In the primary distribution segment of the oil and gas industry, marine liners play a major role in moving petroleum products across the globe in an economical and efficient manner. Coastal movements are the preferred choice of oil organizations for transporting petroleum between continents.

These massive tanker vessels require huge quantities of fuel oil to power their engines. This fuel is specified by the vessel operator. Generally, low sulfur heavy fuel oil (LSFO) is commonly used as fuel for ships, although other fuel oils are also used. The process of filling the vessel's fuel tanks is known as bunkering. Though it appears to be a simple activity, it has a very detailed procedure that must be strictly followed for safety and environmental reasons. (Related reading: The Challenges of Shipping and Distributing Liquefied Natural Gas.)

Before we get into details of the standard operating procedures relating to bunkering, let us briefly describe the different types of vessels used to transport fuel across the globe.

Clean vs Dirty Tankers

The first tanker set sail over a century ago in 1886. Since then oil transportation has grown exponentially both in terms of the number of vessels and the volume of oil transported worldwide. It is interesting to note that nearly half the world's seaborne trade consists of crude oil or petroleum products.

Compared to the total volume of other goods moved via marine transportation, the volume of crude almost doubled from 1952 to 2002. Tankers move approximately 2 billion tons of oil every year. Of this, about three-fourth is crude and one-fourth is refined products. The largest tankers carry up to 4 million barrels of crude oil, or roughly more than 600 million liters. Refined products such as fuel oil, naphtha, jet fuel and lubricating oils travel in smaller tankers but in separate compartments for each product. Hence, multiple products can be transported to the intended destination.

Crude tankers move large quantities of unrefined crude oil from its extraction point to refineries. Product tankers, generally smaller, are designed to move petroleum products, petroleum specialties and petrochemicals from refineries to points near consuming markets. These two types of tankers are also referred to as either "clean" or "dirty." Clean tankers carry refined petroleum products such as gasoline, kerosene, jet fuels and chemicals, while dirty vessels transport products such as heavy fuel oils or crude oil. Larger tankers usually only carry crude oil.

A Good Value for the Investment

Oil tankers are used for both transportation and as floating storage units. This is acceptably done with ships close to the end of their service life. On average, an oil tanker has a lifetime of about 25 years, which does not seem to be very long considering the investment of around $40 million USD for small and $120 million USD for large tankers. On the other hand, the value of a single load easily surpasses the value of a large ship.


Today's oil tankers are enormous vessels. Ultra large crude carriers (ULCCs) are supertankers that can carry more than 330,000 tons of cargo. Such huge supertankers need very large ports to load and unload their cargo. Many ports are offshore terminals served by underwater oil pipelines. Additionally, supertankers cannot pass through the Panama Canal or Suez Canal, so they need to circumnavigate the capes of South Africa and South America. The closure of the Suez Canal in 1967–1975 was one of the factors that encouraged the building of these giant ships because it was not viable to force the smaller vessels to take the longer routes.

How is Bunkering Performed?

Assessing the fuel requirements of marine vessels is analogous to other vehicles like cars and trucks. The concept of vehicle mileage is similar, and upon estimating the total nautical miles of the proposed voyage and based on the fuel required to travel one nautical mile, the total bunkering required is determined.

Once the quantity of fuel and lubricants are estimated, approval from the vessel captain is obtained by the person responsible for running the main engines and auxiliary engines onboard. The fuel and lubricant quantities and specifications are ordered from the bunkering supplier, who will arrange for the supplies at the stated port of call. Because the huge quantities of fuel entail significant costs, one has to take care of key commercial details such as prices, taxes and any additional charges before deciding on the port of call. Also, the facilities available for bunkering should be taken into account before arriving at this decision.

Bunkering is an interesting operation and a very specialized task to perform, particularly when it comes to large carriers like ULCCs. Once the supplier and port of call are determined, the vessel is prepared for the bunkering operation. The engineer responsible for looking after the fuel position of the vessel must assess the initial fuel level present in each of the fuel tanks of the vessel. Large vessels contain large fuel tanks, so an accurate assessment of the fuel, the sequence in which the tanks are to be filled and the amount of fuel to be loaded in each tank while maintaining the ship's balance has to be determined. This is called a bunkering plan, and it is approved by the captain of the vessel, taking the safety of the personnel, vessel and environment into account.

Based on the bunkering plan, arrangements are made by the bunkering supplier. Per the rules of the International Maritime Organization (IMO), a shipboard oil pollution emergency plan (SOPEP) must be prepared and put into effect during a bunkering operation. The SOPEP details the equipment and accessories needed to manage an oil spill incident during bunkering operations.

Delivery Note and Invoices for Bunkering Fuel

Once bunkering is completed, a delivery note with the amount of fuel is prepared, which is approved by the vessel engineer and captain as an acknowledgement of receipt of bunkering. Because petroleum products expand as the temperature increases, a detailed calculation of the actual quantity supplied in volume and weight after due temperature correction (usually to 15°C) is presented. The invoice is created for the quantity, either by volume or by weight, per the terms of the supply contract. It is also a business practice to keep a sample of the fuel for future reference by the vessel's operators.