General Formula For Calculating Energy Transferred (LNG)
The formula for calculating the LNG transferred depends on the contractual sales conditions. These can relate to three types of sale contract as defined by Incoterms 2000: an FOB sale, a CIF sale or a DES sale.
In the case of an FOB (Free On Board) sale, the determination of the energy transferred and invoiced for will be made in the loading port.
In the case of a CIF (Cost Insurance & Freight) or a DES (Delivered Ex Ship) sale, the energy transferred and invoiced for will be determined in the unloading port.
In FOB contracts, the buyer is responsible to provide and maintain the custody transfer measurement systems on board the vessel for volume, temperature and pressure determination and the seller is responsible to provide and maintain the custody transfer measurement systems at the loading terminal such as the sampling and gas analysis. For CIF and DES contracts the responsibility is reversed.
Both buyer and seller have the right to verify the accuracy of each system that is provided, maintained and operated by the other party. The determination of the transferred energy usually happens in the presence of one or more surveyors, the ship’s cargo officer and a representative of the LNG terminal operator. A representative of the buyer can also be present.
In all cases, the transferred energy can be calculated with the following formula:
E =(VLNG × DLNG × GVCLNG) - Egas displaced ± Egas to ER (if applicable)
Where:
E = the total net energy transferred from the loading facilities to the LNG carrier, or from the LNG carrier to the unloading facilities.
VLNG= the volume of LNG loaded or unloaded in m3.
DLNG = the density of LNG loaded or unloaded in kg/m3.
GCVLNG = the gross calorific value of the LNG loaded or unloaded in MMBTU/kg
E gas displaced = The net energy of the displaced gas, also in MMBTU, which is either: sent back onshore by the LNG carrier when loading (volume of gas in cargo tanks displaced by same volume of loaded LNG), Or, gas received by the LNG carrier in its cargo tanks when unloading in replacement of the volume of discharged LNG.
E(gas to ER) = If applicable, the energy of the gas consumed in the LNG carrier’s engine room during the time between opening and closing custody transfer surveys, i.e. used by the vessel at the port, which is:
+ For an LNG loading transfer or
- For an LNG unloading transfer
Read more about this topic: Custody Transfer
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