Numerical Study of Heat Transfer Characteristics in a Subsea Wet Oil Storage Tank

There are two different methods used to store oil in ocean engineering. One method is "dry" storage, which consists of storing the oil in a dry tank within the hull. This is the current method used in offshore production systems such as FPSOs. The other method is "wet" storage, which stores the oil over seawater. During production, the crude oil displaces the seawater, whereas during offloading, the storage compartments are refilled with seawater. In this paper, we developed a new type of wet storage system, which can provide a storage capacity of 100,000∼1,000,000 barrels of oil. This storage system comprises of a steel/concrete storage tank located on the seabed, a water caisson located on an offshore platform, an oil loading line bringing steady oil to the storage tank, an oil offloading line for delivery of crude oil to the shuttle tanker, a seawater displacement line to take displaced water as oil enters the tank back to the water caisson and to flow seawater into the bottom of the storage tank enabling crude oil offloading at design flow rate. As the crude oil stored in the subsea oil storage tank has an initial temperature measurably higher than that of the ambient seawater. It is expected that the hot oil will gradually be cooled by the ambient seawater over time, and the final temperature of the stored oil can even reduce to ambient temperature. This could create serious problems in exporting the stored oil to the shuttle tanker, especially for a crude oil with high viscosity and high condensation point. To investigate the characteristics of heat transfer between the hot oil and the ambient seawater in the subsea oil storage tank, a CFD numerical analysis is conducted in the present study. The storage tank used in the numerical simulation is a vertical cylindrical steel tank and has a storage capacity of 100,000 barrels of oil. The center section of the storage tank is a center well, which is square in cross-section and is fully filled with seawater. Two kinds of structures of tank wall are discussed: one is single wall; the other is double wall with water between the walls. The findings may contribute to determine optimal operating temperature of the incoming crude oil and the storage time limit in particular environment condition and can be used for reference to design insulation system or reheating system to avoid the oil cooling to an unacceptable level.