Maritime Cargo Operations and Safety Procedures: Marine Chemistry, MARPOL, and Tank Cleaning Practice Test

The most pollution risk occurs when topping off because the tank is almost full, leaving very little headspace to absorb any fault or surge. If level sensing or the automatic stop fails, or if the pump rate is still high as the last portion is put in, cargo can overfill and escape through vents, deck drains, or overboard outlets. That small margin means even a brief loss of control can release a relatively large amount of cargo into the sea, making the environmental consequence substantial. Start of loading and mid-loading have more room to accommodate small overfills without immediate spillage, and discharge is managed with containment and disposal controls, but the peak risk point during topping off is tied to the near-full condition and the potential for overfill.