Measurement of LNG Custody Transfer

Estimated reading time: 4 minutes

 

Produced from natural gas and liquefied, liquefied natural gas (LNG) is transported in specially designed ships called LNG carriers. LNG is typically stored for future uses, in anticipation of potential demand peaks. There are many characteristics of LNG, one of which is its hazardous nature due to its flammable vapours. Understand how you can manage the risks of LNG ships and tackle LNG fires here. Another characteristic of LNG is that it is able to reduce the volume of natural gas by 600 times. This means that one ship carrying LNG is equivalent to 600 ships carrying natural gas at atmospheric pressure. LNG has made it possible to transport natural gas in bulk and to transfer natural gas between tanks and tankers; tankers and ships; and other transactions.

 

For LNG transfers, there are 3 main types of sale contracts – all of which will affect the calculation of the LNG transferred. Below are the 3 main types of sale contracts:

1. Free On Board – The calculation and measurement of the energy transferred and invoiced for will be made at the loading port.
2. Cost Insurance & Freight – The calculation and measurement of the energy transferred and invoiced for will be made at the unloading port.
3. Others – For the rest of the sale contracts, such as Delivered at Terminal or Delivered at Place, there are other rules that exist for these maritime transports.

 

Factors affecting measurement

Custody transfer refers to the transactions that involve transportation of physical substance from one operator to another. These physical substances can include raw and refined petroleum. Given that transactions in the LNG market are huge and typically involve up to $4 million of product per hour, it is not surprising to know that LNG custody transfer measurement is challenging.

Here are the 4 essential elements that has to be accurate for a precise measurement of the quantity of energy loaded or unloaded.

1. LNG volume
2. LNG density
3. LNG gross calorific value
4. Energy of the gas displaced during the transfer of LNG

 

1. LNG volume

How the volume is measured is based on the LNG carrier’s instruments, predominantly the use of level gauges and calibration tables. To accurately measure the volume, it is recommended that the LNG piping on the LNG carrier’s deck is in an identical inventory condition during both custody transfer surveys. Before any cooling down operations, the initial level gauging should be made. Immediately after the completion of un(loading) with liquid and drainage of vapour arms, the final level gauging should be made.

2. LNG density

The derivation of the density is from the calculation of the measured composition of the transferred LNG as well as the temperature of the LNG measured in the carrier’s tanks.

3. LNG gross calorific value

To calculate the gross calorific value, the composition of the LNG is used.

4. Energy of the gas displaced during the transfer of LNG

The calculation of the energy is based on the composition and volume of the gas displaced, along with the pressure and temperature of the gas inside the tanks of the LNG carrier before loading or after unloading.

 

In a nutshell, custody transfer of LNG is based on the measurements of the above-mentioned elements. It is important that the method used for custody transfer measurement is an established procedure.

 

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To learn more about the best practices of various LNG operations, the International Group of Liquefied Natural Gas Importers (GIIGNL) has published the latest edition of the LNG Custody Transfer Handbook. The handbook aims to set out the practical issues and requirements to guide and facilitate a skilled operator team to work out a suitable procedure for a specific LNG ship­-shore custody transfer application. It also includes a guide for key LNG operations such as partial (un)loading, ship-to-ship LNG transfer and reloading at LNG import terminals.

Click below to download your free report
“LNG Custody Transfer Handbook (5^th Edition: 2017) ”

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