LNG Custody Transfer · • The “GIIGNL –LNG Custody Transfer Handbook” which is currently...
Transcript of LNG Custody Transfer · • The “GIIGNL –LNG Custody Transfer Handbook” which is currently...
NEL Slide 118-11-07
LNG Custody Transfer
LNG Metrology Training Session
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• Overview
• Volume Measurement
• Temperature and Pressure Measurements
• Sampling
• Density Calculation
• Gross Calorific Value Calculation
• Other Considerations
Contents
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• 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
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• 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
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Custody transfer takes place during loading and offloading of the tankers
LNG Supply Chain
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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
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Current Measurement System
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• 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.
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Volume Measurement
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• 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
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• 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
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• 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
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• 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
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Gauge Tables
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Gauge Tables
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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
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• 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
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• 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
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• List and trim corrections can be either positive or negative.
List and Trim
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• List and trim corrections can be either positive or negative.
List and Trim
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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
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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
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• 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
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• 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
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Volume Transferred
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Current Measurement System
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• 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
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Temperature Measurement
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• 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
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Current Measurement System
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• 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
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Sampling Overview
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• 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
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• 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
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• 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
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• 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
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Vaporiser with water/steam circulation
Vaporisation
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Vaporiser with water heated electrically
Vaporisation
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Vaporiser with direct electrical heating
Vaporisation
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• 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
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• 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)
Composition Measurement
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Composition Measurement
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Composition Measurement
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Composition Measurement
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Current Measurement System
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• 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
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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
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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
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• 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
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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
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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
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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
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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
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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