Answer
From 1 January 2020, MARPOL Annex VI Regulation 14 limits fuel sulphur globally to 0.50% m/m outside SOx ECAs; inside SOx ECAs the limit remains 0.10% m/m.
Core Operational & Technical Issues:
Fuel Instability and Sludging: VLSFOs are typically blended fuels combining paraffinic, aromatic, and naphthenic cutter stocks. If the blend is unstable, heavy asphaltenes precipitate out of suspension, leading to massive sludge accumulation in settling/service tanks, rapid clogging of fuel filters, and purifier choking within hours.
Incompatibility During Fuel Mixing: Mixing two stable VLSFOs from different bunkering ports inside the same storage tank often triggers severe fuel incompatibility. The resulting blend breaks down, causing severe filter blocking, pump starvation, and blackouts.
Cat-Fine Abrasive Wear (Al+Si): Refining processes to produce low-sulfur fuels use aluminum and silicon catalysts. Residual catalytic fines (cat-fines) act as a harsh abrasive paste. If purifiers fail to reduce cat-fines below 15 ppm, they cause catastrophic abrasive scuffing and rapid wear of fuel pump plungers, injectors, piston rings, and cylinder liners.
Cold-Flow Properties & Waxing: Many VLSFOs have high Pour Points and high Cold Filter Plugging Points (CFPP). If fuel temperatures drop below its Cloud Point in storage or transfer lines, paraffin wax crystals precipitate out, completely blocking suction lines, filters, and transfer pumps.
Low Viscosity & Lubricity Issues: Some VLSFO blends have surprisingly low viscosities (<10 cSt at 50∘C). Low viscosity reduces the fuel's lubricating film strength, causing sticking/galling of fuel pump plungers, fuel valve needles, and internal leakage leading to starting difficulties.
Combustion Delays & CCAI Issues: VLSFOs with high Calculated Carbon Aromaticity Index (CCAI) values exhibit poor ignition quality. This causes delayed ignition, rapid pressure rise (ΔP/Δθ), severe engine knocking, dirty combustion, carbon trumpeting on injector nozzles, and high exhaust temperatures.
Cylinder Lubrication Mismatch: Switching from High Sulfur Fuel Oil (HSFO) to 0.50% VLSFO requires matching the Base Number (BN) of the cylinder oil. Using high-BN lubricants (e.g., BN 70/100) with low-sulfur fuel leads to excessive calcium deposit buildup on piston tops and ring grooves, causing bore polishing and ring folding. Conversely, using too low BN leads to cold corrosion.
| Problem Area | Mitigating Onboard Action |
|---|---|
| Fuel Incompatibility | Maintain strict tank segregation—never mix fuels from different bunker barges in storage tanks. Keep tank mixing below 2% to 5% during unavoidable transfers. |
| Cat-Fine Management | Operate fuel purifiers at minimum optimal flow rate and maximum recommended temperature (98∘C) to maximize separation efficiency. |
| Fuel Heating & Storage | Keep fuel storage/settling tank temperatures at least 10∘C above the fuel's Pour Point to prevent wax formation, but below the flash point limits (60∘C). |
| Cylinder Lubrication | Adjust cylinder lube oil BN (typically to BN 40–Line / BN 70 blend depending on engine builder guidance) and optimize feed rates via electronic lube systems (e.g., Alpha ACC). |
| Viscosity Control | Monitor fuel temperature at the engine inlet viscometer carefully to maintain injection viscosity within the builder’s limits (typically 10–15 cSt). |