The gas vessels are designed to carry different types of bulk gasses and the vessels are technically advanced and are often classified into two main segments; LPG and LNG.
LNG(Liquified Natural Gas) Carriers are specifically designed to trade a high volume of LNG. Liquefied natural gas (LNG) is natural gas (predominantly methane, CH4, with some mixture of ethane, C2H6). The LNG ships have a cargo carrying capacity between 125,000 cum to 260,000 cum. The most popular size is up to 180 000 cum.
In order to facilitate transport, natural gas is cooled down to approximately −163 °C (−261 °F) at atmospheric pressure, at which point the gas condenses to a liquid. The tanks on board an LNG carrier effectively function as giant thermoses to keep the liquid gas cold during storage.
Vessels can be classified into four categories in term of the cargo containment system:
Moss tanks (Spherical IMO type B LNG tanks)
This system is named after the Norwegian company which designed them (Kvaerner Moss). Most of these vessels have 4-5 tanks. These tanks have a working pressure of up to 22 kPa (3.2 psi), but the working pressure can be raised foran emergency discharge.
IHI (Prismatic IMO type B LNG tanks)
The self-supporting prismatic type B tank is designed by Ishikawajima-Harima Heavy Industries. Due to several incidents in the past, these tanks were designed to avoid damage due to incidents occurred inside membrane LNG tanks. The design is implemented in very few vessels.
TGZ MARK III
These vessels are designed by Technigas and are a membrane type design. The membrane consists of stainless steel and the tanks have a 1.2mm waffle pattern' to absorb the thermal contraction when the tank is cooled down.
This design designed by Gaztransport consists of primary and secondary membranes made up of a material Invar which has no thermal contraction. The insulation is made out of plywood boxes filled with perlite and continuously flushed with nitrogen gas. The integrity of both membranes is permanently monitored by detection of hydrocarbon in the nitrogen.
Invar, FeNi36 (64FeNi in the US), is a nickel–iron alloy notable for its uniquely low coefficient of thermal expansion (CTE or α).
C-type tanks are quite common in the small scale LNG segments. These tanks are designed as cryogenic pressure vessels, using conventional pressure vessel codes (very often vapour pressure). The design pressure for these tanks is in ranges above 2000 mbar. The most common shapes for these tanks are cylindrical and bi-lobe. Type ‘C’ tanks are used in both, LPG and LNG carriers.
These tanks have a prismatic tanks shape and maximize the use of the vessel’s tanks, ensuring a flat deck without losing cargo capacity. A self supporting tank structure enabling parallel building activities. The design aim to optimize the cargo carrying capacity volume, the thermal insulation performance in tank shape and hull design.
In Maritime Optima we have used the cum. sizes to divide the vessels into sub segments. Later we will fine tune by adding more filtering labels.