What does "thieving" a petroleum cargo refer to?

"Thieving" a petroleum cargo refers specifically to the practice of measuring the amount of water present in cargo tanks. This process is crucial because water can adversely affect the quality of petroleum products and lead to problems in storage, transport, and refining processes. If water is present in the cargo, it can result in corrosion, instability, and thermal issues, thereby impacting the overall efficiency and safety of the operations.

While the other options are related to tank monitoring and quality assessment, they do not pertain to the term "thieving." For instance, removing excess fuel oil is a routine practice but doesn't directly relate to measuring water content. Testing temperature and assessing vapor pressure are essential for understanding the properties and safety of the cargo but do not involve the assessment of water presence in tanks. Thus, the focus on water measurement is what defines "thieving" in this context.