Because of increased liquefied natural gas (LNG) demand, operation of LNG receiving terminals having both above-ground and in-ground LNG storage tanks will soon be required. In such cases, there exists a pressure head difference between the two types of tanks. As such, during the depressurization step of LNG unloading, vapor can be generated at the top of the unloading pipeline of the above-ground tank due to pressure head. Vapor produced in a branch pipeline of the above-ground tank can thereby cause congestion during depressurization, resulting in a pressure difference across the unloading valve. This can, in turn, cause excessive boil off gas inflow into the storage tank. In this paper, we suggest a reliable unloading operation procedure for a mixed operation of above-ground and in-ground storage tanks using dynamic simulation.