New LNG Membrane Containment System Achieves Reduced Boil-Off
French Engineering company Gaztransport & Technigaz, more commonly known as GTT, has upgraded its well-established Mark III LNG membrane containment system to achieve a reduced boil-off rate from LNG carriers by one third.
The new system, called Mark III Flex, was recently qualified by class society DNV and reduces the boil-off rate from 0.15% to 0.10% of the cargo volume per day through increased insulation.
The common industry practice has been to build LNG carriers with boil-off rates of 0.15% of the cargo volume per day, reflecting the typical need for boil-off gas as fuel on board. The Mark III Flex system has been developed in response to the reduced need for fuel gas due to the improved efficiency and changed operational practice of LNG carriers.
For current standard LNG carrier designs with a cargo capacity in the range of 160,000-170,000 m3, the 0.05% reduction in the boil-off rate is equivalent to 85 m3 or 36 tonnes per day.
The improvements have been achieved by increasing the containment tanks’ insulation thickness from 270 mm to 400 mm. This change in thickness affects the design of system elements that penetrate the insulation, such as the lower pump tower support and gas dome. The modified proportions of the insulation panels have a potential effect on the loading of the secondary barrier. The new system also implements GTT’s modified liquid dome design.
GTT has carried out an extensive qualification program for the new system to ensure that the safety and reliability levels remain equivalent to or better than those of the conventional Mark III system.
Approval in Principle (AiP) was granted by DNV in June 2011. In mid-September of this year, the system was given DNV’s General Approval for Ship Application (GASA), which means it is qualified for shipboard use. So far, 27 new vessels with the new Mark III FLEX system have been ordered from Korean yards. Twenty of these will be built to DNV class.
GTT’s other LNG containment system, NO 96, is being developed to meet similar boil-off rate (BOR) requirements, and a similar qualification process by DNV is ongoing.
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