Bunkering is the process of supplying fuel to a ship. The term covers everything involved in transferring marine fuel from a supply source to a vessel’s onboard storage tanks, including the logistics of delivery, quantity measurement, quality testing, and regulatory documentation. It’s one of the most routine yet operationally critical activities in commercial shipping, and it happens every time a vessel needs to refuel.
Why It’s Called Bunkering
The word comes from “bunker,” which originally referred to the coal storage compartments on steamships. Even though coal gave way to liquid fuels over a century ago, the terminology stuck. Today, a ship’s fuel tanks are still called bunkers, the fuel itself is often called bunker fuel, and the act of filling those tanks is bunkering.
How Fuel Gets to the Ship
There are three main ways to deliver fuel during a bunkering operation, and the method depends on where the ship is, what port facilities are available, and how much fuel is needed.
- Shore-to-ship: The vessel ties up at a terminal equipped with fuel pipelines. Fuel flows directly from shore storage tanks through hoses or loading arms connected to the ship’s fuel manifold. This is common at major ports with dedicated bunkering infrastructure.
- Ship-to-ship (barge delivery): A fuel barge pulls alongside the receiving vessel and transfers fuel through hoses. This is the most common method worldwide because it doesn’t require the ship to occupy a berth at a fuel terminal. When bunkering from a barge, the crew must ensure sufficient fendering between the two vessels to prevent metal-to-metal contact, secure mooring, and a safe, well-lit means of access between them.
- Truck-to-ship: Fuel tanker trucks drive to the quayside and pump fuel aboard through hoses. This method works for smaller vessels or ports without pipeline infrastructure, but the volume per truck is limited, so it’s rarely practical for large cargo ships.
Offshore bunkering, where fuel is transferred at sea or at anchorage rather than in port, follows the same safety protocols as ship-to-ship transfer operations. Industry guidance from the Oil Companies International Marine Forum provides the standard procedures for these transfers.
Types of Marine Fuel
The fuel loaded during bunkering depends on the ship’s engines and the environmental regulations in effect for its route. The most common fuels include:
- Heavy fuel oil (HFO): A thick, residual fuel that was the standard for decades. It’s cheap but high in sulfur, so its use is now restricted under international emissions rules unless the ship has exhaust gas cleaning systems (scrubbers) installed.
- Very low sulfur fuel oil (VLSFO): A blended fuel with a sulfur content at or below 0.50%, which became the default for most ships after the International Maritime Organization’s 2020 sulfur cap took effect.
- Marine gas oil (MGO): A cleaner, lighter distillate fuel often used in emission control areas where sulfur limits drop to 0.10%.
- LNG (liquefied natural gas): A growing alternative that produces significantly lower sulfur and particulate emissions. LNG bunkering requires specialized equipment and cryogenic handling.
The IMO is also developing frameworks for evaluating the lifecycle greenhouse gas intensity of newer fuels, including biofuels and hydrogen-based options, as the industry moves toward decarbonization.
The Bunker Delivery Note
Every bunkering operation for ships of 400 gross tonnage and above requires a formal document called the bunker delivery note (BDN). Under MARPOL Annex VI, the international convention governing air pollution from ships, the fuel supplier must provide this document with specific information:
- Name and IMO number of the receiving ship
- Port and date of delivery
- Name, address, and telephone number of the fuel supplier
- Product name and quantity in metric tons
- Fuel density at 15°C
- Sulfur content (percentage by mass)
- Flashpoint temperature
The supplier’s representative must also sign a declaration confirming that the fuel meets the applicable sulfur limits. This paperwork isn’t just bureaucratic. Port state inspectors can board a vessel and ask to see its bunker delivery notes, and discrepancies between what the paperwork says and what testing reveals can lead to fines or detention of the ship.
Fuel Sampling and Quality Verification
Fuel quality disputes are common in shipping, so sampling during bunkering is a critical step. According to IMO guidance and ISO 13739:2020, representative fuel samples should be drawn continuously throughout the entire delivery at the receiving ship’s manifold, using either an automatic sampler or a continuous drip sampling device.
Best practice calls for at least five samples from each delivery. Three go to the receiving vessel: one designated as the official MARPOL sample (which must be kept on board for inspection), one sent to a lab for detailed fuel testing, and one retained as a backup. A fourth sample goes to the fuel supplier, and a fifth may be held by an independent bunker surveyor in case of a dispute.
Crews are advised to record the seal numbers of each sample bottle on the bunker delivery note and maintain photographic or video evidence showing proper sampling procedures were followed. These samples serve as the primary evidence if a ship later experiences engine problems or if the fuel turns out to be off-spec.
Safety During Bunkering
Bunkering involves transferring large volumes of flammable liquid, so safety procedures are tightly controlled. Before any fuel transfer begins, the ship’s crew and the fuel supplier complete a pre-bunkering checklist covering communication protocols, emergency stop procedures, spill containment measures, and fire safety equipment readiness.
Both parties agree on signaling procedures in advance. An audible alarm is typically used to supplement an emergency stop signal so that everyone involved, on the ship and at the supply source, can recognize it immediately. Scuppers (deck drainage openings) are plugged to prevent any spilled fuel from reaching the water. Smoking and hot work are prohibited in the bunkering area, and absorbent materials and containment booms are kept ready.
During the transfer, crew members monitor tank levels continuously to prevent overfilling. The pumping rate starts slow, increases once connections are confirmed secure, and slows again as tanks approach capacity. Communication between the ship and the supplier remains open throughout, with regular updates on quantities transferred and tank levels remaining.
How Long Bunkering Takes
The duration varies widely depending on the volume of fuel, the pumping rate, and the delivery method. A small coastal vessel taking on a few hundred tons of fuel from a barge might finish in a few hours. A large container ship loading several thousand tons could take 12 to 24 hours or more. Ships typically schedule bunkering to coincide with cargo operations in port so that fueling doesn’t add extra time to the voyage schedule.
Who Pays and How Fuel Is Priced
Fuel is the single largest operating expense for most commercial ships, often accounting for 50% or more of total voyage costs. Bunker fuel is priced per metric ton, and prices fluctuate based on crude oil markets, local supply and demand at each port, and the specific grade of fuel. Major bunkering hubs tend to offer more competitive pricing due to higher volumes and competition among suppliers. Ship operators often employ bunker brokers or fuel procurement teams to find the best combination of price, quality, and port availability along a vessel’s route.