Seagoing oil tankers are merchant ships used for the bulk transport of petroleum oils. There are two basic types of oil tankers: the crude oil tanker and the product tanker. Crude oil tankers move large quantities of unrefined petroleum crude oil from its point of extraction to refineries where the crude oil is processed into gasoline and other fuels as well as a variety of other products. Product tankers, which are generally much smaller, are used to move processed products from oil refineries to points near consuming markets.

It should be noted that the above is only a very general description. Both the crude oil tankers and the product tankers come in various sizes as shown in the table below:


As can be seen in the table, the most commonly used tanker classification is based on the oil-carrying capacity in terms of Dead Weight Tonnage (DWT) expressed in metric tons. As also shown in the table, there are two scales used to classify the tankers: the Average Freight Rate Assessment (AFRA) scale and the Flexible Market Scale. Note that some of the tanker classes in the Flexible Market Scale are named after the canals the tankers are intended to travel through. For example, the Panamax class was named after the Panama Canal and the Suezmax class was named after the Suez Canal.

Starting with the largest oil-carrying capacity and descending to the smallest capacity (in terms of DWT ), the major classes in the Flexible Market Scale are the ULCC (Ultra Large Crude Carrier), the VLCC (Very Large Crude Carrier), the Suezmax, the Aframax, the Panamax and the Product tankers.

Similarly, the major classes in the AFRA scale are the ULCC, the VLCC, the LR2 (Large Range 2), the LR1 (Large Range 1), the Medium Range tanker and the GeneraL Purpose tanker.

The following part of this article is based on a specific oil tanker captained by the author of this article. The ship is a so-called "Handysize" oil tanker with a capacity of 38,000 DWT and it would be classified as a "Product tanker" using the Flexible Market Scale classification or as a Medium Range tanker using the AFRA Scale classification. The ship was built in 2006 as an "Oil/Chemical" tanker but it had to be downgraded to only an "Oil" tanker due to the revised IBC code of the International Maritime Organization which went into force in 2007. That limited the number of different cargoes that the ship is allowed to carry so as to prevent carrying cargoes that might be toxic to human or marine life.

The ship's main dimensions are:
  • SDWT (Summer Dead Weight Tonnage): 38,555 MT (metric tons)
  • Draft: 11.617 m (meters)
  • LOA (Length Over All): 182.86 m (meters)
  • Br. mld. (Breadth Moulded): 27.43 m (meters)
  • Dpth. mld. (Depth Moulded): 16.80 m (meters)
  • Main Engine: Man B&W
  • Engine type: 6S50MC-C
  • Engine PS / KW: 12,880 metric horsepower / 9,480kilowatts

Although the ship was built in China, all of the major components came from European or European-licensed manufacturers.

The Propulsion Engine

A Man B&W 6S50MC-C six cylinder diesel engine

The ship's main engine is the Man B&W/6S50MC-C which is a six cylinder, two-stroke, slow speed, turbocharged, reversible diesel engine. It is one of the most popular engines used for the main propulsion of Handysize tankers. It has an extra long stroke which provides a large cylinder volume and thereby high power (9,480 kW) compared to it’s relatively small bore size. It is a rather short engine in the ship's longitudinal direction (the last C is for “compact”), which means that the engine room is smaller and thus allows more cargo to be carried. These engines are quite fuel efficient which gives a long range with relatively small fuel tanks, which again provides more cargo space.

The total fuel volume is approximately 1,200 MT which theoretically makes it possible for the vessel to stay at sea for approximately 48 days provided it is performing at economical speed. These figures may in reality be somewhat restricted by company safety procedures that limit the filling the fuel tanks to 90 %. Other factors like the vessels draft, higher speed demand and adverse weather will affect the action radius, so a safe estimate would be about 36 days at sea, which is still sufficient to make a voyage from the Persian Gulf to U.S. West Coast.

This type of engine has been produced by MAN B&W since the early 1980s and is based on classic well-known, well-proven principles. During the years since the early 1980s, it has been up-graded and up-rated to meet the needs of customers. The specific engine used in this ship was built around 2003 and still features chain-driven camshaft, mechanically controlled fuel injection timing and exhaust valve opening as well as conventional fuel oil pumps ... as opposed to newer versions of similar ships in which the fuel injection timing is electronically controlled.

The main engine's load and revolutions are controlled by an electronic governor, which is connected by a linkage arm to the common fuel rack for all of the fuel pumps.

Lubrication oil for crankshaft bearings and the camshaft is provided by an electric pump. Cylinder lubrication is provided by electronically controlled lubricators which is a retrofitted system that provides better and much more economical lubrication. The system includes the ability to adjust lubrication settings according to the quality of fuel oil being consumed, as this is a major contributing factor to the wear of pistons and liners, a subject of immense interest to both manufacturers as well as ship owners.

The Pistons

Each of the six cylinders contains a piston. Those pistons are the most vital components of a diesel engine. The pistons have two parts, namely a piston crown and a piston skirt. The crown is made of heat resistant steel. The skirt is made of cast iron with a bronze band.

Each piston has four ring grooves which are hard chrome plated on both the upper and lower surfaces of the grooves. The piston rings are cut at one place in their circumference since they are intended to be removable and replaceable. To provide protection against the combustion pressure in the cylinder, the uppermost piston ring lock is of the “Controlled Pressure Relief” type (CPR). The other three piston rings all have an oblique cut in either the right or left direction. All four rings are alu-coated on the outer surface for running-in purposes.

In order to monitor ring wear, all four rings are equipped with a groove where the wear can be measured. In the most recent version, the groove in the top ring has been replaced with several milled passages, as this has proven to reduce the wear and thereby increase the service intervals.

The Fuel Injector

Another vital component of the main engine is the fuel injector. Each of the six cylinders is fitted with two injectors of the so-called "slide valve" type that was introduced in 2002 in order to improve fuel efficiency. It later turned out, as a side effect, that the amount of NOx emissions was also reduced (merely by retrofitting these fuel injectors), making old engine types immediately compliant with Marpol Tier I emission requirements. The Marpol (marine pollution) Tier 1 emission requirements are a directive of the International Marine Organization and limit the NOx emissions to less than 8.6 grams per horsepower-hour.

The Exhaust Valve

The exhaust valve is opened hydraulically by a piston activated by the camshaft and closed by means of air pressure. In order to distribute the wear on the valve seat when in operation, the valve spindle has small vanes attached and these are driven by the exhaust gas causing a slow rotation of the spindle.

The Turbocharger

To improve the fuel combustion and thereby the efficiency of the main engine, it is fitted with a turbocharger driven by the exhaust gas. The turbocharger then pushes air into the scavenging air receiver at a pressure between 1 and 3 bar depending on the main engine's load. The average revolution speed of the turbocharger is about 12,000 to 15,000 rpm.

Publishing Note

This article was written by Thorbjoern Joensen, a member of this wiki, and uploaded by him. It was then edited by Milton Beychok, the organizer of this wiki.