Why should petroleum cargo tanks not be topped off at deck level when loading on a cold day?

The correct answer highlights the relationship between temperature and the volume of petroleum cargo. When loading petroleum products on a cold day, the liquid is cooler and more viscous, meaning it occupies less volume than it would at a higher temperature. If tanks are topped off at deck level under these conditions, the liquid has little room to expand.

Subsequently, if the temperature increases after the loading process, the petroleum will expand due to thermal expansion. This can lead to overflow since the tanks do not have sufficient ullage (empty space) to accommodate the increase in volume. Ensuring adequate ullage is critical in preventing spills and ensuring safety, especially when operating under changing temperature conditions.

The other considerations regarding air pockets, valve function, and loading pressures, while pertinent to safety protocols, do not directly address the primary risk associated with temperature-induced overflow, which is the key aspect captured by the correct answer.