GTT received an order from the South Korean shipyard Daewoo Shipbuilding and Marine Engineering (DSME) for the tank design of two new 173,400 m3 LNG carriers (LNGC). These units will be built on behalf of a European ship-owner.
The tanks will be fitted with the NO96 GW technology and the vessels’ delivery is expected by end of 2020.
NO96 membrane system is a cryogenic liner directly supported by the ship’s inner hull. This liner includes two identical metallic membranes and two independent insulation layers.
NO96 ORIGINAL DESIGN
PRIMARY & SECONDARY INVAR MEMBRANES
The primary and secondary membranes are made of Invar®, a 36% nickel-steel alloy, 0.7mm thick. The primary membrane contains the LNG cargo, while the secondary membrane, identical to the primary, ensures a 100% redundancy in case of leakage. Each of the 500mm wide Invar® strakes is continuously spread along the tank walls and is evenly supported by the primary and the secondary insulation layers.
PRIMARY & SECONDARY THERMAL INSULATION
The primary and secondary insulation layers consist in a load bearing system made of prefabricated plywood boxes filled with expanded perlite. The standard size of the boxes is 1m x 1.2m. The thickness of the primary layer is equal to 230mm; the thickness of the secondary layer is 300mm. The primary layer is secured by means of the primary couplers, themselves fixed to the secondary coupler assembly. The secondary layer is laid and evenly supported by the inner hull through load-bearing resin ropes, and fixed by means of the secondary couplers anchored to the inner hull.
THE NO96 SYSTEM COMPETITIVE ADVANTAGES
Dual Invar® membrane technology continuously improved since 1969 is approved by all the major classification societies. It benefits from more than 50 years of experience at sea.
The two independent insulation spaces are continuously flushed with nitrogen gas. The integrity of both membranes is permanently monitored by the detection of hydrocarbon in the nitrogen.
Minimal thermal stress
The Invar® material offers an extremely low thermal contraction coefficient, thus reducing the thermal stresses in the material.
The membrane strakes are fixed to the boxes by a tongue device enabling the sliding of the membrane on the top of the boxes (no shear stress) and the absorption of the transverse loads through the deflection of the raised edges.
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Smooth transfer of longitudinal loads
Along the transverse bulkhead corners, the two membranes are connected to the hull through an Invar® tube, so that all the dynamic and thermal loads are directly and uniformly transferred to the steel structure.
The boxes are internally strengthened to withstand high impact pressures, and to absorb the energy from liquid motion. Several levels of reinforcements have been designed to adapt the strength of the boxes to the expected sloshing loads inside the tanks.
Plywood and perlite are easily procured, low-cost raw materials, available worldwide.
This modular system calls for standard prefabricated components that can accommodate any capacity of tanks. In addition, they are designed for mass production techniques and easy handling.
LATEST NO96 EVOLUTION FOR LOWER BOIL-OFF SOLUTIONS
In order to meet the requirements for lower boil-off, as a first step, the perlite in the boxes has been replaced by glass-wool: NO96 GW. It has to be noted that this evolution is not a structural change. The bearing plywood structure is kept identical to NO96 system. The guaranteed boil-off rate with NO96 GW system is between 0.125% and 0.13% V/day.
For an even better thermal efficiency, NO96-L03 is proposed: within the usual NO96 thickness, three layers of boxes are assembled. The primary boxes are identical to NO96 system. The NO96 secondary boxes are replaced by intermediate plywood boxes assembled with the third layer of insulation made from R-PUF (foam panel). In the N096-L03 configuration, only standard rectangular boxes are replaced with the foam panel design (except highly reinforced areas at the top of the tank). As a consequence, 66% of the total surface of the tank is covered by NO96-L03 design. The plywood boxes are filled with glass-wool. The guaranteed boil-off rate with NO96-L03 system is 0.11% V/day
These evolutions have been approved by classification societies based on a complete justification dossier and commonly applied on the latest vessel delivered.
LATEST NO96 EVOLUTION FOR STRENGTH INCREASE
In order to meet requirements for systems sustaining higher sloshing loads, a set of different boxes corresponding to different levels of reinforcements are proposed. For classical LNGC applications, the standard, standard-reinforced and ultra-reinforced boxes are sufficient.
Depending on the specification of the project, higher levels of reinforcement can be needed. This is the basis of the development of the Mega-Reinforced design. The plywood structure is reinforced by increasing the thickness of the primary box double cover and secondary box internal bulkhead.
Boil-off and reinforcements can be optimized according to ship-owner specifications.This NO96 design has been successfully applied to the largest FSRU delivered.