Which method is NOT effective for preventing release due to overfilling underground storage tanks?

Drop tubes inside the tank are not considered an effective method for preventing overfilling because they primarily serve to minimize turbulence when fuel is being added and help direct the flow of fuel to the bottom of the tank. While they can help reduce the splashing effect during loading, they do not actively manage the fuel levels or provide feedback about the tank's capacity.

In contrast, options like fill pipe enclosures provide physical barriers that help contain spills, and high-level sensors offer direct monitoring of the fuel level, alerting operators to potential overfilling. Similarly, vent float valves are designed to prevent pressure build-up and overflow by allowing vapor to escape while also signaling the tank's capacity. Each of these methods is specifically designed to either monitor fuel levels or prevent spills during the loading process, highlighting their effectiveness compared to drop tubes.