More than a doubling of impact peak pressure can be expected in LNG carrier membrane tank systems if operating with no filling restriction compared to currently allowed filling rates. These are the findings of a large-scale test recently carried out by DNV.
The world’s use of natural gas is increasing. Although the development of new technology will lead to new solutions and new methods of transportation, the transportation of liquefied natural gas (LNG) by ship will still be the dominating alternative.
But while most LNG trades so far have taken place on fixed routes, with fixed volumes for each voyage and avoiding the toughest seas, things will change. Spot trades will increase. New vessels– almost as big as the largest crude oil tankers - will be built for transporting the LNG.
There will be more cross-trading and activities in the Atlantic Basin. Fatigue damage accumulation is twice as rapid in the North Atlantic than in the comparatively more benign environment where most of today’s routes are.
In addition, offshore LNG import terminals now under construction - at which safe offloading can take place far from populated areas and busy ports - may create more sloshing while offloading. This requires offloading carriers capable of operating without tank filling restrictions – or with filling restrictions determined for the environmental conditions at the actual offloading location.
The market is experiencing an upswing. Orders for new ships keep coming, and new offshore receiving and storage terminals may be established at different locations – e.g. in the US.
All this means that new ships have to be built for transporting gas from the main producer to the main consumer. In order to facilitate offshore discharge, the new tanks have to be prepared for partial fillings. Unrestricted partial fillings may double the impact peak pressure due to sloshing in the tanks.
The same DNV large-scale test also confirmed not only that the impact peak pressure increases dramatically under unrestricted filling operation but also that the duration of the impact pressure pulse is significantly extended.
Fatigue considerations and tank sloshing loads will become much more important design parameters. Sloshing loads inside partly filled LNG cargo storage tanks and cargo transfer in open seas are examples of challenges that have to be addressed.