During the history of LNG transport on the seas, there were many attempts, new projects and ideas for the manner of transport and the design of LNG carriers. But in the last fifteen years, two main types of LNG carriers have been defined; these are carriers with spherical tanks and membrane-type carriers.
LNG Carriers with spherical Storage Tanks (Kvaerner-Moss System)
The first LNG carrier with spherical tanks, the so-called Kvaerner-Moss System, was the "Norman Lady" (87600m3) launched in Stavanger, Norway in 1973. Before this project experienced its economic boom, it passed a number of tests, analyses and studies. The studies encompassed:
Dynamic loads of the ship during navigation with special reference to stress of the hull (bending moments and shearing forces), acceleration and bending
Special reference was accorded to the "equatorial ring" on which the greatest loads are placed since the spherical tank "hangs" on the ring
Thermal stresses of the construction with special analysis of the equatorial ring
Analysis of the storage tank and its insulation
"Fatigue" of the storage tanks/materials which were designed of 9% nickel-steel
Although the first carriers had storage tanks that were made of 9% nickel-steel, that technology was quickly replaced by aluminum tanks. Aluminum storage tanks proved to be more resistant to mechanical stress, rupture, and it was easier to correctly form them into a sphere. The main characteristic of the spherical tanks is the equatorial ring on which the tank "hangs". The greatest mechanical and thermal stresses are precisely on the "equator". That part of the ship's structure must be able to absorb the deflections of the ship's hull on one hand and the thermal and mechanical strains of the tank on the other hand. These storage tanks have an insulation which makes possible only around 0.10% of boil-off. The tanks are mostly insulated with several different layers, some of which are: glass wool, aluminum "foil" (vapor permeable) and various expansion foams. The "storage" in which the tank is located is considered to be a secondary barrier and this area is usually inert or under dry air.
Until 2000, 54% of all LNG carriers were spherical, primarily because Japanese shipyards had a license for the construction of only this type of ships, and since at the same time the Japanese are the largest LNG importers, this was one way to enter the very rich market. Today, membrane type ships have "surpassed" the spherical ships in number. One of the future advantages of the spherical tanks will be for the terminals which will be located in the polar region (Russia).
Membrane-type LNG Carriers (GTT Technology)
GTT technology represents two main types of membrane-type ships and recently, after the union between Gaz Transport and Tehnigaz in 1994, a third type of membrane-type carriers emerged.
The two basic concepts of GTT technology are the Mark III and No. 96 System, and the third one is a combination of these two and is appropriately named CSI (Combine System One). The membrane-type ships were developed during the 1960s, and both concepts use a thin flexible metal "membrane" which is in contact with the cargo. The system has the characteristics of a sandwich where the cargo presses on the membrane; the insulation material presses on the membrane and in the end, everything leans on the ship's inner hull.
No. 96 System - (Gaz Transport System)
The primary and secondary membranes are equal in this system and they are made of invar, and each membrane is only 0,7 millimeters thick. Invar is a material containing 36% nickel and the rest is steel, and as such has a minimal, that is, negligible dilatation coefficient. This system uses plywood boxes filled with perlite as insulation.
Perlite is a material based on treated volcanic rocks and has good insulation characteristics which do not change over time. The system is made of a primary membrane which leans on the first insulation layer, a secondary membrane which leans on the second insulation layer and finally, the second insulation layer leans on the ship's inner hull. The total thickness of the system amounts to 0, 5 meters.
Mark III System - (Technigaz System)
The primary membrane in this system is made of low-temperature stainless steel and has a thickness of 1,2 millimeters. Since steel has a significant dilatation coefficient, these membranes are well-defined and the joints enable them to move in two directions under light loads. As insulation material for these ships, the polyurethane foam is reinforced with fiberglass. The secondary barrier is much cheaper and simpler, and is made of triplex. Triplex is a type of a plastic mass which is vapor permeable but which on the other hand can contain liquid. The total thickness of the system depends on the type of project for which the ship is intended.
In both membrane systems, boil-off of 0, 15% is guaranteed although in reality its quantity is much smaller. Until 2000, there were only a few membrane ships but due to their significant flexibility and changes in the LNG spot market, in the past few years they have "surpassed" spherical ships in number. This type of ship is mostly produced in South Korea, but China has "produced" three membrane LNG carriers in the past three years.