The full name of the SF double wall underground fuel tank is “Steel Reinforced Fiberglass Double-Layer Storage Oil Tank.” It is a double-layer oil tank consisting of a steel inner tank with an additional layer of fiberglass-reinforced plastic (FRP) outer casing for leak prevention. The gap between the steel inner tank and the FRP outer tank creates a space that is continuously monitored by a leakage detection system. The system can detect any leaks in the inner or outer tank by monitoring the liquid level in the gap space. Once a leak is detected, the sensor in the detection system will send an alert when the liquid level at the bottom of the gap reaches a critical level, ensuring the safe use of the oil.