Question 1274 Deck General

The correct answer is C) an above deck datum. Ullages, which refer to the volume of space above the liquid in a tank, are measured from an above deck datum. This is typically a fixed reference point located above the tank, such as the deck or a dedicated measurement point. This allows for consistent and accurate measurement of the tank's contents, regardless of the tank's shape or size. The other options are incorrect because: A) the thievage mark is not a standard reference point for ullage measurements, B) the tank ceiling is not a suitable reference point as it can vary in height, and D) the tank top bushing is a component of the tank itself, not an external reference point.