.038 -0.057

0.81 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.057

0.82 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.057

0.83 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.057

0.84 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.057

0.85 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.057

0.86 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.057

0.87 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.058

0.89 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.058

0.90 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.058

0.92 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.058

0.93 0.077 0.058 0.039 0.019 -0.019 -0.038 -0.058

0.94 0.077 0.058 0.039 0.019 -0.019 -0.039 -0.058

The trim values are in metres, negative values fore, positive values aft.The heights and the corrections are in metres.


GIIGNL - DS TML/Z - CG - 2001/10/09 23

TABLE 11

Inaccuracy on volume measurement (example)

1. Volume at 1.5% (m3) 401.550 2. Volume rating inaccuracy (m3) ± 0.803 3. Height of liquid at 1.5% (mm) 412 4. Volume inaccuracy due to height

measurement at 1.5% (m3)± 7.31

5. Volume at 98% (m3) 26,234.6 6. Tank volume rating inaccuracy

(m3)± 52.47

7. Height corresponding to 98%(mm)

21,969

8. Volume inaccuracy due to heightmeasurement at 98% (m3)

± 7.24

9. Volume loaded (m3) 25,833.05010. Total rating inaccuracy (m3) ± 53.48

1. Volume on arrival in the loading port: 1.5% ofthe total volume indicated in the gauge tables.

2. Rating inaccuracy of the volume indicated in thegauge tables: ± 0.2%.

3. Height of the liquid for 1.5% of the tank volumegiven by the gauge tables.

4. Inaccuracy in the measurement of the LNGlevel by the level gauge, i.e. ± 7.5 mm.This inaccuracy takes the tank configuration atthe height of 412 mm into account.

6. See no 2.

8. See no 4.

10. The overall inaccuracy is obtained as thesquare root of the sum of the squares of theinaccuracies from the quadratic combination ofinaccuracies (2), (4), (6) and (8). It thereforerepresents about 0.21% of the quantity of LNGloaded.

NOTEFor spherical tanks, owing to their geometry, thevolume inaccuracy, as a result of the smallinaccuracies of the level gauge, is significantly lessthan for prismatic tanks.

To illustrate, assuming a nominal 33,500 m3 capacitytank, arrival heel 40 m3, the depth in the tank isabout 0.8 m and the inaccuracy due to level gauge is15 mm (± 7.5 mm). This represents a volume of onlyabout 1.5 m3.

3.4.1. Cargo Liquid Lines

The previous section addresses the inaccuracy ofvolume measurement of the cargo tanks. To have afull picture, the issue of contents of the cargo liquidlines needs to be considered.

For most 'flat-deck' designs of LNG Carrier(membrane and IHI-SPB), the arrangement of cargolines on deck is such that, at completion of cargooperations, all liquid left in the liquid lines can drainby gravity back to a cargo tank.

Once the drainage is completed, custody transfermeasurements can proceed as described and thereis no need to consider liquid in the cargo lines.

For all spherical tank designs and, to a lesser extent,some 'flat-deck' designs where the manifold valvesare below the crossover lines, some consideration isneeded for undrainable liquid.

Since significant volumes of LNG may remain in thecargo manifolds and crossovers after completion ofdelivery, the normal approach is to pre-cool andcompletely fill the cargo lines with LNG prior to thefirst CTS (Custody Transfer Survey) reading onarrival. The assumption is that LNG volume in liquidlines is the same at the time of both CTS readings,and therefore can be ignored in the calculation.

4. TEMPERATUREMEASUREMENT

4.1. LIQUID TEMPERATURE

4.1.1. Device

The LNG temperature is measured by probesplaced at different heights in the tanks. These probesare generally three- or four-wire platinum resistancetemperature sensors, of which there are typically fiveper tank.

The variation in resistance, according to thetemperature, is converted into degrees Celsius withthe help of a data acquisition computer equippedwith a printer (table 12, see page 24).

Table 12 shows an example of a printout of LNGtemperatures when the tanks are filled to 98%capacity with LNG.

Figure 6 (see page 24) shows a diagram oftemperature measuring devices installed on a LNGcarrier.

In this example, 5 probes are immersed in LNG ineach tank.

The liquid temperature is calculated once the loadingoperations are over, when it is a matter ofdetermining the quantity loaded, and before theunloading operations, when it is a matter ofdetermining the quantity unloaded.

Thermocouples are not used for LNG temperaturemeasurement within custody transfer because theyare less sensitive and often give a less accuratemeasurement than platinum resistance probes. Inaddition their installation is more complex(compensation cables… ). They may be installedsometimes for control or simple indication (such ascooling down or heating of the tank).


GIIGNL - DS TML/Z - CG - 2001/10/09 24

TABLE 12

EXAMPLE OF TANK TEMPERATURE RECORDINGS (98% FILLED)

TEMPERATUREPROBE

TANK 1°C

TANK 2°C

TANK 3°C

TANK 4°C

TANK 5°C

T 1 -161.83 -161.90 -161.94 -161.89 -161.84

T 2 -161.80 -161.88 -161.91 -161.89 -161.86

T 3 -161.82 -161.87 -161.92 -161.90 -161.90

T 4 -161.79 -161.86 -161.87 -161.88 -161.81

T 5 -161.82 -161.82 -161.88 -161.91 -161.84

TANK AVERAGE -161.81 -161.87 -161.90 -161.89 -161.85

The liquid temperature in each tank is determined by the arithmetic average of temperatures indicated by theprobes that are dipped in the LNG and that are in working order.

FIGURE 6

DIAGRAM OF TEMPERATURE MEASURING DEVICES ON BOARD A LNG CARRIER


GIIGNL - DS TML/Z - CG - 2001/10/09 25

4.1.2. Testing and accuracy

The probes are tested and recalibrated at regularintervals. The accuracy of the platinum resistanceprobes varies between ± 0.1 and ± 0.2°C fortemperatures ranging between -145 and -165°C.

It is preferable to adjust the measurement foroperation at low temperature.

The overall accuracy of the temperature measuringchain can be estimated at about ± 0.5°C (probes,cable, signal converter display or printer).

The influence of temperature measurementaccuracies on the determination of LNG density (seeparagraph 8.3) is important. For instance, for LNGwith an average density in the range 440 - 470kg/m3, and at a temperature around -162°C, therelative accuracy on density calculation, due to anaccuracy of 0.5°C on temperature measurement, isabout 0.15%.

4.2. VAPOUR TEMPERATUREThe temperature in the gaseous phase of the tanksis used to determine the quantity of gas displacedduring the loading and unloading operations, or thelevel correction of the float gauge due to ribbonshrinkage.

It is the result of the average value of thetemperatures indicated by the probes not immersedin the LNG.

For some LNG shipping contracts, an accuracy of ±1.5 °C (in the range –145 to + 40 °C) is required.

5. VAPOUR PRESSUREMEASUREMENT

Vapour pressure measurements can be taken with apressure gauge, which indicates the pressure in thegas spaces of the cargo tanks.This pressure is needed to calculate the energy ofdisplaced gas, see Section 11. For this, it isnecessary for the pressure to be in absolute terms. Ifthe ship's instrumentation measures pressure in'gauge' terms, then the atmospheric pressure mustbe recorded and added to the gauge pressure.

The pressure value is recorded, with the atmosphericpressure if appropriate, at the time of taking the otherCTS readings.

In some LNG shipping contracts the requiredpressure measurement accuracy is specified as ± 1% FS (1 % of full scale).

ISO 13398 also addresses vapour pressuremeasurement.

6. SAMPLING OF LNG6.1. LNG QUALITYPossible contamination of LNG is a concern becauseit may have consequences to:• the sampling system and analytical instruments,• systems and equipment in which the LNG is to

be processed,• systems and equipment exposed to the

vaporised LNG.

As used here, contamination is meant to includeimpurities at levels greater than expected andunexpected impurities. The source of contaminationmay be at the location where the gas is liquefied, inthe transport container, and even in the systemwhich is processing or sampling the LNG.

Examples of contaminants and potential impactinclude :• water – exposed to LNG, water or water vapour

turns into a solid (ice) which can block samplingsystems, valves and instrument taps as well asdamage equipment

• particulates – metal shavings, welding debris,insulation, sand, wood and cloth are typicalexamples of particulate material. If inert, themost common problem with particulates wouldbe blockages and damage to equipment.

• sulphur – a sampling point that utilizes copperor copper alloys may be damaged bycontamination with sulphur and/or may impairthe measurement of trace sulphur compoundsby chemical reaction with copper.

• mercury – traces of mercury may damagealuminium components by chemical reactionwith the aluminium. A release of gas due to aresultant failure of the aluminium is an exampleof a possible consequence.

• other hydrocarbons – a sampling or pipingsystem that contains, for example, traces ofLPG, may result in erroneous analysis, orotherwise in LNG out of specification. Moreoverdue to the limited solubility of butanes andhigher paraffins in LNG, too high concentrationsof these may also solidify and clog samplingsystems.

• inert gases – nitrogen and air may be present inboth sampling systems and piping systemsfrom, perhaps, inadequate or poor purgingoperations. Moreover the presence of oxygenfrom the air may present a safety hazard.

• acid gases – CO2 exposed to LNG, turns into asolid similarly to water and may block samplingsystems and damage equipment.

In establishing the components for analysis, thepotential exists to ignore a contaminant because it isnot normally present at levels that exceed tolerance.The consequence may be damage to equipmentand, possibly, customer rejection of the LNG.

6.2. SAMPLING PRINCIPLESIn order to determine the quality of the LNG it is firstnecessary to undertake particular operations to


GIIGNL - DS TML/Z - CG - 2001/10/09 26

condition the fluid sampled from its initial condition,liquid at low temperature, to a final condition, gas atambient temperature, without partial vaporisation orloss of product.

Sampling of LNG includes three successiveoperations:

- taking a representative sample of LNG,

- complete vaporisation,

- and conditioning the gaseous sample beforetransporting it to the analyser,

Sampling is the critical point of the LNGmeasurements chain: each step must always beundertaken without any modification of itsrepresentativity. It is the most complicated phase ofthe measurements and most of the troublesobserved in determination of the energy loaded orunloaded come from the sampling system. Thesampling system is not easily changeable during atransfer, so many operators duplicate the system toensure sample collection in the event of failure of themain system.

Evolution in the LNG industry tends to normalize thesampling processes. The "spot (discontinuous)sampling system" described in the previous edition ofthis handbook has meanwhile become obsolete inthe LNG industry. It is therefore recommended to beused only as a back-up system, in case of failure ofthe main device and for the limited period of itsunavailability. For completeness, description of thissystem is given at the end of this section (paragraph6.12).

The sampling processes used in the LNG industryare now mainly of two types. Please note that theterminology continuous/discontinuous is differentfrom the terminology used in the previous edition.This is to reflect the terminology used in currentstandards, such as EN 12838):

- Continuous sampling: sampling processinvolving continuously collecting LNG during theloading/unloading operation from the main LNGflow, which is subsequently vaporized andstored in a gasholder; gas bottles are then filledwith the gas mixture in this gasholder, which isrepresentative of the average composition ofregasified transferred LNG, and connected tothe analyser. The continuous sampling system isspecified in ISO 8943.

- Discontinuous sampling: sampling processinvolving continuously collecting LNG from themain LNG flow; the LNG is subsequentlyvaporized and either analysed bychromatography at regular intervals or sampledin gas bottles at regular intervals which are thenconnected to the analyser.

European Standard EN12838 specifies the tests tobe carried out in order to assess the suitability ofthese two LNG sampling systems.

The various elements and options of a LNG samplingchain are summarised in figure 7.

6.3. SAMPLING POINTThe sampling point is generally located:

- on the main loading pipe, after the LNG pumpssend-out,

- on the main unloading pipe, after the unloadingarms.

LNG must be sampled on the whole flow of LNGtransferred. It is preferable to install the samplingpoint as close as possible to the transfer point (armflanges) so that the characteristics of LNG do notchange before it is actually transferred to thepurchaser due to heat input. However, generally theinfluence of heat input is limited, when the flow doesnot vary too much, in an insulated pipe.

In addition, LNG must be in a subcooled condition atthe sampling point. The LNG subcooled conditioncan be determined by using the method proposed inISO 8943 (annex A).

6.4. SAMPLING PROBESTwo basic options can be found:

- direct connection of the sampling tube on theperiphery of the LNG header ,

- LNG sampling tube protruding inside the LNGheader.

This second option is to be preferred as it avoids thepossible effect of a boundary layer at the surface ofthe main pipe, which could affect the representativityof the LNG sampling. However its design should takeinto account the risk of possible damages, due toflow-induced vibrations.

Sampling probes and tubes, transferring the sampleflow of LNG to the vaporiser, are generally made ofstainless steel.

In order to keep the LNG sampling flow in asubcooled condition, the ambient heat input shouldbe minimised. The following lay-outs can be used:

- a straight sampling tube inside the main LNGheader (figure 8a),

- a direct connection on the main pipe (figure 8b).Remark: both above lay-outs require appropriateinsulation around the sampling tube,• vacuum insulation around the Pitot

sampling tube (figure 8c) completed withappropriate insulation of the top part of theprobe,

• LNG Pitot sampling tube cooled by apermanent internal sidestream flow initiatedwith natural LNG circulation going back tothe LNG header (figure 8d),

• Optional: ambient heat input can be furtherminimised by a permanent externalsidestream flow to a purge line of the plant.


GIIGNL - DS TML/Z - CG - 2001/10/09 27

FIGURE 7

ELEMENTS OF LNG SAMPLING CHAINS


GIIGNL - DS TML/Z - CG - 2001/10/09 28

FIGURE 8

EXAMPLES OF SAMPLING PROBES

Direct connection on main LNG line (cross section view)

With a Pitot tube (transverse section view)


GIIGNL - DS TML/Z - CG - 2001/10/09 29

6.5. PIPING ARRANGEMENTBETWEEN SAMPLING PROBEAND VAPORISER

It is important to have the liquid sample line, betweenthe sampling probe and the LNG vaporiser, as shortas possible, with a small inside diameter (4 or 6 mmfor instance) and with good insulation, so that theLNG is kept in a subcooled condition until it reachesthe vaporiser.

The maximum recommended length of the liquidsample line between the sampling point and thevaporizer can be calculated by the following formula:

qHW

l∆= .

where:l = the length of pipeline (m)

W = the flow rate of sample LNG (kg/s)

H∆ = the degree of subcooling at inlet of sampleprobe (J/kg)

q = the heat input (W/m)

In calculating this maximum length of the pipe, thevalue of q is the most difficult value to estimateaccurately. The value chosen should be larger thanthe design value to take into account the ageing ofthe insulation.

6.6. LNG VAPORISER ANDCONTROL DEVICES

6.6.1. Main devices

The vaporisation of the LNG must be as complete aspossible, so that the gas obtained is representativeof the quality of the LNG.

The vaporiser must be designed in order to avoidfractionation, especially if the gas sample is directlytaken for analysis. When the device includes agasholder, filled during a great part of the transferoperation, it enables the components coming fromvaporisation to be mixed.

Vaporisation is achieved by heat exchange, mainly inone of the following devices:

- atmospheric vaporiser, but in which the heatingflow may not be enough to avoid fractionation;this device should only be used to fill a gasholder,

- water vaporiser, the heat flow being provided bywater at ambient temperature, or more often byhot water (hot water flow or water heated by anelectrical resistance, for instance),

- steam vaporiser, steam being used to warm up awater bath where a coil of metal piping, in whichLNG flows, is submerged, or steam warming the

coil up directly (Shell-tube LNG SampleVaporiser) (see figure 9c),

- electric vaporiser, the coil of piping beingwarmed up by Joule effect.

6.6.2. Description of vaporising devices

- Water or steam vaporiser

Figure 9 shows diagrams of vaporisers using watercirculation (ambient temperature or, preferably, hotwater), or water warmed by steam as heating fluid, orwater warmed by an electrical resistance submergedin the vessel, or direct low pressure steam.

The LNG sample flows in a tubing coil installed in thevessel and vaporises in it. The coil is usually made ofstainless steel (recommended) or, sometimes,copper.

- Electric vaporiser

Figure 10 shows an example of an electric vaporiser,which consists of a tubing coil in which LNG flowsand vaporises, this coil being the short-circuitedsecondary winding of a transformer.

When the primary winding is supplied by electricitythe Joule effect, which develops in the coil, producesthe energy necessary to vaporise the LNG sampleflow.

The tubing coil can be made of copper or stainlesssteel. Stainless steel is recommended and is used inthe new generation of electrical vaporiser.

6.6.3. Auxiliary vaporisation control devices

Control devices must be installed to supervise theconditions of vaporisation and to protect theequipment; some of the following ones are mainlyfound:

- on regasified LNG outlet:• pressure regulator, or gas flowrate regulator,

to control the LNG flow to be vaporisedindependently of pressure or flowrate in themain LNG pipe

• anti-pulsation bottle, or mixing accumulator,to absorb the pressure pulses and to createa temporary retention of gas homogeneity

• impingement chamber, to prevent theentrainment of possible fine droplets ofliquid

• flow meters• pressure meters• temperature detection switches,

corresponding to very high gas temperature(example: no more LNG flow) or to very lowtemperature (example: failure of heatingdevice)

• and associated devices, such asoverpressure safety valve, electrovalve toisolate the vaporiser, etc…


GIIGNL - DS TML/Z - CG - 2001/10/09 30

FIGURE 9

EXAMPLES OF WATER OR STEAM VAPORISERS

a) With water or steam circulation

b) With water warmed by electrical resistance

c) Shell-tube type vaporiser


GIIGNL - DS TML/Z - CG - 2001/10/09 31

FIGURE 10

EXAMPLE OF ELECTRIC VAPORISER

- on LNG inlet:• check valve, to prevent a possible retro-

diffusion of vaporised components to themain LNG pipe,

• restriction orifice,• needle valve, to control the flow of LNG

(however, it is better to control gas phaseflow in order not to create a disturbance ofthe state of the LNG sampled),

• filter

- on heating fluid (water or steam or electricity):• temperature regulator, or thermostatic

control, to keep constant vaporisingconditions according to the LNG flow,

• thermometer and thermostats in case offailure of heating devices,

• control of the electrical supply of thetransformer or of the submerged resistance;

- auxiliary safety devices, such as:• protection of electrical supply and electrical

devices, which must be of a type designedfor hazardous conditions (explosion proof,pressurised box).

6.6.4. Operating parameters

Among the operating parameters, the following onescan be particularly recommended:

- sampling rate greater than 1.0 m3(n)/h ofregasified LNG,

- sample pressure at the sampling point greaterthan 2 bars,

- vaporiser outlet temperature not below about+20°C.

6.7. COMPRESSOR FORTRANSFERRING GAS SAMPLE

According to the pressure and piping conditions,compressors may be used to transfer the gassample:

- from the vaporiser directly to the analyser,

- or from the vaporiser to the gas sample holder,

- or from the vaporiser or the gas sample holder tothe gas sample bottle filling station.

They must be of the oil-free type and stand-by unitsare also necessary.

6.7.1. GAS SAMPLE HOLDER

A gas sample holder may be used to store regasifiedLNG during the sampling period of the LNG transferoperation; the characteristics of the gas containedafter completion and mixing is representative of thecharacteristics of the LNG loaded or unloaded.

The volume of gas sample holder must be enough tofill the gas sample bottles and to purge theconnecting lines. It is often between 0.5 and 1 m3

(figure 11a and b).


GIIGNL - DS TML/Z - CG - 2001/10/09 32

Gasholders can be of two types:

- water-seal type, the sealing water beingsaturated with gas by bubbling regasified LNGthrough it before filling the holder,

- or waterless type.

6.8. GAS SAMPLE CONDITIONINGTwo main devices can be found:

6.8.1. Gas sample bottles

They are filled:

- either directly, at the outlet of the vaporiser,periodically during the transfer operation (figure11a),

- or at the outlet of gas sample line coming fromthe gas sample holder, possibly through acharging compressor (figure 11b).

These bottles have capacity enough for the analysesthat follow, for instance 0.5 l.

They are generally made of stainless steel withvalves at both ends, as shown on figure 11c.

Quick connectors are preferable instead of screwconnectors due to frequent handlings of the bottles.

After purging of piping has been performed severalbottles may be filled simultaneously or successively,according to the installed manifold. It is extremelyimportant to take care that no air enters the bottleand that the bottle is sufficiently purged before takingthe gas sample to be analysed.

6.8.2. Direct piping to gas analyser

This device allows analyses to be carried out asfrequently as the analyser permits, and to get rid ofpossible air ingress during gas sample handling.

In this case, a prefarably stainless steel pipe with asmall diameter directly connects the outlet of thevaporiser to a manifold at the inlet of gas analysersinstalled in a laboratory of the loading/unloadingfacilities. A gas compressor may be required in orderto make up for the pressure drop in the gas line.

FIGURE 11

EXAMPLE OF METAL GAS SAMPLE CONTAINER

a) Example of water-seal type gas sample holder


GIIGNL - DS TML/Z - CG - 2001/10/09 33

b) Example of waterless-type gas sample holder

c) Example of metal gas sample container


GIIGNL - DS TML/Z - CG - 2001/10/09 34

FIGURE 12

a) EXAMPLES OF DISCONTINUOUS SAMPLING DEVICES (first example)


GIIGNL - DS TML/Z - CG - 2001/10/09 35

b) EXAMPLES OF DISCONTINUOUS SAMPLING DEVICES (second example)


GIIGNL - DS TML/Z - CG - 2001/10/09 36

c) EXAMPLES OF CONTINUOUS SAMPLING DEVICES

6.9. EXAMPLES OF GENERALARRANGEMENT OF SAMPLINGDEVICES

As examples, figures 12a and 12b show diagrams ofseveral possible arrangements of discontinuoussampling devices and figure 12c shows diagrams ofan arrangement of continuous sampling device.

The various elements are described in the previousparagraphs.

6.10. PERFORMANCES OF THEDEVICES

The following remarks can be made about the designof such sampling devices and the choice of theelements which constitute them:

- to prevent or to limit possible contamination oradsorption of the heavy components (C5+)stainless steel is the preferred material for allparts in contact with LNG/NG flowing betweenthe sampling point and the analyser, since it isless reactive than other materials. It is alsorecommended that lines, bottles, valves, etc...between the vaporising device and the analyser

are insulated. During the filling of the gasholderor during the transfer to the analyser/bottles, thegas flow should be regulated to limit outsidetemperature and gas velocity effects on theadsorption/desorption phenomenon;

- low temperature LNG vaporisers (atmosphericor water at ambient temperature for instance)should not be used, because they aresusceptible to creating fractionation, or theyshould only be used with a gas sample holder asshown in figure 11a or b;

- the continuous and discontinuous samplingdevices consisting, for the first one, of filling agas holder during the sampling period in order toobtain a gas mixture equivalent to the averagevaporised transferred LNG (figure 12c), and thesecond one, of direct repetitive analyses ofvaporised LNG performed automatically duringthe sampling period (figures 12a and 12b),provide the best representativity of the LNGtransferred;

- it is important to ensure that the sampling andvaporising parameters remain as constant aspossible during the whole sampling period,mainly LNG or gas flowrate, LNG pressure, andtemperature of vaporisation.


GIIGNL - DS TML/Z - CG - 2001/10/09 37

As the sampling devices have not gone trough thesame tests, it is difficult to compare their operatingperformances. Examples of calculation of operatingperformances of continuous and discontinuoussampling devices are given in European StandardEN12838.

Yet, considering the distribution of the calorific valuescalculated from the composition determined bychromatographic analysis of regasified LNGsamples, it is reasonable to consider that the grosscalorific value can be evaluated with an accuracy ofabout ± 0.25% to ± 0.30%, including the averageaccuracy of analysis itself (± 0.10%) with goodsampling devices complying with the previouscomments (see § 13.3).

6.11. SAMPLING PROCEDURE6.11.1. Sampling period

It is recommended that the LNG be sampled whenthe LNG transfer flowrate is sufficiently settled. It isnecessary to exclude the initial period, correspondingto the starting of transfer pumps and increase of LNGflowrate, until the main pipe is completely full of LNGand biphasic or overheated LNG contained at thebeginning of the operation has been eliminated, oruntil the full flowrate is obtained.

It is also necessary to exclude the final period whenLNG flowrate decreases before stopping.

When significant changes in pressure or flowrateoccur in the transfer line, it is better to suspendsampling temporarily.

6.11.2. Sampling frequency

As far as filling of a gas holder is concerned,sampling is continuous during the sampling period, ata fixed flowrate; spot samples can be collected inaddition during this operation, in order to control LNGquality and to monitor the transfer operation, but thecorresponding analyses are not used for energycalculation.

When gas samples are taken in bottles during LNGtransfer it should be done on a regular basis,depending on the characteristics of transfer lines andequipment, the organisation of operation in the plant,the duration of gas sample analysis, etc.

Example: frequency often around 1 hour, whichmakes about 8 samples for a normal LNG transferduration of 12 hours, sampling starting about 2 hoursafter the beginning of transfer and ending about 2hours before the end of transfer.

When regasified LNG is sent directly to thelaboratory for analysis, gas sample analysisfrequency depends on the available analyser (seeparagraph 7.6). Example: 1 chromatographicanalysis every 15 to 20 minutes during samplingperiod, if a chromatograph is dedicated for such anoperation and if components higher than C6 are notseparated.

6.11.3. Purging

It is recommended that purging of sampling devices(probe, line, vaporiser, gas holder) and sampleconditioning equipment (line, bottles,… ) is carried outbefore any LNG or gas sample is taken into account:

- before starting sampling:• purging of sampling probe if necessary

(double-flow, … ),• circulation of LNG, vaporisation and

circulation of regasified LNG in vaporiser,pipe and either to atmosphere (small gasflowrate) or to a boil-off gas pipe of theplant, if there is no gasholder, or in gasholder with possible gas bubbling in thesealing water, and then evacuation toboil-off gas pipe;

- before filling as gas bottle:• connection of the bottle(s),• successive operations of filling and

emptying each bottle (3 times or more)before gas sample is collected,

• isolation and removal of the bottle(s).

If samples are taken periodically in gas bottles, it isbetter to keep the sampling system in servicebetween operations, so that the equipment iscontinuously purged and ready for a new samplingwith the same operating parameters.

6.11.4. Sampling parameters

It is important that the operating parameters of thesampling device (pressure, temperatures, flowrates)are kept as constant as possible throughout thesampling period, in order to obtain a smoothoperation which enables representative andrepeatable sampling.

6.11.5. Utilisation of gas sample bottles

Gas samples collected in bottles are:

- on the one hand, directly analysed in order todetermine the average composition of LNGtransferred,

- and, on the other hand, possibly given to theother party concerned with the transfer(purchaser or seller according to the type of gaspurchase contract,) or even kept for furtherinvestigations, in case of dispute for instance,during a period defined in the contract (severalweeks).

When the sampling device includes a line wherebythe regasified LNG is directly piped to the gaschromatograph, an additional system may bedesigned to collect spot samples (gas sample bottlefilling station) which are then only used for control,these samples being taken on a diversion pipe at theoutlet of the vaporiser with the sampling parametersbeing adjusted accordingly.


GIIGNL - DS TML/Z - CG - 2001/10/09 38

6.12. SPOT SAMPLING DEVICEAn appropriate quantity of LNG is injected in aproperly purged chamber previously cooled down byLNG circulation. The chamber is thus partially filledwith LNG, then isolated. The LNG sample is thenbrought to ambient temperature and vaporises. Thusthe regasified LNG filling the whole volume of thechamber which is designed to withstand thecorresponding pressure increase. Gas samples arethen withdrawn from the chamber via pressurereducing valves to fill gas sampling bottles.

7. GAS ANALYSISJust like all natural gas, regasified LNG, is analysedby gas chromatography in order to determine itscomposition. The reason for this is to be able tocalculate the energy content based on itscomponents. A direct energy content measurementby e.g. calorimeter would be less precise and wouldalso not give the useful compositional information tocalculate other properties (like density or Wobbeindex).

Also gas chromatography can be used to determinesome of its impurities like sulphur components. Adifferent set up is often required than for its maincomponents.

Other trace impurities, like mercury, require adifferent analytical technique. With most impuritiessampling is critical and special precautions andsampling materials are required.

Gas chromatography is a technique, which isnormally applied on a comparison basis; the qualityof the analyses is generally only as good as thequality of the calibration gas.

Although it is not the aim of this handbook todescribe in full detail what gas chromatography is, ageneral description of the important aspects involvedare mentioned.

This technique is classical to determine thecomposition of gases and can be directly applied incase of regasified LNG.

Many methods exist in the open literature, forexample in international standards series, like ISO(e.g. ISO 6974), national institutes like BS (BS 3156)or methods from institutes like ASTM (ASTM D 1945)or GPA (e.g. GPA 2261).

This chapter describes the general set up of gaschromatography systems, on line or in the laboratory,which can be used in LNG facilities, both from buyerto seller, to determine the quality of transferred LNG,their operation and data processing.

7.1. TYPE OF GASCHROMATOGRAPH

7.1.1. General arrangement

Among the various arrangements that can be found,the following ones are mentioned as examples:

- 1 chromatograph with 2 or 3 columns toseparate selectively the components, forinstance: 1 column for N2, C1 to C5 and 1 for C6+,or 1 column for N2, C1, C2 and CO2 and 1 for C3,C4 and C5;

- two or more chromatographs, each onespecialising in the analysis of a few components;for example: 1 for C1, 1 for C2 to C5 and CO2,and 1 for O2 and N2.

It should be noted that the significant heaviestcomponents in LNG are generally limited to C5 andthat the need for good resolution betweenhydrocarbons is obvious.

The primary purpose in resolving oxygen peaks is todetect contaminated samples and failures of thesampling device or leaks in the gas chromatographtubing.

It should be realised that some methods do notresolve oxygen (and argon) from nitrogen; for lowoxygen level (ppm level) a dedicated meter shouldbe used.

In addition to the determination of the previouslynamed components, the detection and quantitativeanalysis of impurities, such as sulphur components,like mercaptans COS or H2S, may also be required;they are carried out according to standardisedmethods (e.g. ISO 6326 under review and replacedsoon by ISO 19739) and may use a specificchromatographic detector.

Figure 13a and b (see page 39) shows twoarrangements of gas chromatography systems,respectively with one and several chromatographs.

7.1.2. Columns

The choice of the type and stationary phase in thecolumns depends on the general arrangementchosen for gas chromatographic analysis and on theconstituents to be analysed.

The following types of columns can be found:

- packed columns, consisting of tubes, generallymade of stainless steel, filled with a stationaryphase (packing material) which may be:• a support made of solid particles

impregnated (coated) by a stationary liquidphase,

• or a solid adsorbing material (not coated),such as molecular sieves;


GIIGNL - DS TML/Z - CG - 2001/10/09 39

FIGURE 13

TWO GAS CHROMATOGRAPH SYSTEMS

a) with one gas chomatograph

b) with several gas chromatographs


GIIGNL - DS TML/Z - CG - 2001/10/09 40

- capillary columns, consisting of open tubes ofvery small diameter, in which the stationaryliquid phase is directly coated on the wall. Thereare different types: wall coated open tubularcolumns (WCOT), support coated open tubularcolumns (SCOT) and porous layer open tubular(PLOT) columns; both in glass (fused silica) andin steel.

Packed columns typically have lengths between 0.5and 9 m and internal diameters between 2 and 6mm, whereas normal capillary columns generallyhave lengths between 5 and 100 m and internaldiameters between 0.1 and 0.5 mm. Packed columnsare well adapted for the qualitative and quantitativeaspect of analysis. Packed columns are well adaptedfor the analysis of natural gas and are applied inestablished methods (GPA 2261 and ISO 6974 part.3, 4 and 5).

Capillary columns (WCOT columns) generallyprovide the best separation of constituents, and canhandle only a very low quantity of gas sample.Splitting of the sample is often applied (ratio 1:10 to1:50).

The PLOT column has a relatively high capacity withgood separation characteristics. Molsieves (forseparation of e.g. He, H2, N2 and O2) are now usedin both capillary and packed column applications.The latest methods (ISO 6974 part 6) are now usingcombinations of WCOT, PLOT and capillarymolsieve columns to optimise separationcharacteristics in analysis of natural gas. GPA 2186now applies a combination of both capillary (WCOT)and packed columns.

The operating temperature may be controlled eitherat constant temperature or using temperatureprogramming. Temperature programming can reducethe duration of analysis, but isothermal conditionsare preferable when thermal conductivity detectorsare used because flow variations caused bytemperature variations may create baseline drift withthese detectors. In any case, the temperature of theoven containing the columns has to be wellcontrolled.

7.1.3. Detectors

Thermal conductivity detectors (TCD) are oftenapplied in gas chromatographs used in LNGfacilities, mainly because they are sensitive to all thecomponents of natural gas and have a fairly linearresponse. They are kept at a constant temperature.Detection levels nowadays can go as low as 0.001mol%.

Flame ionisation detectors (FID) can also be used.They generally have a greater sensitivity, and candetect much lower percentages than thermalconductivity detectors. This however, is not alwaysnecessary for the composition of LNG. FID's areuseful for the heavy ends (C6+ fraction, identificationand quantification of hydrocarbons up to C12), butcannot detect inert gases (N2, O2, He, Ar, etc.).

7.1.4. Carrier gas

Helium and hydrogen are normally used as carriergases. Their purity must be higher than 99.995%v.Helium is recommended for safety reasons.

The following examples of packing columnapplications can be given:

- according to ISO 6974 part 4:2000 (the formerISO 6568-1981), a standard describing a simplemethod for the analysis of natural gas (nitrogen,carbon dioxide and hydrocarbons up to pentaneand C6+), using 2 columns whereby:• column 1 = 9 m long, 4.75 mm internal

diameter, stainless steel or copper,• column 2 = 0.45 m long, 4.74 mm internal

diameter, stainless steel or copper,Stationary phase of both columns:• support Chromosorb PAW (porous polymer,

particles of 250 to 315 m),• liquid phase = silicone oil DC200,• degree of loading = 28 g per 100 g of

support,Operation of the application at a constant oventemperature of 110°C ± 2°C,The application uses helium as carrier gas and aTCD as detector.

- according to ISO 6974 part 3:2000, a standarddescribing a method for the analysis of naturalgas (hydrogen, helium, nitrogen, oxygen, carbondioxide and hydrocarbons up to octane), usingtwo columns whereby:• column 1 = 3 m long, 2 mm internal

diameter, stainless steel,packing = Porapak R with particle sizebetween 150 and 180 µm (for separation ofhydrocarbons),

• column 2 = 3 m long, 2 mm internaldiameter, stainless steel,packing = Molsieve 13X, particle sizebetween 150 and 200 µm (for separation ofHe, H2, N2 and O2),

Temperature control between 40°C and 200°C,with a linear programmer providing a rate oftemperature change of 15°C/min,The application uses both helium and argon ascarrier gas and two detectors (TCD and FID).(TCD = Thermal Conductivity Detector; FID =Flame Ionisation Detector).

7.1.5. Quality of the separation of components

The following parameters, which influence the qualityof the separation of the components, must be takencare of when a chromatographic system is installed:

- operating temperature: when temperatureincreases the retention time decreases, but theresolution decreases at the same time,

- carrier gas flow control by inlet pressure and thecarrier gas flow rate should be optimisedtowards the highest resolution of peaks,

- valve switching, a problem can createcontamination or cutting off of components in thechromatogram.


GIIGNL - DS TML/Z - CG - 2001/10/09 41

7.2. INTEGRATOR AND DATAPROCESSING

7.2.1. Integrator system

An integrator is connected to one or more gaschromatographs in order to determine the retentiontime, the area of each peak obtained by the detectorand to print the results, or send them to an auxiliarydata processing system.

Integrators can also, according to the sophisticationof their software programs, take into account thecalibration of chromatographs, detect and identify thepeaks, calculate the percentage of each componentin the gas mixture and even control the operation ofthe chromatographs.

The components of a gas are identified by theirretention time. Integration of a peak is sometimesdifficult when peaks are not fully separated. It isrecommended to use equipment where the actualintegration line can be verified.

Another function of the integrator may be to generatealarm.

7.2.2. Data processing

Additional processing of the chromatographic datamust be done before the analysis is reported. Bothintegration function and data processing arenowadays carried out in a computer (standard PC).Also additional calculations of physical properties arenormally done in this computer (heating value,density, etc., according to ISO 6976-1995).

In calculating the end result (expressed in mol% percomponent) the computer normally reports both raw(not normalised) data and normalised data. The raw(not normalised) data are the result of the calculationof the sample with the calibration table (resultingfrom the analysis of the calibration gas). The total ofthe raw results should normally be between 98 and102 mol%. If the total (of not normalised data) isoutside this window, it normally means thatsomething in the analysis is wrong. The calibrationmay not be valid or there is a problem in the analysis(injection, column switching, peaks missing etc.).Careful examination of the equipment is thanrequired. If the accuracy is found, a re-calibrationshould be carried out.

7.3. CALIBRATIONThe application of gas chromatography in natural gasis a technique that requires calibration. Calibration iscarried out with standard gases of a similarcomposition as the samples (often-single point).Validation of the GC equipment can be doneaccording to ISO 10723.

7.3.1. Calibration procedure

After the initial set up of the gas chromatograph andthe installation of the integrator/computer software,the gas chromatograph has to be calibrated. Thisoperation consists of successive analyses (4 - 6 for

instance) of a certified calibration gas (see paragraph7.3.2.) in order to determine:

- the retention time of each component in therespective columns: identifying the components,

- the response factors (arithmetic average offactors calculated after several analyses, 10 ormore) or the parameters corresponding toreference components (see paragraph 7.4.):quantifying the components gas sample.

These parameters are stored in theintegrator/computer system.

The quality of calibration gas is determining thequality of all the related measurements. A traceablegas, preferably of Primary Reference Material(PRM), needs to be used from a reliable source (seeISO/DIS 13275).

In addition to these determinations, a repeatabilitytest is recommended to evaluate the randomaccuracy of the system. Several analyses are carriedout successively (at least 20) in the same conditionson the same gas. The distribution of the results(percentages of each component or calculatedheating value) is then examined and the standarddeviation calculated.

If this parameter is too high, for instance more than11 kJ/m3(n) on calculated gross calorific valuearound 44.4 MJ/m3(n), it must be interpreted as ananomaly of the conditioning of the system, orsomething wrong with the integration parameters, ora failure of an element of the system; and the systemthen needs to be repaired or its operating conditionsneed to be modified.

7.3.2. Calibration gas/working standard

The gas mixture used as a calibration gas mustinclude all the components found in the regasifiedLNG to be analysed, within close percentages.

The preparation of such a gas mixture must beundertaken with great accuracy. It is recommendedthat gravimetric standard methods are used. Thepreparation of the calibration gas may be performedaccording to standards such as ISO/DIS 13275.

The preferred quality of the calibration gas is primaryreference material (PRM). This gas is expensive, andcareful use is advised in view of the relatively highcost (about USD 6,000 for a cylinder of 5 litre at 50bar).

It is useful to have a working standard, which isanalysed against the calibration standard. Thisworking standard needs to be stable and is used tocheck the performance of the analyser system on aregular basis (e.g. weekly, or prior to each cargo).

Working standards can be made by filling a bottlewith gas taken on the LNG facilities (e.g. 50 litrescylinder filled at 50 bar). Of course also lower gradereference gases can be purchased for this purpose.

If the analysis of the working standard indicates aneed for re-calibration, the PRM should be used to


GIIGNL - DS TML/Z - CG - 2001/10/09 42

recalibrate the system (after proper analyses of thecause of this). After each change on the hardware orsoftware, re-calibration should take place. Typicallyre-calibration should not be needed more than onceper 3 months or so.

7.4. QUANTITATIVE ANALYSISThe basic data used for quantitative analysis are theareas of the peaks corresponding to the variouscomponents. The most common method is describedbelow according to the ISO. 6974 part 2.

7.4.1. Response factors

It also supposes that the calibration gas contains allthese components in the same concentration level.

The percentage Xi of the component is calculated bythe following formula:

100.).(

.ii

iii

AKAKX

Σ=

where Ai is the area of the peak of the component i inthe gas mixture analysed and where Ki is theresponse factor of component i determined bycalibration as follows:

Si

Sii

AXK =

SiX = the known percentage of component i in thecalibration gas,

SiA = the arithmetic average of the surface of thepeak of component i in the calibration gas(at least 10 or more analysis).

This methodology can only be used if the responseof the detector is linearly changing withconcentration, or if the concentration of thecalibration gas is very close to that of the componentbeing analysed. After determining the concentrationof all components, the total is calculated (raw result,nor normalised). Ideally the total is close to 100%.Small deviations may occur from randominaccuracies (e.g. injection volume by variation dueto atmospheric pressure). Normally the sum is notexactly 100%. Normalisation (to make the totalexactly 100%) is a way to deal with this problem. Theassumption with normalisation is that the problem isthe same for all components. All components aredivided by the raw total and multiplied by 100, givena total (normalised) result of 100%. The window inwhich this allowed is 98 – 102 percent for the notnormalised total. Higher or lower levels are anindication of a problem.

It should be noted that there might also be otherproblems, which are related to individualcomponents. An integration problem can give a falsepeak area; integration of peaks should be the sameas for the calibration gas. This is why the calibrationgas should have about the same composition as thesample to be analysed.

7.5. ENVIRONMENT FOR A GASCHROMATOGRAPHIC SYSTEM

The practical requirements for the installation of sucha system are the same as those required for anyhigh-accuracy analysis device and mainly involve:

- installation in a closed and temperate (notnecessarily air-conditioned) building, shelteredfrom the sun, heating sources or draughts, oroutside for process chromatogrpahs typicallybetween –10°C and +50°C,

- appropriate and constant temperature ofcalibration gases and sample (injectedmass/constant volume),

- permanent and secured electrical supply,without interferences,

- shrouding and earth connections of the electricalconnections between the chromatograph and theintegrator.

7.6. ANALYSIS OF REGASIFIED LNGAND RETAINED SAMPLES

During a normal LNG transfer operation (forinstance, transfer duration of 12 hours, samplingperiod duration of 8 hours), the following analysisprocedures can be carried out:

- in the case of direct connection betweenvaporiser and chromatograph: the analyses canbe made successively during the whole samplingperiod, with a frequency equal to the duration ofeach analysis by the chromatographic system,Example: with 1 analysis every 20 minutes, 24analyses are available during the samplingperiod. For the new generation ofchromatographs, the duration of each analysis isreduced to 5 minutes;

- in case of periodic filling of sampling bottles: 1 ormore (often 2) analyses can be carried outsuccessively on each gas sampling bottle, with acomparison of results, and possible additionalanalysis or new filling of sampling bottles in caseof important thresholds, and then calculation ofthe arithmetic average of the percentages of thecomponents determined by the analysesconsidered or the determination of the averagecomposition of the sample.Example: 2 analyses on each bottle filled everyhour and calculation of the average, so 8average analyses during the sampling period;

- in the case of filling of a gas holder (ISO 8943):at the end of the sampling period, 3 bottles arefilled, 1 for each of both parties (Seller, Buyer)and 1 kept for further investigations (e.g. a thirdparty, in case of a dispute); 1 or more (generally2) analyses can then be carried out on the samesample and retained, if there is no significantthreshold.


GIIGNL - DS TML/Z - CG - 2001/10/09 43

Then, the average composition is calculated(arithmetic average of the percentage of the variouscomponents) with all of the results taken into account(direct result of analysis or mean of several analyseson the same sample). This value can be used incalculations of any additional parameters (LNGdensity, calorific value) for instance by the dataprocessing unit (see sections 8 and 9). It is commonpractice that the analysis resulting from the samplecollected by continuous sampling is only applied tothe quantity of heat calculation.

7.7. INACCURACY OF GASANALYSIS

The inaccuracy is related to the level of a componentin the sample. Provided the system is chosen,installed and operated with care and in compliancewith the method and recommendations, calibrationwith a high quality calibration gas (PRM), the totalinaccurancy in the computed gross calorific valuecan be as low as ± 0.1% (relative) for the maincomponents (level 10-100%).

For lower concentration levels, the inaccuracy (interms of repeatability) is 1% (relative) for a level of1-10% and 10% (relative) for a level of 0.1-1%. On alevel between 0.01 and 0.1 percent, the repeatabilityis around 30% relative.

7.8. IMPURITIESThe impurities in regasified LNG which sometimesare specified in LNG contracts are carbon dioxide,sulphur components (hydrogen sulphide, carbonylsulphide and mercaptans) and mercury. The sulphurand mercury components are normally traceimpurities at respectively 0-1 mg/m3 or, in case ofmercury as low as 5 ng/m3. Sampling cannot bedone in normal cylinders for these trace impuritiessince they are chemically reactive and will beabsorbed by the wall of the sample cylinder.

For sulphur impurities, special materials to minimiseabsorption are commercially available (silicosteel).

The determination of trace impurities requires aspecial approach. This can hardly beunderestimated; set-up, operation and maintenanceare an area for specialists. All aspects are critical:sampling, calibration and analysis. Validation andverification of results is strongly advised before usingthe results of the analyses for commercial purposes.

7.8.1. Carbon dioxide

The carbon dioxide content is normally determinedby GC analysis; the specification limit is often around0.01 mol%. The GC is capable of analysing down to0.01% or even lower.

7.8.2. Sulphur

Sulphur can be specified as total sulphur and/or asspecific sulphur containing components. Hydrogen

sulphide (H2S), carbonyl sulphide (COS) andmercaptans (RSH, where R is an alkyl group; e.g.methylmercaptan, CH3SH or ethylmercaptan,C2H5SH) are mentioned. The level of these impuritiesis normally on a 0-25 mg/m3(n) level.

Sampling for trace sulphur components is not soeasy; special precautions are needed in order toavoid absorption of sulphur components to the wallof the sampling system devices. Sampling in bottlesis preferably made according to the standard methoddescribed in the ISO 10715. The incrementalsampling is very delicate for sulphur components.The interior face of cylinders must be made out of amaterial which doesn't react with sulphurcomponents.

The sample cannot be conserved for more than 8days. The material "silicosteel" is suitable for thisapplication but is very expensive.

7.8.2.1 Total sulphur

Total sulphur can be determined by combustiontechniques, like the ISO 4260 (Wickboldcombustion), where all sulphur is converted into SO2

which is trapped and quantified. Instrumentaltechniques like microcoulometry (ASTM D 3146),pyrolyses/chemoluminescence, or hydrogenolysis/rateometric colorimetry (ASTM D 4045) aresometimes applied. ISO 4260, which is oftenspecified in contracts, is cost effective but not thesafest to apply. Many accidents have alreadyoccured due to the rather violent combustion of gasin an oxygen/hydrogen flame.

Therefore the use of the instrumental techniques isrecommended.

7.8.2.2 Sulphur components

In order to determine the sulphur componentsseparately in a gas, these components must beseparated first. A gas chromatograph can be usedfor this in combination with a sulphur specificdetector. The commonly used TCD (not sensitiveenough for traces levels) and FID (no signal at all)are not suitable. Several detectors are available, themost common are the flame photometer detector, achemoluminescence detector (ASTM D 5623) andthe microcoulometer detector. This equipment isexpensive and requires specialist skills. On linemeasurement is possible, in principle.

In principle the total sulphur can also be calculatedfrom the components present, under the assumptionthat all sulphur components are eluting from the GC(which is normally the case, of course).

7.8.3. Mercury

Mercury can be determined by ISO 6978, which iscurrently being revised (ISO/DIS 6978 – part 1-3,1998). For low levels of mercury the proceduredescribed is in part 3 (formerly section B of the 1992version). Sampling of mercury by amalgamation ongold/platinum is described in the range 10-100,000ng/m3 (sampling at atmospheric conditions).


GIIGNL - DS TML/Z - CG - 2001/10/09 44

Commercial equipment is available on the market butdoesn't fit strictly with the recommendations of ISO6978 which has been developed mainly for highpressure gas measurements. In the case of LNGmeasurements, commercial equipment are efficient ifprecuations are taken.

Impurities in general, and in case of mercurysampling in particular, are very critical because of theabsorption properties of mercury. Sampling linesshould be as short as possible (see also ISO 10715for materials, preferably less than a metre in length),preferably heated (80°C) especially if the gaspressure is high (above some 20 barg) to preventpossible condensation of hydrocarbons and shouldbe flushed for a very long time (preferably evencontinuously at a low rate, e.g. 2 l/min). Sampling formercury (basically the same applies to low sulphurimpurities) in a cylinder is difficult in case of gaseoussamples and not reliable for mercury at low levelswhich will be absorbed by the cylinder wall in a veryshort period of time and any readings will beerroneous.

Mercury is normally in the form of its metal or lightorganic mercury components (e.g. methyl mercury).These forms are trapped effectively by the gold trap.

8. DENSITY8.1. GENERALThere are two ways of determining density:

- the first consists of measuring its average valuedirectly in the LNG carrier's tank by means ofdensimeters,

- the second enables the density to be calculatedon the basis of an average composition of LNG.

In-situ measurements with the help of a densimetertakes into account the LNG state of equilibrium andcomposition, which means that one no longerdepends on product sampling and analysis. It wouldtherefore be the best method for measuring the LNGdensity. Unfortunately, technological progress hasnot reached the stage where it is possible for areliable apparatus to be available on board a LNGcarrier under normal operating conditions. This iswhy the second method, which enables the densityto be calculated from the LNG average composition,is the one that has been selected here.

8.2. DENSITY CALCULATIONMETHODS

A variety of calculation methods exists [6], such as:

- state equations in their integral form;

- method of extended corresponding states,

- hard sphere model method,

- WATSON method,

- ELF-AQUITAINE method,

- graphic method of RC MILLER,

- HIZA method

- revised KLOSEK-McKINLEY method (K1, K2tables in degrees Kelvin: K)

- ISO 6578, also using the revised KLOSEK-McKINLEY method (K1, K2 tables in degreesCelcius: C°)

In this handbook, the method selected is the revisedKLOSEK-McKINLEY method, described in N.B.S.Technical note 1030 December 1980 [10]. It is easyto apply and only requires the LNG temperature andcomposition to be taken into account. The limits ofthe method also encompasses the composition ofmost of the LNG produced. Its accuracy is ± 0.1%,when either the nitrogen or butane content does notexceed 5%. For these density calculations anelectronic spreadsheet or a computer programme isoften used.

Note: comparison between the revised KLOSEK-McKINLEY method using tables in degrees Kelvin(NBS) and tables in degrees Celcius (ISO6578:1991) indicates to a relative difference of about10-4.

8.3. REVISED KLOSEK-Mc KINLEYMETHOD

8.3.1. Limits of the method

The method can be used within the following limitson composition and temperature:

CH4 > 60% moliC4 + nC4 < 4%iC5 + nC5 < 2%N2 < 4%T < 115 K


GIIGNL - DS TML/Z - CG - 2001/10/09 45

TABLE 13

COMPONENT MOLAR VOLUMES

Molar volume, l/molComponent

118 K 116 K 114 K 112 K 110 K 108 K 106 K

CH4 0.038817 0.038536 0.038262 0.037995 0.037735 0.037481 0.037234

C2H6 0.048356 0.048184 0.048014 0.047845 0.047678 0.047512 0.047348

C3H8 0.062939 0.062756 0.062574 0.062392 0.062212 0.062033 0.061855

iC4H10 0.078844 0.078640 0.078438 0.078236 0.078035 0.077836 0.077637

nC4H10 0.077344 0.077150 0.076957 0.076765 0.076574 0.076384 0.076194

iC5H12 0.092251 0.092032 0.091814 0.091596 0.091379 0.091163 0.090948

nC5H12 0.092095 0.091884 0.091673 0.091462 0.091252 0.091042 0.090833

N2 0.050885 0.049179 0.047602 0.046231 0.045031 0.043963 0.043002

Source: N.B.S. - Technical note 1030 December 1980.

TABLE 14

VOLUME CORRECTION FACTOR - K1 x 10-3

Volume reduction, l/molMolecularweight

of mixtureg/mol 105 K 110 K 115 K 120 K 125 K 130 K 135 K

16 -0.007 -0.008 -0.009 -0.010 -0.013 -0.015 -0.017

17 0.165 0.190 0.220 0.250 0.295 0.345 0.400

18 0.340 0.375 0.440 0.500 0.590 0.700 0.825

19 0.475 0.535 0.610 0.695 0.795 0.920 1.060

20 0.635 0.725 0.810 0.920 1.035 1.200 1.390

21 0.735 0.835 0.945 1.055 1.210 1.370 1.590

22 0.840 0.950 1.065 1.205 1.385 1.555 1.800

23 0.920 1.055 1.180 1.330 1.525 1.715 1.950

24 1.045 1.155 1.280 1.450 1.640 1.860 2.105

25 1.120 1.245 1.380 1.550 1.750 1.990 2.272



GIIGNL - DS TML/Z - CG - 2001/10/09 46

TABLE 15

VOLUME CORRECTION FACTOR - K2 x 10-3

Volume reduction, l/molMolecularweight

of mixture 105 K 110 K 115 K 120 K 125 K 130 K 135 K

16 -0.010 -0.015 -0.024 -0.032 -0.043 -0.058 -0.075

17 0.240 0.320 0.410 0.600 0.710 0.950 1.300

18 0.420 0.590 0.720 0.910 1.130 1.460 2.000

19 0.610 0.770 0.950 1.230 1.480 1.920 2.400

20 0.750 0.920 1.150 1.430 1.730 2.200 2.600

21 0.910 1.070 1.220 1.630 1.980 2.420 3.000

22 1.050 1.220 1.300 1.850 2.230 2.680 3.400

23 1.190 1.370 1.450 2.080 2.480 3.000 3.770

24 1.330 1.520 1.650 2.300 2.750 3.320 3.990

25 1.450 1.710 2.000 2.450 2.900 3.520 4.230


8.3.2. Formula

This method is based on an empirical evaluation ofthe molar volume of the mixture in the thermodynamicstate of the LNG considered. The density of LNG iscalculated as follows:

mix

mixLNG

VMD =

where:

LNGD = density of LNGmixM = molecular weight of the mixturemixM = ii MX .Σ

iM = molecular weight of purecomponent i

iX = molar fraction of component i

mixV = molar volume of the mixture expressed inl/mol

= [ ] 42 .)0425.0/)(( 121 CHN XXKKKXiVi −+−Σ

Xi = molar fraction of component i

Vi = volume of the component i at thetemperature of LNG

21, KK = correction factors

8.3.3. Charts available for calculation

Table 13 (see page 45) gives the molar volume inl/mol for CH4 to C5 and N2, and for temperatures from106 K to 118 K.

Tables 14 and 15 (see page 45, 46) give the volumecorrection factors, K1 and K2 in l/mol, for variousmolecular weights and temperatures from 105 K to135 K.

8.3.4. Example of LNG density calculation

A worked out example of LNG density calculationaccording to this revised Klosek-McKinley (KMK)method using molecular weight of individualcomponents from ISO 6976 is given in Appendix 1(tables A1-1, A1-2, A1-3, A1-4).

8.3.5. Rounding off

The calculations can be made to six decimal placesand the final result can be rounded off to threedecimals.

9. GROSS CALORIFIC VALUE

9.1. GENERALThe gross calorific value (GCV) of gas corresponds tothe quantity of heat produced by completecombustion in air of a unit of volume or mass of thegas, at a constant absolute pressure of 1.01325 barand at a temperature Th, the water formed during thecombustion being condensed at temperature Th; inthe case of volumetric gross calorific value, the unit ofvolume of gas is considered at the conditions oftemperature Tv and pressure Pv.


GIIGNL - DS TML/Z - CG - 2001/10/09 47

One of the following reference conditions aregenerally taken into account:

- for the determination of GCV:hT = 0°C or 15°C or 15.56°C (60°F) or 25°C

- for the volume conditionvP = 1.01325 bar absolute or, sometimes,

1.01560 bar absoluteand

vT = 0°C or 15°C or 15.56°C (60°F) or 25°C

As an example, the reference conditions consideredin this handbook will be:

hT = 25°CvP = 1.01325 bar absolute andvT = 0°C,

referred to as "normal conditions", for which thevolume is indicated in m3(n).

In addition, the unit of energy will be kJ, which can bethen converted to BTU according to the conversionfactor table of Enclosure 1.

9.2. METHOD OF DETERMINATIONOF THE GROSS CALORIFICVALUE

The gross calorific value can be determined:

- by measurement with calorimeters,

- by computation on the basis of the composition ofthe gas and the reference data.

9.2.1. Determination with the help ofcalorimeters

Several types of calorimeters can be used, amongthem:

- manual calorimeters• deflagration calorimeters• with auxiliary fluid circulation exchanger

- automatic calorimeters• water flow calorimeters• air circulation calorimeters

- empirical calorimeters• with indirect measurement of the heat

produced• infra-red absorption calorimeters• sonic velocity calorimeters• optical interferometer type calorimeters• Rauter gas analysers

A detailed study of these different types of devicescan be found in references [7] and [8].

These devices enable the volumetric gross calorificvalue to be measured but, as LNG Custody Transferrequires the mass gross calorific value of LNG, thedensity of LNG must also be determined.

Since the composition of LNG is necessary for thecalculation of the density of LNG (according to

paragraph 8), the gross calorific value is alwayscalculated as well. This explains why calorimeters arenot used for the purpose of LNG Custody Transfer.

9.2.2. Determination of GCV by calculation

9.2.2.1 Examples of formula

a) GCV (volumetric) expressed in coherent units,e.g. in kJ/m3(n) or in kJ/m3(st) can be calculated,with one of the following formulae depending onwhether the ideal gas or the real gas calorificvalues are considered:

i) Ideal gas GCV (volumetric)

ii

ii

MVXmolGCVXvolGCV

.)(.)(

ΣΣ=

or

v

v

TRpmolGCViXivolGCV.

).(.)( Σ=

where:iX = molar fraction of component i

)(molGCVi = molar gross calorific value ofcomponent i, expressed inkJ/mol

iMV = molar volume of componenti, expressed in m3(n)/mol

vv Tp ; = gas volume meteringconditions

R = molar gas constant= 8.314510 J/mol/K

the physical constants GCVi(mol) beingspecified in standards,

ii) Real gas GCV (volumetric)

ZvolGCVXvolGCV ii )(.)( Σ=


)(volGCVi = molar gross calorific value ofperfect component i,expressed in kJ/m3(n)

Z = compressibility factor of thegas mixture

the values of GCVi(vol) and the method ofcalculation of Z being specified in coherentstandards.

b) GCV(mass) expressed in kJ/kg can becalculated by one of the following formulae:

ii

ii

MXmassGCVMiXmassGCV

.)(..)(

ΣΣ=


GIIGNL - DS TML/Z - CG - 2001/10/09 48

where:iX = molar fraction of

component i

)(massGCVi = mass gross calorific valueof component i, expressedin kJ/kg

iM = molecular mass ofcomponent i, expressed ing/mol

the physical constants GCVi(mass) and Mi beingspecified in coherent standards,

Note: In Japan the above formula is preferred.

or:

ii

ii

MXmolGCVXmassGCV

.)(.)(

ΣΣ=


)(molGCVi = molar gross calorific value ofperfect component i,expressed in kJ/mol

iM = molecular mass ofcomponent i, expressed ing/mol

the physical constants GCVi(mol) and Mi beingspecified in coherent standards.

9.2.2.2 Examples of charts of basic physicalconstants

Among others, one of the following standards may beused to provide tables of physical constants andmethods of calculation of factors necessary todetermine the gross calorific value:

ISO 6976ASTM 3588GPA 2145GPA 2172IP 251

These standards, or other publications which areused, refer to the results of works enjoyinginternational recognition, and most of them, at leastpartly, to the works published by American PetroleumInstitute (A.P.I.), Research Project 44.

9.2.2.3 Example of calculation

A worked out example is given in Appendix 2 (tablesA2-1, A2-2).

The worked out example is based on the use ofreference tables and methods of calculation specifiedin ISO 6976 standard.

The calculations carried out, result in the mass grosscalorific value used for the determination of theenergy of LNG transferred (table A2-1, see Appendix

2 page 64), and the volumetric gross calorific valueused for the determination of the energy of the gasdisplaced (table A2-2, see Appendix 2 page 65).

The formulae chosen here are as follows:

• For LNG:

ii

ii

MXmolGCVXmassGCV

.)(.)(

ΣΣ=

• For gas displaced

ii

ii

MVYmolGCVYvolGCV

.)(.)(

ΣΣ=

Note: In Japan the above formula is preferred, alsofor gas displaced.

with Xi, GCVi(mol), Mi, MVi as defined in paragraph9.2.2.1. and Yi = molar fraction of component i in gasdisplaced.

It should be noted that the molar composition of thedisplaced gas is different from the LNG composition.It is determined either by gas analysis, or calculated.Practical calculation is possible with a simple formulabased on empirical Ki values for each component. Kifor each component is defined as an empiricallydetermined ratio between the molar fraction in thegaseous state and the molar fraction in the liquid stateof that particular component.

iii xXKY =For practical displaced gas calculations, these Kivalues are usually limited to the most significant onesi.e. of the components with the lowest atmosphericboiling points: nitrogen, methane and sometimesethane. Considering the relatively small energycontent of gas displaced vs. LNG (un)loaded (typicallyless than 1% of total), it is generally viewed that thissimple calculation approach results in a sufficientlyaccurate overall calculation of energy transferred.

The calculations in the example shows that theethane vapour to the GCV of gas displaced is verymarginal. Therefore it can be argued that for simplicityit may even be dropped in the calculation of this GCV.Nitrogen is then the only component of which themolar fraction should either be measured in the gasreturned flow, or be calculated as shown in theexample in Appendix 2, table A2-2, the followingtypical Ki values for nitrogen, methane and ethane atnear atmospheric pressure and temperatures around-160°C are:

2NK = 23 (range: 20 – 26)4CHK = 1

62HCK = 0.005

The Ki values for any other component are assumedto be zero.

In this way, starting from the molar composition in theliquid state determined by LNG sampling and gasanalysis, the molar composition in the gaseous statecan be calculated. In the event that the sum of of thethus obtained fractions, does not equal 100%, a linearcorrection of the value of each component must becarried out.


GIIGNL - DS TML/Z - CG - 2001/10/09 49

COMPOSITION OF THE LNG LOADED/UNLOADED

Component Contractuallimits %mol

Mean composition%mol Out of limits

CH4

C2H6

C3H8

i-C4H10

n-C4H10

i-C5H12

n-C5H12

N2

For the LNG unloaded, and in the case of an F.O.B. agreement, two additional columns should be added; they couldbe entitled:- composition at loading- loading/unloading variation

10. ANALYSIS REPORTA standard analysis report could be as follows:

10.1. IDENTIFICATION- Name of the ship.- Beginning of loading/unloading.- End of loading/unloading.- Reference no.

10.2. BASIC DATA- Volume loaded/unloaded.- Mean temperature of the LNG.- Continuous/discontinuous sampling (cf.

paragraph 6).- Number of (discontinuous) samples.

10.3. RESULTSMolecular weight (g/mol) :Pseudo-molar volume (l/mol) :Corrected molar volume (l/mol) :Density (kg/m3) :Gross calorific value (kJ/kg):Gross calorific value (BTU/kg) :Gross calorific value (kJ/m3(n)) :Wobbe Index (kJ/m3(n)) :Expansion ratio (m3(n)/m3LNG) :

The mass gross calorific value is expressed inBTU/kg in order to calculate the energy transferred.

The volumetric gross calorific value and the WobbeIndex may be expressed in other units, here kJ/m3(n)for instance, in order to verify the qualityspecifications regarding these two properties.

11. ENERGY OF GASDISPLACED DURINGLOADING OR UNLOADINGOPERATION

To complete the calculation of Gross Calorific Valueof the LNG transferred, the figure derived from theliquid volume calculation may be adjusted for thefollowing reasons:

- The gas sent back from the ship during theloading operation or the gas transferred to theLNG carrier during unloading operationexpressed in energy terms is determined asdescribed in paragraph 11.1. This may bereferred to as 'buy-back' gas.

- and the gas possibly consumed by the LNGcarrier as fuel during the operation, determinedas described in paragraph 11.2.

11.1. ENERGY OF GAS DISPLACEDFROM THE TANKS OF THE LNGCARRIER

This quantity is determined by the following formula,whether it is received or sent back by the ship.

gasLNGdisplacedgas GCVxPxT

VE01325.115.273

15.273+

=

where:

gasE = quantity of energy in gaseous formdisplaced during loading or unloading,expressed in MMBTU.


GIIGNL - DS TML/Z - CG - 2001/10/09 50

LNGV = volume of the LNG loaded or unloaded,expressed in m3.

p = absolute pressure in the tanks,expressed in bar.

T = mean value of the temperatures of theprobes not immersed in LNG,expressed in Celsius degrees.

gasGCV = GCV of the gas in gaseous statecontained in the ship's tanks,expressed in MMBTU/m3(n) inaccordance with ISO 6976.

As already indicated in paragraph 2.3.3., theparameters p, T and GCVgas can be either measured,estimated, or taken as constants determined byexperience. Any inaccuracy that could be made in themeasurement would only concern the energy of gasdisplaced which represents about 0.3% of the quantityof energy transferred; this inaccuracy would thereforebe negligible. In Japan it is common practice toassume the return gas to be 100% methane in thecalculation of the energy of gas displaced.

For example, typical parameters for Algeriandisplaced gas are:

T = -140°CGCVgas = 0.0341 MMBTU/m3(n)p = 1.150 bar

and in this case, the formula is simplified andbecomes with energy expressed in MMBTU:

Egas displaced = 0.073Vgas diplaced

11.2. ENERGY OF GAS CONSUMEDAS FUEL BY THE LNG CARRIER

The LNG carrier, subject to agreement of Buyer andSeller, may use gas as fuel in its engine room duringthe loading operation (FOB cargo) or unloadingoperation (CIF or DES cargo). The energy consumeddepends on the characteristics of the plant, the levelof total consumption and the amount of gas in thetotal fuel consumption. For instance, for a 125,000 m3

LNG carrier, it may represent up to 0.02% of the totalenergy of LNG transferred.

This amount of energy can be estimated:- either by the measurement of the total volume of

gas Vg consumed (gas flowmeter on board theLNG carrier) and the evaluation of the GCV ofgas as described in paragraph 9.2.

gasggasfuel GCVVE .=

- or by a formula agreed by the Seller and theBuyer and based on the average gasconsumption experienced for a given ship at theoccasion of her first unloading and resulting infuel-oil economy. This formula may take intoaccount the duration of the operation, or acontractual fixed amount for any operation maybe acceptable due to the small percentage of the

total transferred energy it represents and hencethe negligible of its inaccuracy on the overallinaccuracy of the energy transfer measurement.

12. ENERGY TRANSFERMEASUREMENT

All elements enabling the determination of the energytransferred are known and the formula mentioned insection 2.1. can be applied within the framework oftable 16 (see page 51).

13. OVERALL INACCURACY OFTHE ENERGY TRANSFERMEASUREMENT

13.1. VOLUMEIn section 3.4., it was shown that the measuringinaccuracy with regard to the volume is ± 0.21%.

13.2. DENSITYThe inaccuracy of the calculated density depends on:- the computation methods used, the uncertainty of

which is estimated by NBS in its LNGMeasurement study [8] at ± 0.10%,

- the gas analysis, where the inaccuracy in thedetermination of its composition is estimatedaccording to the same source at ± 0.09%,

- the LNG temperature measurement, theinaccuracy of which is ± 0.5°C, the result beingan uncertainty concerning density of about ±0.15%.

The quadratic combination of the inaccuracies ininteraction (gas analysis and temperature) yields aninaccuracy of ± 0.17%.

The total inaccuracy as to density can therefore beestimated at ± 0.27%.

13.3. GROSS CALORIFIC VALUEPossible sources of inaccuracy in determining thegross calorific value are as follows:- The inaccuracy of the LNG sampling and

vaporisation, is estimated by NBS [8] at ± 0.3%,- in the chromatographic analysis, the use of the

calibration gas and in more precise terms, theweighting process of the various gases enteringinto the composition of this calibration gas, is thesource of an inaccuracy estimated at ± 0.03%according to NBS,

- the gross calorific value of the components usedin the mixture is also a source of inaccuracy, andis estimated at ± 0.04% according to NBS,


GIIGNL - DS TML/Z - CG - 2001/10/09 51

TABLE 16

E = VLNG x DLNG x GCVLNG - Egas displaced(5) (1) (2) (3) (4)

ENERGY TRANSFER MEASUREMENT

QUANTITY UNITS

Liquid volume before loading/unloading m3

Liquid volume after loading/unloading m3

Liquid volume transferred (1) m3

Density (2) kg/m3

Weight of the liquid transferred kg

Gross calorific value (3) MMBTU/kg

Energy of LNG MMBTU

Quantity of gas displaced (optional, see paragraph 11.1)(4)

MMBTUin kg equivalent LNGin m3 liquid equivalent LNG

Quantity of gas consumed as fuel on board ship during(un)loading operation (optional, see paragraph 11.2)

MMBTUin kg equivalent LNGin m3 liquid equivalent LNG

Quantity transferred (5)MMBTUin kg equivalent LNGin m3 liquid equivalent LNG

The quantities expressed in kg equivalent LNG or in m3 equivalent LNG are used for customs purposes only.(Please refer to flowchart page 5


GIIGNL - DS TML/Z - CG - 2001/10/09 52

The quadratic combination of the two aforementionedinaccuracies results in an inaccuracy of ± 0.05%.

The overall inaccuracy regarding the gross calorificvalue is ± 0.35%, according to the study referred tohere above.

13.4. GAS DISPLACEDThe gas displaced represents about 0.3% of the totalquantity transferred and the total inaccuracy on theparameters used in the calculation is ± 1%. Theinaccuracy resulting from the calculation of gasdisplaced in the energy transferred is ± 0.003% andcan therefore be considered as negligible.

13.5. TOTAL INACCURACY IN THEDETERMINATION OF THEENERGY TRANSFERRED

The following table sums up the different inaccuraciesof the measured and calculated parameters:

Element calculated Total inaccuracy (±%)

Volume ±0.21

Density ±0.27

Gross calorific value ±0.35

The total inaccuracy obtained from the quadraticcombination may therefore be estimated at ± 0.49%

14. LNG SALES CONTRACTCUSTODY TRANSFERCHECKLIST

The following is a tabulation of custody transfer issuesto consider for inclusion in an agreement for purchaseand sale of LNG.

• Definitions- Energy units: British Thermal Unit, BTU (or

equivalent, e.g. kJ)- Gross (Net) Calorific Value, volumetric and

mass, reference temperature for combustion- Standard (or Normal) Conditions (reference

temperature and pressure)- Volume units: Standard or Normal Cubic

Metre or Standard Cubic Foot- Wobbe Index

• Units of Measure- Temperature (i.e. degrees Celsius)- Pressure (i.e. Pa or bar, or psi)- Density (i.e. kg/m³)- Volume (i.e. Standard or Normal m³)- Mass (i.e. kg)- Calorific Value (i.e. BTU/kg, BTU/m3(n))

- Mole fraction- Base or reference temperature, pressure

(Normal, Standard conditions)- Significant figures

• LNG Quality Specifications- Calorific Value

Upper and lower limitsGross/Net (higher/lower)Wobbe Index

- Density at temperature- Composition (limits)

methaneethanepropanei-butanen-butanei-pentanen-pentanehexane plushydrogen sulphidecarbonyl sulphidemercaptan sulphurtotal sulphuroxygencarbon dioxidenitrogen

- Contaminantsparticulateswatermercuryoil

• Quantity Determination- Ship provisions- Shore provisions- Calculation method (ISO, revised Klosek-

McKinley, HIZA, etc.))

• Boil-off, Vapour Displacement, optional : Gas toEngine Room- Consideration in quantity determination- Impact of delay of transfer

• Measurement- Ship measurement devices- Shore measurement devices- Measurement device inaccuracy,

repeatability (certificates)- Calibration of measurement devices,

analyzer, tank gaugingStandard, methodFrequency of calibration

- Independent surveyor- Standards and guides (GIIGNL Custody

Transfer Handbook, ISO, etc.)- Condition of ship, shore at gauging prior to

and at completion of LNG transfer (e.g.ship's trim and list)

- Level measurement device specification,identify primary and secondary level gauge

Verification of consistency incomparisonTolerance of difference

- Pressure measurementMeasurement device specificationEach ship tank

- Witnesses to measurement (opening andclosing custody transfer surveys)


GIIGNL - DS TML/Z - CG - 2001/10/09 53

- Method of recordingNumber of gauge readings, timeintervalManual, electronic

- LNG SamplingLocationTiming, frequencyFlow-proportional(?)Basis for rejection (inaccuracy)Method of sample vaporizationMethod of analysis (gaschromatograph)Location of analyzer (i.e. laboratory)Reserve samples – quantity,retention period

• Buyer/Seller obligations and rights- Ship tank gauging – level, temperature,

pressure, list, trim, correction (tables) forsecondary level gauge

- Barometric pressure gauging- LNG sample collection- Boil-off sample collection- Sample analysis- Certification - tank gauge tables, instrument

calibration- Witness calibration, measurement- Independent surveyor- Costs- Notification- Records preservation

• Actions Upon Deviation- Failure of instruments- Inaccuracy in readings, calculations- Off-specification


GIIGNL - DS TML/Z - CG - 2001/10/09 54

ENCLOSURE 1

CONVERSION FACTOR TABLE FOR ENERGY UNITS

MM BritishThermal Units

106 BTU(MMBTU)

Gigajoule106 joules

(GJ)

Kilowatt-hour

(kWh)

Gigacalorie106 calories

(Gcal)

MMBTU 1 1.05506 293.071 0.251996

GJ 0.947817 1 277.778 0.238846

kWh 0.00341214 0.0036 1 0.000859845

Gcal 3.96832 4.1868 1163.00 1

(for information only, please refer to ISO 1000)


GIIGNL - DS TML/Z - CG - 2001/10/09 55

ENCLOSURE 2

METHODS FOR LNG AND NATURAL GAS FOR CUSTODY TRANSFERISO methods, being the highest level of international methods, are to be recommended. Equivalent alternatives, asindicated, from other institutes, like GPA, IP or ASTM can also be used, and may be agreed between all partiesinvolved. It is the basic intention to use the latest issues of a method. In order to avoid too frequent updates, latestissues are not represented in the following list.

METHOD TITLE (alternative method)

General methodsISO 7504 Gas analysis – VocabularyISO 13686 Natural gas – Quality designationISO 14111 Natural gas - Guidelines to traceability in analysesISO 14532 Natural gas – TerminologyEN 1160 Installations and equipment for Liquefied Natural Gas – General

characteristics of Liquefied Natural GasEN 437 Test Gases – test Pressures – Appliance Categories

Sampling LNG/natural gasISO 8943 Refrigerated light hydrocarbons fluids – Sampling of liquefied natural gas –

Continuous methodISO 10715 Natural gas – Sampling guidelinesEN 12838 Installations and equipment for liquefied natrual gas – Suitability testing of

LNG sampling systems

Calibration Gas ChromatographicEquipmentISO 6141 Gas analysis – Requirements for certificates for calibration gases and gas

mixturesISO 6142 Gas analysis – Preparation of calibration gas mixtures - Gravimetric

methodISO 6143 Gas analysis – Determination of composition of calibration gas mixtures –

Comparison methodsISO/DIS 13275 Natural gas – Preparation of calibration gas mixtures – Gravimetric

methods

AnalysisComposition/heating valueISO 6568 Natural gas – Simple analysis by gas chromatographyISO 6974-1 Natural gas – Determination of composition with defined uncertainty by gas

chromatography – Part 1: Guidelines for tailored analysisISO 6974-2 Natural gas – Determination of composition with defined uncertainty by gas

chromatography – Part 2: Measuring system characteristics and statisticsfor processing of data

ISO 6974-3 Natural gas – Determination of composition with defined uncertainty by gaschromatography – Part 3: Determination of Hydrogen, Helium, Oxygen,Nitrogen, Carbondioxide and Hydrocarbons up to C8 using two packedcolumns

ISO 6974-4 Natural gas – Determination of composition with defined uncertainty by gaschromatography – Part 4: Determination of Nitrogen, Carbondioxide and C1

to C5 and C6+ Hydrocarbons for a laboratory and on-line measuring systemusing two columns

ISO 6974-5 Natural gas – Determination of composition with defined uncertainty by gaschromatography – Part 5: Determination of Nitrogen, Carbondioxide and C1to C5 and C6+ Hydrocarbons for a laboratory and on-line process applicationusing three columns

ISO 6974-6 Natural gas – Determination of composition with defined uncertainty by gaschromatography – Part 6: Determination of Hydrogen, Oxygen, Nitrogen,Carbondioxide, and Hydrocarbons up to C8 using three capillary columns

ISO 6975 Natural gas – Extended analysis – Gas chromatographic method (GPA2286)

ISO 10723 Natural gas – Performance evaluation for on-line analytical systems


GIIGNL - DS TML/Z - CG - 2001/10/09 56


ImpuritiesISO 4260 Petroleum products and hydrocarbons – Determination of sulfur content –

Wickbold combustion methodISO 6326-1 Natural gas – Determination of Sulphur compounds – Part 1: General

introductionISO 6326-2 Gas analysis – Determination of Sulphur compounds in natural gas – Part

2: Gas chromatographic method using an electrochemical detector for thedetermination of odoriferous sulphur compounds

ISO 6326-3 Natural gas – Determination of Sulphur compounds – Part 3: Determinationof hydrogen sulphide, mercaptan sulphur and carbonyl sulphide sulphur bypotentiometry

ISO 6326-4 Natural gas – Determination of Sulphur compounds – Part 4: Gaschromatographic method using a flame photometric detector for thedetermination of hydrogen sulphide, carbonyl sulphide and sulphur-containing odorants (available in English only)

ISO 6326-5 Natural gas – Determination of Sulphur compounds – Part 5: Lingenercombustion method

ISO 6327 Gas analysis – Determination of the water dewpoint of natural gas –Cooled surface condensation hygrometers

ISO 6570-1 Natural gas – Determination of potential hydrocarbon liquid content – Part1: Principles and general requirements

ISO 6570-2 Natural gas – Determination of potential hydrocarbon liquid content – Part2: Weighting method

ISO 6570-3 Natural gas – Determination of potential hydrocarbon liquid content – Part3: Volumetric method

ISO/CD 6570 Natural gas – Determination of potential hydrocarbon liquid content –Gravimetric methods

ISO/CD 15972-1 Natural gas – Measurement of Prperties – Single Components andcondensation properties – Water content and water dewpoint determination

ISO 6978 Natural gas – Determination of mercuryISO 10101-1 Natural gas – Determination of water by the Karl Fischer method – Part 1:

IntroductionISO 10101-2 Natural gas – Determination of water by the Karl Fischer method – Part 2:

Titration procedureISO 10101-3 Natural gas - Determination of water by the Karl Fischer method – Part 3:

Coulometric procedureISO 11541 Natural gas – Determination of water content at high pressureISO 13734 Natural gas – Organic sulphur compounds used as odorants –

Requirements and test methods

Calculations/conversions of propertiesISO 1000 SI units and recommendations for use of their multiples and of certain other

unitsISO 6976 Natural gas – Calculation of calorific values, density, relative density and

Wobbe index from composition (GPA 2145).ISO 12213-1 Natural gas – Calculation of compression factor – Part 1: Introduction and

guidelinesISO 12213-2 Natural gas – Calculation of compression factor – Part 2: Calculation using

molar-composition analysisISO 12213-3 Natural gas – Calculation of compression factor – Part 3: Calculation using

physical propertiesISO 13443 Natural gas – Standard reference conditions

Quantity related methodsISO 8309 Refrigerated light hydrocarbon fluids – Measurement of liquid levels in

tanks containing liquefied gases – Electrical capacitance gaugesISO 8310 Refrigerated light hydrocarbon fluids – Measurement of temperature in

tanks containing liquefied gases – Resistance thermometers andthermocouples

ISO 8311 Refrigerated light hydrocarbon fluids - Calibration of membrane tanks andindependent prismatic tanks in ships – Physical measurement

ISO 9091-1 Refrigerated light hydrocarbon fluids - Calibration of spherical tanks inships – Part 1: Stereophotogrammetry

ISO 9091-2 Refrigerated light hydrocarbon fluids - Calibration of spherical tanks inships – Part 2: Triangulation measurement

ISO 10574 Refrigerated light hydrocarbon fluids – Measurement of liquid levels intanks containing liquefied gases – Float type level gauges


GIIGNL - DS TML/Z - CG - 2001/10/09 57


ISO 13398 Refrigerated light hydrocarbon fluids – Liquefied natural gas – Procedurefor custody transfer on board ship

ISO 13689 Refrigerated light hydrocarbon fluids – Measurement og liquid levels intanks containing liquefied gases – Microwave type level gauge


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ENCLOSURE 3

OTHER RELEVANT STANDARDS & REFERENCES

1. GPA 2261-72

Gas chromatography: "Analysis for Natural Gas anad Similar Gaseous Mixtures by Gas Chromatography"

2. IP 251/76

GCV of the gas

3. GPA 2145 – 86

GCV of the gas

4. INCOTERMS 2000

RELATED GENERAL WEBSITES FOR FURTHER INFORMATION

http://www.aga.org: American Gas Associationhttp://www.eurogas.org: European Natural Gas Companieshttp://www.gas.or.jp: Japan Gas Association (JGA)http://www.giignl.org: information on G.I.I.G.N.L.http://www.gastechnology.com: information on USA LNG topicshttp://www.igu.org: information on IGU, the International Gas Unionhttp://www.jnoc.go.jp: Japan National Oil Corporationhttp://www.iso.ch: information on ISO standardshttp://www.sigtto.org: information on LNG shipping practices


GIIGNL - DS TML/Z - CG - 2001/10/09 59

LIST OF FIGURES

Page

Figure 1-2: Correction according to theposition of the LNG carrier

10

Figure 3: Electrical capacitance typelevel gauge

13

Figure 4: Float type level gauge 14

Figure 5: Microwave type level gauge 15

Figure 6 Diagram of temperaturemeasuring devices on boarda LNG carrier

24

Figure 7: Elements of LNG samplingchains

27

Figure 8: Examples of samplingprobes

28

Figure 9: Examples of water or steamvaporisers

30

Figure 10: Example of electric vaporiser 31

Figure 11: Example of metal gassample container

32-33

Figure 12a: Examples of discontinuoussampling devices (firstexample)

34

Figure 12b: Examples of discontinuoussampling devices (secondexample)

35

Figure 12c: Examples of continuoussampling devices (thirdexample)

36

Figure 13: Examples of gaschromatographic systems

39

LIST OF TABLES

Page

Table 1: Example of gauge tables withzero list and trim (tank n° 5)

7

Table 2: Example of bottom fine gaugetable

8

Table 3: Example of top fine gaugetable

9

Table 4: Example of volume correctionsaccording to tank servicetemperatures for self-supporting tanks

11

Table 5: Volume determination beforeloading LNG

18

Table 6: Volume determination afterloading LNG

19

Table 7: Example of gauge correctionsfor low temperatures

20

Table 8: Float gauges - Example offloat buoyancy/dip corrections

21

Table 9: Example of list correction table 21Table 10: Example of trim correction

table22

Table 11: Inaccuracy on volumemeasurement (example)

23

Table 12: Example of tank temperaturerecordings (98% filled)

24

Table 13: Component molar volumes 45Table 14 Volume correction factor K1 x

10345

Table 15: Volume correction factor K2 x103

46

Table 16: Energy transfer measurement 51Table A1-1: Calculation of molecular

weight in g/mol61

Table A1-2: Calculation of the molarvolume in l/mol by temperatureinterpolation

61

Table A1-3: Calculation of the correctionfactor K1 in l/mol –temperature interpolation

62

Table A1-4: Calculation of the correctionfactor K2 in l/mol –temperature interpolation

62

Table A2-1: Gross calorific value (kJ/kg) 64Table A2-2: Gross calorific value (kJ/m3(n))

for Egas displaced calculation65


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REFERENCES

[1] Le volume du gaz naturel liquéfié – AssociationTechnique de l'Industrie du Gaz en France - F.Dewerdt et B. Corgier - Janvier 1987.

[2] Gauge table of the LNG carrier Methania issuedby Meteorological Service – Brussels.

[3] Gauge table of the LNG carrier Mourad Didoucheissued by the Meteorological Service – Paris.

[4] Calibration of containers and gauges – Journal ofthe Institute of Petroleum - Volume 58 n°561 –1972.

[5] Gas quality – Proceedings of the congress of "GasQuality Specification and measurement of physicaland chemical proporties of natural gas" –Groningen The Netherlands 22-25 April 1986 –Edited by GJ van Kossum.

[6] La masse volumique du gaz natural liquéfié –Association Technique de l'Industrie du Gaz enFrance - F. Dewerdt – Mars 1980.

[7] Le pouvoir calorifique du gaz naturel liquéfié, parM. F. Dewerdt – Association Technique del'Industrie du Gaz en France - Mars 1983.

[8] LNG measurement – NBSIR 85-3028 - Firstedition 1985.

[9] Standard for metric practice - American Society forTesting Material E380-79.

[10] Four mathematical models for the prediction ofLNG densities – NBS Technical Note 1030 –December 1980.


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APPENDIX 1: EXAMPLE OF LNG DENSITY CALCULATION (cf. section 8.3.4.) FOR LNG WITH COMPOSITIONAS INDICATED IN THE COLUMN MOLAR FRACTION, AT A TEMPERATURE OF 112.38 K

TABLE A1-1

1) CALCULATION OF MOLECULAR WEIGHT IN g/mol (in accordance with ISO 6976:1995)

Molar composition Molecular weight

Component Molar fractiong/mol

Componentmolecular weight

g/mol

Molecularweight fract.

g/mol

Methane CH4 0.89235 16.043 14.315971

Ethane C2H6 0.08267 30.070 2.485887

Propane C3H8 0.01313 44.097 0.578994

Isobutane Ic4H10 0.00167 58.123 0.097065

n-Butane nC4H10 0.00277 58.123 0.161001

Isopentane iC5H12 0.00011 72.150 0.007937

n-Pentane nC5H12 0.00000 72.150 0.000000

n-Hexane nC6H14 0.00000 86.177 0.000000

Nitrogen N2 0.00730 28.0135 0.204499

1.00000 17.851353

TABLE A1-2

2) CALCULATION OF THE MOLAR VOLUME IN l/mol BY TEMPERATURE INTERPOLATION

Molar volume fromtable n° 13Component Mole

fractionat 114 K at 112 K

Differentialmolar

volume for2 K

Differentialmolar volume

for 0.38 K

Molarvolume at112.38 K

Pseudomolar

volume

(3) (4) (5)=(3)-(4) (6)=(0.38K/2K)x(5) (7)=(4)+(6) (10)=(7)x(2)(1) (2)

l/mol l/mol l/mol l/mol l/mol l/mol

CH4 0.892350 0.038262 0.037995 0.000267 0.000051 0.038046 0.033950

C2H6 0.082670 0.048014 0.047845 0.000169 0.000032 0.047877 0.003958

C3H8 0.013130 0.062574 0.062392 0.000182 0.000035 0.062427 0.000820

iC4H10 0.001670 0.078438 0.078236 0.000202 0.000038 0.078274 0.000131

nC4H10 0.002770 0.076957 0.076765 0.000192 0.000036 0.076801 0.000213

iC5H12 0.000110 0.091814 0.091596 0.000218 0.000041 0.091637 0.000010

N2 0.007300 0.047602 0.046231 0.001371 0.000260 0.046491 0.000339

1.000000 0.039421


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3) CALCULATION OF VOLUME REDUCTION FACTORS K1 AND K2

3.1) Calculation of K1

The K1 values are given for different temperatures and molecular weights in table 14.

Two interpolations must be carried out:1) on the temperature,2) on the molecular weight.

TABLE A1-3

CALCULATION OF THE CORRECTION FACTOR K1 IN l/mol - temperature interpolation

Molecularweight

K1 at115 K

K1 at110 K

Differentialvalue for K1 for

5 K

Differential valuefor 2.38 K

K1 at112.38 K

(11) (12) (13)=(11)-(12) (14)=(2.38K/5K)x(13) (15)=(12)+(14)g/mol

l/mol l/mol l/mol l/mol l/mol

MW1(17 g/mol) 0.000220 0.000190 0.000030 0.000014 0.000204

MW2(18 g/mol) 0.000440 0.000375 0.000065 0.000031 0.000406

- Molecular weight interpolation factor: MWI = 148647.00.170.18

851353.170.18 =−

−

- Volume correction factor K1 at T = 112.38 K and with M.W. = 17.851353 g/mol

( ) ( ) )),(),((,, 1121211 TMWKTMWKxMWITMWKTMWK −−= = 0.000406 – 0.148647 x (0.000406 – 0.000204) = 0.000376

K1 (MW,T) = 0.000376 l/mol

3.2) Calculation of K2

The K2 values are given for different temperatures and molecular weights in table 15.

TABLE A1-4

CALCULATION OF THE CORRECTION FACTOR K2 IN l/mol - temperature interpolation

Molecularweight

K2 at115 K

K2 at110 K

Differentialvalue for K2 for

5 K

Differential valuefor 2.38 K

K2 at112.38 K

(17) (18) (19)=(18)-(17) (20)=2.38K/5K)x(19) (21)=(18)+(20)g/mol

l/mol l/mol l/mol l/mol l/mol

MW1(17 g/mol 0.000410 0.000320 0.000090 0.000043 0.000363

MW2(18 g/mol) 0.000720 0.000590 0.000130 0.000062 0.000652


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- Molecular weight interpolation factor: same as for as K1 = 0.148647.

- Volume correction factor K2 at T = 112.38 K and with MW = 17.851353 g/mol

( ) ( ) )),(),((,, 1222222 TMWKTMWKxMWITMWKTMWK −−= (24)

= 0.000652 – 0.148647 x (0.000652 – 0.000363) = 0.000609

K2 (MW,T) = 0.000609 l/mol

4) CORRECTED MOLAR VOLUME AND DENSITY

Application of the formula mentioned in paragraph 8.3.2. yields the following result;ΣIXiVi = 0.039421 g/molK1 = 0.000376 l/molK2 = 0.000609 l/molXN2 = 0.0073XCH4 = 0.89235

Vmix = ΣIXiVi – [K1 + (K2 – K1) x (XN2/0.0425)] x XCH4

= 0.039050 l/mol

Mmix = 17.851353 g/mol

DLNG = Mmix/Vmix= 457.140922 kg/m3

which, when rounded off to the nearest thousandth, gives: 457.141 kg/m3.


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APPENDIX 2: EXAMPLE OF GCV CALCULATION (cf. section 9.2.2.3)

TABLE A2-1

GROSS CALORIFIC VALUE (kJ/kg)

Molarfraction

Molecularweight

component

Molecularweightfraction

GCVcomponent

GCVfractionkJ/mol

(2) (3)=(1)x(2) (4) (5)=(4)-(1)Component

(1)g/mol g/mol kJ/mol kJ/mol

Methane CH4 0.89235 16.043 14.315971 890.63 794.753681

Ethane C2H6 0.08267 30.070 2.485887 1,560.69 129.022242

Propane C3H8 0.01313 44.097 0.578994 2,219.17 29.137702

Isobutane iC4H10 0.00167 58.123 0.097065 2,868.20 4.789894

n-Butane nC4H10 0.00277 58.123 0.161001 2,877.40 7.970398

Isopentane iC5H12 0.00011 72.150 0.007937 3,528.83 0.388171

n-Pentane nC5H12 0.00000 72.150 0.000000 3,535.77 0.000000

Nitrogen N2 0.00730 28.0135 0.204499 0.00 0.000000

S 1.00000 17.8513539 966.062088

- The constants used in columns (2) and (4) are taken from ISO 6976:1995.

- The gross calorific value is calculated at 25°C

GCV = 966.062088/17.85135354.117023 MJ/kg54.117 MJ/kg

or = 54.117023 x 0.947817/1000.05129 MMBTU/kg


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GROSS CALORIFIC VALUE (kJ/m3(n)) FOR EGAS DISPLACED CALCULATION

In this example the displaced gas molar fractions are estimated by calculation as described in section 9.2.2.3.

− First stepN2 gas = 23 N2 liq = 23 x 0.0073 = 0.1679CH4 gas = 1 CH4 liq = 0.89235C2H6 gas = 0.005 C2H6 liq = 0.005 x 0.08267 = 0.000413

Other components in the gaseous state are assumed to equal zero.

− Second stepLinear correction so that the sum equals 1.Correction factor= 1/(0.1679 + 0.89235 + 0.000413)

= 0.942807.

This yields the final adjusted values:N2 gas = 0.1679 x 0.942807 = 0.158297CH4 gas = 0.89235 x 0.942807 = 0.841314C2H6 gas = 0.000413 x 0.942807 = 0.000389

TABLE A2-2

GROSS CALORIFIC VALUE (kJ/m3(n)) FOR EGAS DISPLACED CALCULATION

Molar fraction GCV component GCV fraction

(2) (3)=(1)x(2)Components(1)

MJ/m3(n) MJ/m3(n)

Methane CH4 0.84131 39.735 33.429612

Ethane C2H6 0.00039 69.630 0.027086

Propane C3H8 0.00000 99.010 0.000000

Isobutane iC4H10 0.00000 127.960 0.000000

n-Butane nC4H10 0.00000 128.370 0.000000

Isopentane iC5H12 0.00000 157.440 0.000000

n-Pentane nC5H12 0.00000 157.750 0.000000

Nitrogen N2 0.15830 0.000 0.000000

1.00000 33.456698

- The constants used in columns (2) and (4) are taken from ISO 6976.

- The gross calorific value is calculated at 25°C (gas volume at 0°C and pressure 1.01325 bar a).

GCV = 33.457 MJ/m3(n)orGCV = 33.456698 x 0.947871/1000

= 0.03171 MMBTU/m3(n)


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G.I.I.G.N.L. 2001

LNG Custody Transfer Handbook_2E_2001

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