KR Webzine Vol.144

Introduction
Liquefied Natural Gas (LNG) is transported at cryogenic temperatures near its saturation temperature of -163oC, which implies a net heat transfer from the environment, in spite of the well-insulated cargo tanks. Like other liquids, LNG evaporates when heated past its boiling point. Heating LNG beyond its boiling point generates boil-off gas (BOG). The amount of BOG depends on the design and operating conditions of LNG tanks and ships. The generation of BOG increases the pressure inside the LNG tank. To keep pressure inside the tank within a safe range, BOG needs be continuously removed. As a safety measure, most LNG tanks are have a safety valve that can alleviate the pressure of the tank by releasing BOG as required.

Most of the BOG is generated during the transportation of LNG by ships. BOG released during the voyage of an LNG carrier may occur due to the following reasons;

 ● The ingress of heat into cargo tanks due to the difference between the temperature in the cargo

     tanks and the temperature of the environment

 ● Due to the cooling of a ship’s tanks during ballast voyages, achieved by occasional spraying of LNG

     in the upper part of the tank
 ● Due to the sloshing of cargo in partially filled tanks, due to the action of waves, causing friction on

     the inner wall of the tank creating an additional thermal effect.

Heat ingress is the primary reason for the generation of BOG on ships. In the maritime transportation of LNG, a quantity of evaporated cargo is typically presented as loss, expressed as a percentage of the total volume of liquid cargo, during a single day, i.e. as Boil-off rate (BOR). 

BOR Calculation Conditions:
The following assumptions and conditions apply to BOR calculations. The thermal design conditions are based on IMO recommendations.

The external ambient conditions such as Aft and Forward cofferdam temperatures should be calculated, using temperature distribution analysis and central cofferdam equipped with heating conditions could be assumed as 5 oC.

Calculation Equation:
 The evaporation rate of LNG is evaluated by calculating the BOR, which signifies the percent of evaporated LNG per day, compared to the initial LNG loaded amount. Assuming all the penetrated heat is used for the evaporation of LNG for one day, BOR can be expressed as shown in the following equation;

Where,

Calculation of Heat Ingress into LNG Tanks:
 The total amount of heat flow that penetrates LNG tanks can be calculated by the analytical method or numerical method. In the analytical approach, a temperature distribution analysis, for IMO warm design condition is performed to calculate the inner hull temperatures following the procedures given in KR guidelines on Heat transfer analysis for ships carrying liquefied gases in Bulk/Ships. Figure 1 illustrates an example of inner hull temperatures, calculated by the analytical method considering IMO warm design condition.

Figure 1: Example of inner hull temperatures calculated by the analytical method

Once the temperature of the inner hull is known, the heat leak is evaluated using the following equation.

Where,
      Q   = heat flux that penetrates from outside to inside LNG tank
      U   = Overall heat transfer coefficient
      A   = Area for heat transfer
      ΔT  = Difference in temperatures between inner hull and LNG
      t   = Thickness
      k   = Thermal conductivity

The total heat flux from outside to inside the LNG tank should be the sum of all the local heat transfers through surface areas and corner areas.

Where,
      Qs = heat flux that penetrates through surface areas
      Qc = heat flux that penetrates through corner areas

Figure 2 presents an example for an LNG tank with area numbering in a membrane-type LNG carrier. The heat leaks from each of the numbered area, which in turn consists of surface and corner areas, should be calculated.

Figure 2: Example of area numbering for a membrane-type LNG tank

Generally, the BOR calculation for LNG tanks will be performed by the cargo containment system (CCS) designer. So far, Gaztransport and Technigaz (GTT) is doing this task for their CCS types. However, Korean Shipyards, Korea Gas Corporation (KOGAS) and many other companies around the world are developing their own CCS system. Therefore, this study will provide a complete guideline to the new CCS and LNG carrier designer for BOR calculation.