Researchers stated that 457 million litres of oil enter the oceans annually from regular shipping activities. The majority of ocean-going ships use oil-lubricated stern tubes, and stern tube leakage is an important source of lubricant oil going into the ocean. The International Maritime Organization (IMO) estimated that every year, 80 million litres of oil-based lubricants are lost at sea from stern tubes alone. Stern tube leakage varies by vessel type, condition, age and maintenance. For example, IMO estimated that a barge carrier may consume 20 litres of oil per day, a hopper dredger may consume 3 litres of oil per day and a passenger ferry may consume 2 litres of oil per day.