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Bunkering — the supply of fuel to a ship — is one of the largest single commercial transactions a vessel undertakes in port. Fuel is the dominant operating cost for most vessel types (often 40–60% of voyage cost), and the quality of what goes into the tanks has a direct impact on engine performance, maintenance costs, and regulatory compliance. A bad bunker delivery can damage machinery, trigger port-state detention for an off-spec fuel certificate, and expose the operator to claims from charterers for off-hire.
The legal framework is set by MARPOL Annex VI Regulation 18, which requires a Bunker Delivery Note (BDN) for every fuel delivery, and by ISO 8217 (the internationally recognised marine fuel quality standard). Above those minimum requirements, the commercial, technical, and contractual management of bunkering is the joint responsibility of the shipowner/operator, the charterer (where the charterparty places bunkering obligations on the charterer), and — on board — the Master and Chief Engineer.
MARPOL Annex VI Regulation 18 requires the bunker supplier to provide a BDN for every delivery of marine fuel, and requires the ship to retain it for at least three years. The BDN must state:
Port state control officers inspect BDNs routinely. Missing, incomplete, or inconsistent BDNs are a deficiency that can lead to detention. The BDN figures are also the starting point for any commercial short-delivery dispute.
ISO 8217:2017 (currently the standard; a 2024 revision under finalisation) defines two families of marine fuel: residual (the RM series, from refinery bottom products) and distillate (the DM series, cleaner middle distillates). The most commercially significant change in recent editions was the addition of parameters relevant to VLSFO blends, including limits on pour point, cold filter plugging point, and acid number.
Lower-viscosity residual grades; RMA 10 and RMB 30 are common in warm-weather ports. Sulphur content varies — must meet the applicable cap for the area where the fuel will be burned.
Higher-viscosity residual grades — RMG 380 ('380 cSt fuel') is historically the most traded marine residual fuel. RMK 700 is used in vessels with heating capability and high-power main engines. These grades have higher potential for cat fines (aluminium-silicate particles from refinery catalytic cracking) which damage engine components.
ISO 8217 residual grades blended to meet the 0.50% m/m global sulphur cap in force since 1 January 2020. VLSFO is not a single standard grade — compatibility issues between different suppliers' blends can cause sludging and fuel system blockages. The ISF/IACS/CIMAC Compatibility Guidance applies.
Marine Gas Oil (MGO) — the standard ECA-compliant distillate fuel (max 0.10% sulphur). Used when transiting ECAs or operating vessels that cannot use residual fuel. Clean, low-viscosity, no preheating required.
Marine Diesel Oil — similar to DMA but permits a trace of residual blending. Less common.
Ultra Low Sulphur Fuel Oil — distillate-equivalent grades for ECA use; ULSFO (max 0.10%) used in ports and ECAs where residual fuels are impractical.
Since 1 January 2020, MARPOL Annex VI Regulation 14 limits the sulphur content of marine fuel used outside ECAs to 0.50% m/m. The alternative to using compliant fuel is an approved exhaust gas cleaning system (EGCS / scrubber) that reduces SO₂ emissions to the equivalent level. Inside Emission Control Areas (ECAs), the limit is 0.10% m/m.
MARPOL Annex VI Regulation 18 requires a representative sample of the delivered fuel (the MARPOL sample) to be retained on board for not less than 12 months. The sample must be taken using a continuous drip sampler at the point of delivery (the bunker manifold of the receiving vessel), sealed, and signed by both the supplier's representative and the Chief Engineer (or officer in charge of the bunker operation). A second "commercial sample" may be taken simultaneously for the owner's or charterer's records.
If a fuel quality dispute arises — engine damage, off-spec PSCO check, or a supplier claim — the MARPOL sample is the primary physical evidence. A sample sealed and signed only by one party is significantly less persuasive in arbitration than one signed by both.
Air or nitrogen is injected into the bunker hose or pipeline during delivery, creating a frothy emulsion that occupies volume in the flow meter without delivering the mass of fuel. The meter records a higher quantity than is actually delivered. Also known as 'aerated bunkers'. Mitigation: independent flow-meter witness, mass flowmeter (Coriolis type) preferred over volume flowmeter.
Fuel that meets the sulphur limit but fails ISO 8217 limits for viscosity, water content, density, flash point, acid number, or cat fines. Cat-fine damage (from aluminium oxide particles) can destroy injectors and piston rings within hours of operation. Testing the fuel before use and treating (centrifuging) it on board is essential.
In some ports, the BDN presented does not reflect the fuel actually delivered — quantity, density, or sulphur figures are falsified. Where the ship subsequently fails a port-state fuel check or sustains engine damage, the counterfeit BDN leaves the operator exposed. Independent sample testing and a contemporaneous density measurement on board are the defence.
Lowering the declared density of the delivered fuel in the BDN reduces the calculated mass (tonnes = volume × density). A density understated by 0.010 t/m³ on a 1,000-tonne delivery produces a 10-tonne shortfall in the record but the volume meter shows the agreed figure.
LNG bunkering is established for newbuilding dual-fuel vessels; LNG is sulphur-free and reduces NOₓ by approximately 85% and CO₂ by approximately 20–25% vs HFO. Methanol bunkering is operational on a growing number of vessels (particularly container ships and ferries). Ammonia is under active development as a zero-carbon fuel; the first ammonia-fuelled vessels are expected in commercial service by 2026–2027. Hydrogen (green) remains at the pilot stage. Each alternative fuel requires dedicated safety procedures, bunkering infrastructure, and crew training not yet covered by a single unified IMO code — the IGF Code (covering LNG and low-flashpoint fuels) is the current regulatory basis, with further amendments expected.
The Master and Chief Engineer have both the right and the duty to refuse a bunker delivery that is visibly or documentarily off-specification — incorrect sulphur figure, wrong grade, suspicious density, visible contamination, or lack of a valid BDN. The refusal should be recorded in a letter of protestserved on the bunker barge master, and the owner, manager, and P&I club should be notified immediately. Where delivery is accepted under protest (for commercial necessity — the vessel cannot sail without fuel), the protest letter must be served before hoses are connected, and both the MARPOL sample and a commercial sample must be taken and dispatched for independent analysis at an accredited laboratory.