Why shouldn’t petroleum cargo tanks be topped off at deck level on a cold day?

Petroleum cargo tanks should not be topped off at deck level on a cold day primarily because a subsequent rise in temperature can cause the cargo to overflow. When petroleum products are loaded into tanks, they achieve a certain volume at the ambient temperature during loading. If the air temperature rises after the tanks have been filled, the liquids inside will expand due to thermal expansion. This expansion can result in the liquid reaching beyond the tank's capacity, particularly if the tank was filled to the brim at a cooler temperature.

It's important to account for this expansion when loading cargo, especially in colder conditions where the initial temperature is lower. Ensuring that there is enough space in the tank for the liquid to expand is critical in preventing spills and potential environmental hazards while also adhering to safety regulations.

Other options suggest different issues that may arise, but the significant concern remains the expansion of the cargo as temperatures rise leading to overflow if the tanks are filled to capacity when they are cold.