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More flexibility for the cooling down of membrane tank

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STUDIES

More flexibility for the cooling down of membrane tank

In order to deal with specific situations that vessels may encounter before LNG loading, GTT has revised the cargo tank cooling down criteria.

This has been carried out to provide more flexibility for cooling-down at sea when the remaining heel in tanks is insufficient or has warmed up due to an increase in the proportion of heavy component, or, on the other hand, to allow a shorter time and reduce overall loading duration when the cooling-down is performed at the terminal.

Throughout a complete analysis covering all parts of the cargo tank system, membrane NO96 and Mark III Cargo Containement Systems (CCS) technologies have been assessed considering more constrained operational parameters. GTT membrane technologies have been exposed to different cooling-down profiles, shorter and faster than usual practice, and have shown a remarkable flexibility to withstand potential thermal shock. Membrane CCS NO96 and Mark III are even able to receive LNG without any cooling-down.

Due to the insulating properties of the CCS, and the thermal diffusivity throughout the depth of the CCS, the main part is not impacted by the cooling-down. Whatever the CCS, the thermal shock mainly impacts a thin layer under the primary barrier. While the primary barriers of NO96 and Mark III are insensible to transient state; all areas and junctions have been studied very deeply.

For NO96, the behaviour of the primary boxes and staple junctions of the cover, identified as the most loaded part, have been validated while, at the same time, maintaining a high safety factor and without any increased fatigue level.

For Mark III, the knots around the Pump Tower Base Support and its secondary plate with the bounded junction have also been deeply studied. Even with direct loading stress, the analysis demonstrates a sufficient safety factor and no fatigue damage for over 40 years of operation.

Bureau Veritas has already given an Approval in Principle (AIP), which validates the fact that NO96 and Mark III membrane containment systems are able to withstand fast cooling-down and direct LNG loading without prior cooling-down.

The remaining main limitations are the cooling down criteria for the cargo pumps. At this time, cargo pump makers have enhanced their criteria by raising temperatures before loading. As a result, GTT also proposes to add a temperature sensor fitted on the pumps to monitor the most relevant temperature for the end of cooling-down, and consequently save up to six-hours, if necessary.

Temperature gradients through Mark III depending on cooling-down parameters

Temperature gradients through Mark III depending on cooling-down parameters

 

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