More than half the cargo carried by sea is oil carried in tankers. Since an oil cargo can be pumped on and off a tanker at a much higher rate than dry cargoes, tankers spend less time in port and in the space of one year can carry more cargo than a dry cargo ship of the same tonnage. There are two basic types of tanker:

The large type of tanker (ie in the vicinity of 250 000 dwt). This has become the standard type for transporting crude oil. These are known as VLCC’s – very large crude carriers. Tankers above 300 000 tons dwt are known as ultra large crude carriers. 78% of oil transported by sea is made up of crude oil.

Very large crude oil carrier (VLCC).

The smaller type of tanker (ie in the vicinity of 30 000 dwt). These tankers are known as
product carriers and are used for carrying the refined product from the refineries to the consumer. The main difference between the VLCC and the product carrier is the number of tanks. The VLCC has fewer whereas the product carrier carrying a variety of petroleum products needs more, usually about nine from forward to aft with each tank being divided into three. In order to reduce the risk of pollution, IMO (International Maritime Organisation) restricts the size of any single tank.

Product tanker Breede lying alongside.

Design features.

• The superstructure containing the bridge, accommodation, workshops and engine room is right aft.
• Just forward of the engine room is the pump room.
• The tanks just forward of the pump room are smaller than the rest and can be used as slop tanks when washing out the tanks during a ballast voyage. These tanks can then be discharged into the loading port’s oily water reception facilities. The tanks are separated from the rest of the ship by coffer dams.
• A tanker usually has one small crane or derrick to lift flexible hoses to connect them with the shore pipelines.
• Because the openings on deck are very small they are much less vulnerable to seas breaking over the decks and therefore tankers are allowed to load much deeper.
• As a result of the tankers being allowed to load deeper, some form of catwalk is provided for the crew to move between forward and aft in bad weather.
• Since the Exxon Valdez disaster, new tankers are required to be double hulled.
• The cargo is loaded by being pumped aboard by shore pumps or by having the shore tanks higher than the ship and letting it flow aboard under gravity.
• It is discharged by the ship’s pumps.
• Since some oils can be difficult to pump when cold, steam heating coils in the tanks may be necessary.
• When a tanker has discharged her cargo she is at her most dangerous since the dregs left would vaporise and create an explosive mixture. To help overcome this danger in the ballast condition, new tankers over 20 000 dwt are fitted with inert gas systems which replace s the oil with inert gases as they are discharged.

Floating Production, Storage and Offloading (FPSO) vessels.

A FPSO is a floating unit used for the production, processing and storage of oil. It is designed to receive oil produced by itself or from nearby oil rigs/platforms or directly from the sub-sea oil wells themselves.
It processes the oil and stores it until it is off loaded onto a tanker or via a pipeline to ashore facilities. They are easy to install and do not require a local pipeline infrastructure. They can either be converted tankers or specially designed vessels/platforms.

There are also other types of floating platforms such as the FLNO (floating liquefied natural gas) vessels and the FSO’s (floating storage and off loading vessels). The latter type are mostly converted single hull tankers.
FPSO vessels are mostly used in remote or deep water locations where seabed pipelines are not cost effective. Another advantage of FPSO’s is the fact that when an oil field dries up, the vessels can be towed to a new location.

FPSO vessel Petrojarl Cidade De.