IMO - International Maritime Organization, a United Nations agency that issues international trade standards for shipping.

In Class - A vessel currently meeting all the requirements of its Classification Society is "in-class".

Inert Gas (IG) - A gas used by marine tank vessels to displace air in cargo tanks to reduce oxygen content to 8 percent or less by volume and thus reduce possibility of fire or explosion. The inert gas used is usually nitrogen, carbon dioxide or a mixture of gases such as flue gas.

Inert Gas System (IGS) - A mechanical method of introducing inert gas into a vessel's tanks. An inert gas is one which has little or no ability to react with other gases, or to heat. Examples of inert gases are nitrogen and CO2. Shipboard inert gas systems utilize CO2, either from flue gas sources or from inert gas generators.

Inerting - A procedure used to reduce the oxygen content of a vessel's cargo spaces to 8 percent or less by volume by introducing an "inert" gas blanket such as nitrogen or carbon dioxide or a mixture of gases such as flue gas.

Innage - The amount of space within a tank that is occupied by oil. Innages are sometimes called soundings or body gauges.

Inshore Traffic Zone - A designed area between the landward boundary of a traffic separation scheme and the adjacent coast intended for coastal traffic.

Inspector - A person assigned to determine the quantity and/or the quality of a commodity.

Company Inspector - A Company employee given the responsibility of determining the quantity and/or the quality of a volume of oil being moved or stored.

Independent Inspector (Cargo Surveyor) - A person or organization of persons acting independently, but on behalf of, one or more parties involved in the transfer, storage, inventory or analysis of a commodity for purposes of determining the quantity, and/or quality of a commodity. They may also be assigned to the calibration of various measurement instruments and/or storage tanks ashore or on vessels.

Intake Certificate - A document issued by the shipper indicating the amount of cargo loaded aboard the vessel as calculated from the shore tank gauges. Freight is paid on the basis of these figures.

Intermediate Fuels - Light, residual-type fuel oils with characteristics between bunker fuel and marine diesel fuel, typically used in motor ships. It is quoted in terms of Redwood per second.

International Loadline Certificate - A document issued by a classification society stating the minimum freeboard granted to a vessel and giving the position of the loading disc on the ship's side.

Intertanko - International Association of Independent Tanker Owners.

IP - Institute of Petroleum

ISGOTT - International Safety Guide for Tankers and Terminals

Isherwood System - A method of framing a vessel which employs closely spaced longitudinals with extra heavy floors spaced further apart. Most tankers use this type of framing system.