Why diesel and jet fuel need a clean tanker
Crude and refined products rarely share a ship. What makes a tanker clean, why jet fuel is the strictest cargo on the water, and which ship sizes we use for products.
Tankers come in two trades. Dirty tankers carry crude oil and fuel oil. Clean tankers carry refined products: diesel and gasoil, gasoline, jet fuel and naphtha. A clean ship can go dirty at short notice; going back takes tank cleaning and usually several voyages on ever cleaner cargoes, so owners rarely do it. That is why our calculator prices products on product tankers only.
Coated tanks
Product tankers usually have coated cargo tanks, most often epoxy. A coated tank cleans faster and holds less residue than bare steel, which matters when the next cargo is jet fuel or gasoline. A typical MR product tanker has 6 to 14 cargo tanks served by 2 to 7 separate pump and line systems, so it can carry several grades without mixing them.
Jet fuel: the strictest cargo
Jet fuel may contain no more than 50 mg/kg of FAME, the biodiesel component, under the ASTM D1655 and DEF STAN 91-091 specifications; some pipelines still apply 5 mg/kg. European EN 590 diesel may contain up to 7 % FAME. A diesel cargo directly before jet is therefore the riskiest change of cargo in the clean trade. Charterers usually forbid FAME diesel, vegetable oils and chemicals among a ship's last three cargoes before jet, and ask for tank cleaning, wall-wash tests and flushed lines.
The arithmetic shows why traces matter. 0.6 tonnes of diesel left in tanks and lines, at 7 % FAME, spread through 30,000 tonnes of jet fuel gives about 1.4 mg/kg: well under the limit when it mixes evenly. Real cargoes do not always mix evenly, so a single pocket of residue can still fail a sample.
Other things that go wrong
- Diesel after gasoline. A little gasoline lowers diesel's flash point, and EN 590 diesel must flash at 55 °C or above. Lines and pumps are drained, and the flash point is tested at loading.
- Sulphur. Diesel and gasoline sold at 10 ppm sulphur have little margin; a few ppm carried over from gasoil, marine fuel or fuel oil can put them off-spec.
- Water. Jet fuel must be clear and bright. Free water is drained at the terminal and filters are checked.
An independent inspector usually samples the shore tanks and the ship at both ends and checks the tanks before loading, so a dispute can be traced to the shore or to the ship.
Ship sizes for products
- MR, about 37,000–52,000 tonnes deadweight: typically 38,000 tonnes of diesel. The workhorse for Baltic and Black Sea diesel, transatlantic trade and intra-Asia.
- LR1, 60,000–80,000 tonnes: about 64,000 tonnes of diesel. Middle East and India to Europe, Asia to Australia.
- LR2, 80,000–120,000 tonnes: about 95,000 tonnes of diesel. Long hauls from the Gulf and India to Europe, and naphtha to Japan.
Bigger ships cost less per tonne on long routes but need deeper berths and larger buyers. Our calculator chooses among these classes by the size of the cargo and prices the freight from the day rates listed product-tanker owners report.
Our catalog lists the products we sell, each with its specification limits: EN 590 diesel, marine gasoil, EN 228 gasoline, Jet A-1 and naphtha.