#insights

Analysing boil-off gas (BOG) is important because it is the most requested value, together with the useful capacity, as the guarantee in all refrigerated tanks. The determination of the BOG in full containment LNG tanks is generally carried out using the analytical method.

BOG is defined as the LNG that has been evaporated or boiled-off, producing an increase in the pressure inside the tank. This phenomenon is caused by the heat input into the LNG tank during storage and operation. There are several sources of heat input in this type of cryogenic tanks, such as: solar radiation, natural convection, and soil conduction. The outcome of this heat input into the tanks is a loss of energy – it is necessary for this to be minimised by installating the different required thermal insulations inbetween the outer and inner tank.

Whether the thermal insulation has been designed to meet the requested BOG value is usually and traditionally assessed analytically, considering 1D heat transfer calculation and other approximations and assumption, as constant thermal conductivities of the materials.

To double check and develop a more detailed calculation method of the BOG, Sener carried out an investigation based on numerical methods (Finite Element Method) to calculate the (BOG) in full containment LNG tanks.

The used model is overcoming the geometry simplifications and using the thermal conductivities depending on temperature variations. The model is implemented parametrically in aim of getting immediately the result when any other cryogenic tank is considered.

The results obtained from the same tank using both analytical and numerical methods have been compared. The objective of that results comparison was to highlight which of the two approaches is more optimum and which are the consequences of such optimization.