LNG Custody Transfer · • The “GIIGNL –LNG Custody Transfer Handbook” which is currently...

NEL Slide 118-11-07

LNG Custody Transfer

LNG Metrology Training Session

NEL Slide 218-11-07

• Overview

• Volume Measurement

• Temperature and Pressure Measurements

• Sampling

• Density Calculation

• Gross Calorific Value Calculation

• Other Considerations

Contents

NEL Slide 318-11-07

• LNG tankers carry LNG cargoes of up to 270,000 m3

• That is equivalent to 162,000,000 m3 of natural gas

• Using a gas price of $3.50 per MMBTU this equates to $20m

• Huge amounts of money are at stake and therefore accurate measurement is essential

Overview

NEL Slide 418-11-07

• ISO standard (10976- 2012), developed with input from

• The “GIIGNL – LNG Custody Transfer Handbook” which is currently used as a guidance document (currently 5th Edition 2017)

• A new standard on “Dynamic Measurement of LNG” is currently developed under ISO TC28/WG20, CD21903 (Rev 4). The group aim to proceed to DIS stage later this year.

Standards

NEL Slide 518-11-07

Custody transfer takes place during loading and offloading of the tankers

LNG Supply Chain

NEL Slide 618-11-07

LNG quantity is traded in the form of energy transferred,

E = (VLNG * ρLNG * GCVLNG) – E gas displaced ± E gas to ER

Where:

E = Total energy transferred (MMBTU)

V = Volume transferred (m3)

ρ = Density of LNG (kg/m3)

GCV = Gross calorific value (MMBTU/kg)

E gas displaced = Net energy displaced during loading or unloading (MMBTU)

E gas to ER = Energy consumed by LNG tanker engine during loading or unloading (MMBTU)+ve for an LNG loading

-ve for LNG unloading

Energy Equation

NEL Slide 718-11-07

Current Measurement System

NEL Slide 818-11-07

• Volume measurement is currently carried out using level gauges and gauge tables.

• The level is taken before and after loading/offloading in the LNG tanks.

• Corrections are made to these level measurements based on trim, list and temperature.

• The volume before and after loading or unloading is then calculated using the ships gauge tables.

• The volume transferred is the difference between these two volumes.

.

Volume Measurement

NEL Slide 918-11-07

• Level Measurements are made with a float hanging from a tape or ribbon

• The tape/ribbon is rolled or unrolled from a drum which measures rotation.

• This allows the probe position and hence LNG level to be known

• Corrections need to be made for shrinkage of the ribbon and buoyancy of the float

• The uncertainty is estimated to be 4 - 8 mm

Float Level Gauges

NEL Slide 1018-11-07

• A radar transmitter/receiver is mounted externally above the tank

• The transmitter sends microwaves vertically down to the LNG surface through a waveguide.

• The microwaves are reflected at the LNG surface and returned to the receiver.

• The signal is then processed to determine the level in the tank.

• The uncertainty is estimated to be 5mm or lower.

Radar/Microwave Level Gauges


• Capacitive level sensors consist of two concentric tubes which run the height of the LNG tank.

• Concentric insulators are placed at regularly spaced intervals along the length of the tubes.

• The LNG fills the space between the tubes and affects the dielectric characteristics of the capacitors.

• Therefore by measuring the change in capacitance the height can be inferred.

• The uncertainty is estimated to be 5 mm.

Capacitance Level Gauges


• Gauge tables relate the level of LNG in a tank to the tank volume.

• They are produced for every tank using macro-metrology with tapes, laser or photogrammetric measurement systems.

• They are produced for every cm and interpolation is used between these points.

• They can be produced to mm resolution at the heights where custody transfer takes place. This removes the need for interpolation.

• Uncertainty of the gauge tables is estimated to be 0.05% to 0.1%.

Gauge Tables


Gauge Tables


Gauge Tables


Gauge tables require the following corrections:

• Correcting for the position of the vessel from port to starboard (List)

• Correcting for the position of the vessel from bow to stern (Trim)

• Correcting for the expansion or contraction in the tank caused by temperature changes.

• Correcting for gaseous phase temperature effects on the level gauge measurement.

• Corrections for LNG density effects on level gauge measurement.

Correction Tables


• The list is represented by an angle α in degrees to port.

• The trim is represented by metres or fractions of metres according to differences in bow and stern drafts.

• The vessel’s ballast will often be used to minimise list and trim.

List and Trim


• The list is represented by an angle α in degrees to port.

• The trim is represented by metres or fractions of metres according to differences in bow and stern drafts.

• The vessel’s ballast will often be used to minimise list and trim.

List and Trim


• List and trim corrections can be either positive or negative.

List and Trim


• List and trim corrections can be either positive or negative.

List and Trim


Corrections are made to allow for contraction/expansion of the LNG tanks caused by changes in temperature.

Vt = K * V-160

Where

Vt = Volume of tank at measured temperature.

K = Expansion coefficient.

V-160 = volume at -160°C

• Corrections are also made for the temperature of the vapour phase and the density (if a float gauge is used)

Temperature Corrections


Corrections are made to allow for contraction/expansion of the LNG tanks caused by changes in temperature.

Vt = K * V-160

Where

Vt = Volume of tank at measured temperature.

K = Expansion coefficient.

V-160 = volume at -160°C

• Corrections are also made for the temperature of the vapour phase and the density (if a float gauge is used)

Temperature Corrections


• Corrections are also made for the temperature of the vapour phase and the density of the LNG (if a float gauge is used)

Other Corrections


• Corrections are also made for the temperature of the vapour phase and the density of the LNG (if a float gauge is used)

Other Corrections


Volume Transferred




• LNG temperature is measured using 3 or 4 wire PRT’s

• There are usually 5 PRT’s set at different heights in each tank

• One PRT is always left in the liquid phase and one is always left in the vapour phase.

• The temperature of each phase is determined by the average temperature of all probes within the phase.

• The uncertainty is generally around 0.5 °C for liquid temperature and 1 °C for gas temperature measurements.

Temperature Measurement


Temperature Measurement


• Pressure measurement of the gaseous phase is needed to determine the volume of gas displaced during the custody transfer process.

• A pressure gauge is used for this and if necessary another transmitter is used to measure atmospheric pressure.

• The uncertainty of pressure measurement is generally around 1%

Pressure Measurement




• Sampling is required in LNG custody transfer operations so that composition can be measured and hence density and GCV can be calculated.

• Sampling is carried out using continuous or discontinuous sampling

• The following 3 operations are essential to obtain accurate samples:

1. Taking a representative LNG sample.

2. Complete and instant vaporisation.

3. Ensuring constant pressure and temperature of the gaseous sample.

Sampling Overview


Sampling Overview


• The sampling probe is located as close as possible to the custody transfer point.

• The sampling probe should ideally protrude into the LNG header.

• Pitot tubes are sometimes used in conjunction with sampling probes

Sampling Probe


• The sampling probe is located as close as possible to the custody transfer point.

• The sampling probe should ideally protrude into the LNG header.

• Pitot tubes are sometimes used in conjunction with sampling probes

Sampling Probe


• It is essential that no partial vaporisation takes place before the vaporiser.

• Therefore the vaporiser should be located as close as possible to the sample point and effective insulation should be used upstream of the vaporiser to keep the LNG sub-cooled.

Good Installation Bad Installation

Vaporising


• To ensure a representative sample is obtained vaporisation should be as complete as possible and fractionation should be avoided.

• Therefore the LNG should be heated to 50°C or more in the vaporiser.

• The heat exchange is usually achieved with water heated electrically or with steam

• Direct electrical heating is also used in some vaporisers.

Vaporisation


Vaporiser with water/steam circulation

Vaporisation


Vaporiser with water heated electrically

Vaporisation


Vaporiser with direct electrical heating

Vaporisation


• Once the LNG has been vaporised it is sent to a gas chromatograph for measurement of composition.

• In gas chromatography a sample is carried through a number of columns in a carrier gas known as the mobile phase.

• The columns are filled with a liquid or solid material known as the stationary phase

• The different components in the gas travel through the column at different rates and are therefore separated.

Composition Measurement


• A detector such as a thermal conductivity detector (TCD) is used to monitor the components at the outlet of the column.

• Therefore the order at which they emerge from the column and their retention time can be determined.

• This enables the determination of the composition of the different components within the LNG.

• The uncertainty can be as low as 0.1% for the main components (components > 10% concentration)











• In situ densitometers as used commonly in the oil industry have not yet proved reliable for LNG measurements.

• Therefore density of LNG is generally calculated based on it’s composition and temperature.

• Many equations have been developed for LNG density calculation but in the GIIGNL the revised Klosek-McKinley method is recommended.

Density Calculation


The equation is valid in the following conditions.

• This equation has been shown to have an uncertainty of 0.1% when Nitrogen or Butane content does not exceed 5%.

Revised Klosek-McKinley Method


Where

ρLNG = Density of LNG

Mmix = Molecular weight of mixture= ΣXiMi

Xi = Molar fraction of component iMi = Molecular weight of component i

Vmix = Molar volume of the mixture (l/mol)= ΣXiVi – [(k1+(k2-k1)*(XN2/0.0425)]*XCH4

Vi = Molar volume of component i at LNG temperaturek1,k2 = Correction factors

Revised Klosek-McKinley Method


• The gross calorific value (GCV) is defined as the heat energy produced by the complete combustion of a unit volume or mass of gas.

• It can be determined by calorimeter although this is not common for LNG custody transfer.

• It is generally calculated based on the composition of the gas and the GCV of the individual components.

GCV Calculation


Where

Xi = Molar fraction of component i

GCVi(mol) = Molar gross calorific value of component i (kJ/mol)

Mi = Molecular mass of component i (g/mol)

GCV Calculation


E gas displaced = Energy of gas displaced during loading/unloading (MJ)

VLNG = Volume of LNG loaded/unloaded (m3)

P = Absolute pressure in the tanks (Bar)

T = Mean temperature of the probes not immersed in LNG (°C)

GCV gas = GCV of gas in gaseous state (MJ/m3)

Energy of Gas Displaced Equation


Where

Efuel gas = Energy of gas consumed as fuel during loading/unloading (MJ)

Vg = Total volume of gas consumed as fuel (m3)

Note: this can be measured with a flowmeter

GCVgas = Gross calorific value of the gas (MJ/m3)

Energy of gas consumed as fuel


E = (VLNG * ρLNG * GCVLNG) – E gas displaced ± E gas to ER

From level gauge measurements, correction factors and gauge tables

From sampling, vaporisation, composition measurement and calculation

From GCV, volume of LNG measurements and T, P in tanks

From volume of fuel gas measured and GCV

Summary


The overall uncertainty is determined from the following sources

• The combined standard uncertainty can then be calculated;

U = (0.212+0.232+0.302)1/2 = 0.43%

• Therefore the combined expanded uncertainty is

0.86% at 95% confidence level (K=2)

Element Calculated Standard Uncertainty

Volume ±0.21%

Density ±0.23%

GCV ±0.30%

E gas displaced negligible

E fuel gas negligible

Overall Uncertainty

Date post:	27-Apr-2020
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LNG Custody Transfer · • The “GIIGNL –LNG Custody Transfer Handbook” which is currently...

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