With the IMO global sulphur cap just around the corner, most ships will be expected to use low sulphur fuel oil in compliance with MARPOL Annex VI by 2020. In response to the industry-wide concerns over the quality and stability of compliant fuels and the Members’ enquiries over the handling strategies, this article is compiled to help understand the characteristics and management of the 0.50% fuels.

I.      Marine Fuel Classification

Marine bunker fuels, mainly obtained from petroleum, can be divided into distillate and residual. A third type is a mixture of these two, also known as marine diesel oil (MDO) or intermediate fuel oils (IFO).

Distillate fuel, commonly called marine gas oil (MGO), is composed of petroleum fractions that are separated from crude oil in a refinery with a distillation process. Under Marine Fuel Oil ISO 8217:2017, distillate fuels are further graded as DMX, DMA, DFA, DMZ, DFZ, DMB and DFB. Fatty acid methyl ester, FAME, generally used as fuel in diesel engine vehicles can also be used on ships if no more than 7% v/v FAME is blended in the fuel according to ISO 8217 requirements.

Residual fuel, or heavy fuel oil (HFO), has an asphaltene content of between 3-10%. It also includes other products such as high sulphur fuel oil (HSFO), low sulphur fuel oil (LSFO) and ultra-low sulphur fuel oil (ULSFO). It was further graded as RMA, RMB, RMD, RME, RMG and RMK in ISO 8217 depending on their kinematic viscosity.

II.     Standards of Compliant Fuel

1.      Statutory requirements: SOLAS Chapter II-2, Part B, Regulation 4 includes limitations in the use of oils as fuel and arrangement for oil fuel, such as no oil fuel with a flashpoint of less than 60ºC shall be used. Mandatory specific requirements for fuel oil contents, additives and safety were also made clear in MARPOL Annex VI Regulation 18.3. Verification of compliance with these requirements falls within the jurisdiction of the Port State in which the suppliers are registered.

2.      General standards: ISO has published a Publicly Available Specification (PAS) 23263:2019 in September 2019 with only updates to the 0.5% m/m fuel specifications. It’s used in conjunction with ISO 8217, which defines the limit for fuel oil suppliers in respect of physical properties, composition and performance indicators as given in Table 1 and Table 2 requirements. ISO 8217 Clause 5 also requires the fuel delivered to be a homogenous blend and free from materials that could cause harm to an engine or people.

III.     New Fuel Risks

1.      Stability issues: Stability is the potential for a fuel to change condition in storage in certain circumstances, depending on its resistance to breakdown. It is dependent on the nature of the liquid hydrocarbons in which the asphaltenes are suspended. If the medium is aromatic (hydrocarbons in ring formation) then they will remain in suspension but if it is paraffinic (linear hydrocarbon formation) the asphaltenes may have a propensity to coalesce into sludge.

Once a fuel has chemically broken down there is no way to satisfactorily reverse the process. Precipitated asphaltene cannot be re-dissolved. So, bulk fuel stored for long periods can precipitate out of solution causing the formation of sludge, then blocking filters and pipes.

2.      Compatibility issues: Compatibility is the tendency of fuels to produce deposits when mixed. The issue may immediately occur when fuels comingle. Individually, the new fuels may pass the stability criteria of the ISO 8217 but become unstable when mixed – therefore incompatible.

The composition of blends is anticipated to vary significantly between regions. These differences raise a serious concern of incompatibility; and not just incompatibility between different products but even between batches of the same product. Incompatibility results in sludge formation caused by precipitation of asphaltenes. This leads to blocking of filters, centrifugal separators and, in extreme cases, fuel pipes. The risk of losing propulsion or electrical power becomes very high.

In practice it is often not possible to ensure against a degree of comingling and although the rule of thumb is not to comingle or load on top in excess of 20% mix ratio, the only way to ensure there are no issues is to test a sample mix.

IV.   Testing for Stability and Compatibility

Although aspects like compatibility, viscosity, density, water and catalyst fines can be tested on board with portable test kits, such indications do not provide a basis on which to challenge whether the specification has been met, unless the results are confirmed by an analysis at an accredited shore-based laboratory.

One of the good ways to measure the compatibility of marine fuels on the ship is the ASTM D4740 spot test. For this test a blend composed of representative volumes of the sample fuel and the blend stock is heated and homogenized. The ideal percentage mix is 50/50 as this is the worst-case scenario. A sample is put on a test paper, placed in an oven and heated to 100°C. After 1 hour, the resultant spot is examined.

1.      Homogeneous spot with no inner ring: the fuels are compatible and stable.

2.      Faint or poorly defined inner ring: the fuels will deposit some sludge and should be handled carefully. Do not overheat, increase purifier and blow down frequency.

3.      Well-defined inner ring: as for 2 but increased sludge potential.

4.      Well-defined inner ring and somewhat darker than the background: the fuels are incompatible and unstable.

5.      Very dark solid or nearly solid area in the center: the fuels are incompatible and unstable.

V.     Other Considerations

1.      Cold flow properties – New blended products with a high distillate content are more paraffinic in nature and could be prone to wax formation at lower temperatures.

2.      Lubricity – new blended products containing desulphurised components are expected be prone to lubricity problems (eg sticking of fuel pump plungers) due to the aggressive nature of the desulphurization process. These issues may result in accelerated wear of fuel pumps and injection equipment; therefore, additives may need to be added to the fuel.

3.      Cat fines – Elevated levels of cat fines may lead to accelerated abrasive wear of engine fuel pumps, injectors and cylinder liners.

4.      Viscosity – large difference in viscosity (may range between 10~380 cSt) will affect fuel injection and combustion efficiency. Decrease in fuel oil viscosity may also cause an increase in fuel oil leakage between the fuel pump plunger and barrel of diesel engines. Internal leakages in the fuel injection system may result in reduced fuel pressure to the engine, which may have consequences for the engine performance.

5.      Density – the new fuel blend densities will in general be lower than conventional residual fuel oils. This may require adjustment of centrifuges to ensure adequate cleaning of the fuel oil.

6.      Total Acid Number (TAN) – some compliant fuels derived from plant and animal matter may not contain sulphur but may still have a high acidic content since the fuel is derived from fatty acids. Fuel oil with a high acid number may cause accelerated damage to various parts of the combustion unit including the fuel injection equipment.

VI.     Advice to Members

1.      Carefully consider the ship tank configuration and fuel system adjustment. A fully segregated fuel system for distillate fuels and blended fuels is recommended.

2.      Fuel oil with very low viscosity may lead to leakages, increased wear or seizure of fuel oil pumps. Members may consider installing fuel pumps and injection nozzles suitable for such fuels. Engine and boiler makers should be consulted to ensure its safe and efficient operation. Also, some parts of the fuel oil supply system may need to be overhauled to ensure integrity. Implications for validity of NOx certification (EIAPP Certificate) should be considered.

3.      Prudently arrange heating for different types of compliant fuels according to their characteristics.

4.      Make sure the lubricating oil and/or cylinder oil in use is appropriate for the sulphur level of the fuel being used.

5.      Be careful with the lubricity problems, micro-bled contamination, high acid corrosion, and cold flow properties of distillate fuels.

6.      Check with the bunker suppliers on the extent to which biodiesel is in the fuel when procuring distillates containing FAME. Advice on whether such fuels are applicable should also be sought from manufacturers of engines, boilers, and other auxiliary equipment such as oily water separators and filters. Such fuels should be analyzed to check the total acid number (TAN) and oxidation stability, as well as checks on presence of microbial contamination. It is recommended to avoid storage periods in excess of six months to avoid fuel oxidation and degradation that could damage equipment if used in this condition.

For further information, please contact your manager at the Club.