Abstract: The central drain pipe is an essential device for large size floating roof tanks. The basic requirements for the drain pipe are: capable of heaving freely with the pontoon, reliable sealing in its swivels and flexible rotation, . corrosion resistant in crude oil and rainwater. However, some faults with all kinds of drain pipes took place during its operation due to the different structures and different service environments, one of the faults is the leakage of dynamic seal on the swivel. The analyzing results of the leakage showed that the swivel was compressed due to mishandling in its installation and uneven settlement of tank foundation during operation, that the seal is not applicable to the crude oil stored and the tank volume, and that the structure of drain pipe is not reasonable. Improvements are put forward, which are altering the tensional state of dynamic seal of swivel, and connecting the swivel with expansion loop at water-level for localized pipe section. The investigation indicated that drain pipe with swivel is suitable for large size oil tanks, while drain pipe with metal hose is applicable to product oil tanks.