Currently accepted LNG custody transfer determination requires the accurate measurement of the volume of LNG in the ship’s storage tanks at the beginning and end of LNG transfer, taking account of ship’s list and trim, the chemical composition of the LNG and its temperature. From these basic measurements, the density and gross calorific value are calculated by standard formulae, leading by multiplication to the total energy transferred. Corrections can be made for the energy of boil-off gas that is generated in and displaced from the receiving tank(s) and returned to source and also for any gas used in a ship’s engine room to give a more accurate figure of the net energy transferred.
The GIIGNL LNG Custody Transfer Handbook provides practical guidance on the equipment and methodology for determining the thermal energy of liquefied natural gas (LNG) transported by ship for the specific purpose of title transfer.
For this updated (2017) version of the GIIGNL LNG Custody Transfer Handbook, GIIGNL has mandated its Technical Study Group to consult with the LNG industry on a worldwide basis. The international group of experts responsible for the update, led by Fluxys LNG, comprised a diversified panel of professionals from companies involved in custody transfer operations. The fifth (2017) edition of the GIIGNL LNG Custody Transfer Handbook was written by a panel of experts headed by Stijn Maelfeyt (Plant Operations Manager at Fluxys LNG) with the support of Enagas, Engie, Gas Natural Fenosa, Gate Terminal, National Grid, Shell Global Solutions, Statoil and Tokyo Gas.
The goal was to respond to the rapidly changing market conditions and to address the latest commercial practices. The new version of the Handbook includes measurement procedures and equipment in relation to new technical operations such as:
More than pointing at the differences and highlighting the points of attention when dealing with these new operations, this 2017 version provides answers and solutions for setting up slightly altered or new custody transfer procedures.
For small-scale operations for instance, specific attention must be paid to the difference in tank pressure (small scale vessels mainly consist of type C tanks and can maintain much higher pressure than atmospheric tanks onshore) which may require to use a revised method for density calculations.
In reloading operations, the temperature and pressure of the reloaded LNG are typically higher than at a liquefaction terminal. This has operational implications which may introduce systematic errors in the estimation of the energy transferred, thus requiring to use alternative calculation methods.
In each case, the reason for the proposed changes is of truly technical nature and GIIGNL considers it as its duty to inform the LNG industry about this and its impact.
The Handbook is intended to serve as a reference manual but not as a standard or a specification. It has been updated to include the most advanced measurement technology, operational practices and international standards available to or used by all parties involved in custody transfer operations.
The 2017 GIIGNL LNG Custody Transfer Handbook can be downloaded free of charge here on the GIIGNL website.
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Image Source: Courtesy of GIIGNL (International Group of LNG Importers)
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