The most common cause of oil pollution by ships comes from what are called operational oil spills. These are caused mostly by human error or sometimes intentionally when the ship's crew does not follow the strict regulations and break the law.
At the bottom of the ship under the engines is a space called the bilge. It collects water, oil and grease. When the ship's crew pumps out the engine room bilges, the oil is separated from the water. The waste oil is put into a special holding tank to be offloaded in the next port.
The remaining water, which may have traces of oil, is pumped overboard through an oily water separator. This makes sure only the tiniest amount of permitted oil goes into the sea. The amount is so small it cannot be seen by the naked eye. If an oil slick can be seen behind a ship, it means that the ship has broken the law and has discharged more oil than is allowed.
Other operational spills may happen when a ship is loading bunker oil or lubricating oil for its engines. A hose can break spilling oil. If someone is not watching the level of oil going into the ship's tanks, the tanks could overflow.
An operational oil spill can also happen after the crew of an oil tanker has cleaned the cargo tanks before loading a new cargo of crude oil.
After a cargo oil tank has been cleaned with water and chemicals the oil residue will float on the wash water in the bottom of the cargo tank. This water can be siphoned off and put through an oily water separator leaving only cargo oil residue in the bottom of the cargo tank. The new crude oil cargo can be loaded on top of the remaining old cargo oil. However, sometimes the crew does the wrong thing and illegally pumps the oily waste overboard.