The Korean Register issued the Guidance of Heat Transfer Analysis for Ships Carrying Liquefied Gases in Bulk/Ships Using Liquefied Gases as Fuels, highlighting that the cargo tank of the vessels transmitting liquefied gases in bulk or the fuel tank of ships using liquefied gases as fuels is likely to make the hull structure cold due to the low temperature of the liquefied gas.
Generally, steels increase brittleness at low temperature, so brittle fracture of ships carrying liquefied gases in bulk/ships using liquefied gases as fuels through proper steel selection should be avoided. For this purpose, the IGC Code requires that the heat transfer analysis of ships carrying liquefied gases in bulk be carried out and the steel of the hull structure be selected based on the analysis.
The Guidance applies two heat transfer analysis methods; The first is the analytical heat transfer analysis method and the second is the finite element heat transfer analysis method.
Its up to the user to decide which method to use between the two above.
In addition, the analytical heat transfer analysis method is a method of solving the basic thermal equilibrium equation to obtain the temperature, which is easily applicable and can predict the temperature value.
The calculated temperature means the average temperature of each section. Based on this, the steel grade is selected and finally the welding consumable is also selected.
Air temperature, sea water temperature and wind speed are selected based on the navigation area of the ship for program input. Air temperature, sea water temperature and wind speed in IMO IGC Code or USCG may be used. The material properties for heat transfer analysis should be defined and the properties of steel, insulation, seawater and air should be selected in consideration of temperature.
After all input for analytical heat transfer analysis is prepared, the analysis is performed. The steel grade is selected based on the temperature of each part obtained as a result of the analysis. Also, the welding consumable is selected based on the selected steel grade.
Moreover, finite element heat transfer analysis method can estimate hull temperature distribution by considering complex hull structure. Since each section of the hull is divided into several elements to perform the analysis, it is calculated to have a temperature distribution along one structural member.
Two different results can be obtained depending on how this temperature distribution is processed:
One is to calculate the average temperature of the structural member and select steel grade and welding consumable based on this. The other is to use the average temperature of the structural member only for the steel grade and to select the welding consumable based on the temperature of each joint.
In the possibility that finite element heat transfer analysis method is selected, general finite element analysis software can be used. Two-dimensional analysis modeling or three-dimensional analysis modeling work should be performed considering the hull structure to be analyzed. Air temperature, seawater temperature and wind speed should be selected based on the navigation area of the ship for program input. In this case, IMO IGC Code or USCG can be used.
The material properties for the heat transfer analysis must be entered into the software.
The steel grade is selected based on the temperature of each part obtained as a result of the analysis. At this time, the steel grade is selected using the average temperature of the members.
Concluding, the temperature for welding consumable selection is available in two ways:
- The first method is to select a welding consumable based on the steel grade determined using the average temperature of the member.
- The second method is to select the welding consumable based on the calculated temperature of the joint rather than the average temperature of the member. In this case, the steel grade based on the joint temperature is temporarily selected and the welding consumable is selected by using it.
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