The key idea is that a tanker’s double hull creates an outer hull and an inner cargo-holding hull with a space between them that’s used as ballast tanks. That interspace is specifically designed to hold seawater to provide stability and trim for the ship. Cargo liquids like crude oil or refined products are stored in the inner cargo tanks, not in the ballast interspace. So, the liquid that the double hull section is meant to carry is water (ballast). Crude oil and refined products go in the vessel’s cargo tanks inside the inner hull.

Starting flow from static accumulators should begin at a conservative speed to minimize static electricity buildup and sudden pressure changes. ISGOTT specifies a start velocity of 1 metre per second. This slow, controlled start lets you verify bonding and grounding, check for leaks, and observe equipment response before increasing flow. Higher initial velocities raise the risk of generating static charges, vapor release, and mechanical stress on hoses, while going much slower is inefficient; 1 m/s is the recommended balance for safety and control.

MARPOL Annex II uses two linked ideas: a hazard-based classification for the cargoes and a vessel-type framework that matches ship design to those hazards. Noxious liquid substances carried in bulk are divided into four categories that reflect how dangerous they are if discharged, which sets the level of discharge restrictions, tank cleaning, and onboard treatment required. To support safe handling of different cargoes, the Annex also defines three ship types for chemical tankers, describing the vessel’s design and equipment relative to cargo hazard. Type 1 ships carry the most hazardous cargoes and face the strictest discharge and cleaning requirements; Type 3 ships carry the least hazardous cargoes and have the least stringent requirements. Taken together, there are four cargo categories and three ship types.

Heating coils are used to raise the temperature of the cargo to lower its viscosity, but they cannot be run at any and every tank level. The heat transfer depends on having enough liquid around the coils to absorb the heat. If the tank is too low, overheating of the remaining cargo or the coils themselves can occur, increasing the risk of damage, scorching, or dangerous vapor generation. If the tank is nearly full, excessive heating can still cause unwanted high vapor pressure and other safety issues. Because of these factors, heating coils must be operated within the safe limits specified for the cargo and the vessel, not regardless of how much cargo is in the tank. In practice, you follow the cargo heating plan and monitor levels, temperatures, and pressures to ensure heating is appropriate for the current tank condition.