Abstract: With the rapid development of oil storage construction, the tank size is developing towards large size and being able to adapt to extreme conditions. In order to avoid oil solidification in the tanks and other safety accidents caused by too low oil temperature, the law of oil temperature field in tanks needs to be calculated accurately. Based on the law of conservation of energy, the heat transfer coefficient of tanks can be solved by fractional steps numerical algorithm, then the unsteady heat transfer discrete equation is solved by numerical method which is set by the Taylor series expansion method. The application analysis of the $10\times 10^4$m$^{3}$ floating roof tank in Daqing Oilfield shows that, the oil temperature drop rate increases gradually with the decrease of ambient temperature, and the oil temperature is higher and the temperature drop rate is smaller in the tank with higher tank level and larger volume. The research and analysis provide the important data for optimizing the storage design of large floating tanks and ensuring the safety and economic operation of oil depot.

Key words: floating roof oil tank, heat transfer coefficient, discrete equation, numerical simulation, temperature drop law