DNV GL issued a technical update to publication “Sulphur Limits 2015” regarding the introduction of new “hybrid” fuel types, which may not be fully compatible with ordinary heavy fuel oils, can pose potential technical challenges in operation in connection with the changeover.

For ships passing through or operating in Emission Control Areas (ECAs), new and stricter emission regulations came into force on 1 January 2015.

As regulated by MARPOL Annex VI, the maximum sulphur content of any fuel used on board may not exceed 0.10% m/m S when sailing or operating in an ECA designated for control of SOx emissions from ships. Alternatively, the installation of exhaust gas cleaning systems, proven to be least effective in terms of emission reductions, is seen as an equivalent measure.

However, switching from heavy fuel oil (HFO) to low-sulphur fuel is likely to pose challenges. If not handled with care, the changeover process can cause operational problems in the ship’s propulsion chain and put equipment at risk.

The DNV GL publication “Sulphur Limits 2015 – Guidelines to ensure Compliance”, issued in November 2014, provides an overview of the regulatory background and describes potential difficulties associated with fuel changeover.

DNV GL has also developed and made available a ship-specific Fuel Change-Over Calculator (FCO) to help ship owners and operators determine the ideal parameters for their vessel’s fuel change-over. This enables them to safely manage challenges associated with switching from HFO to marine gas oil (MGO), which has less than 0.10% sulphur content.

The changeover to MGO requires keen attention, for instance, to variables such as:

  • Vessel’s fuel system layout limitations
  • Risk of thermal shock to the injection components
  • MGO low viscosity to avoid fuel pump failure or seizure
  • Risk of HFO/MGO incompatibility that may clog filters and potentially lead to power loss or even cause the engine to shut down

New "hybrid fuels" entering the market

Since the DNV GL guideline was issued, a range of completely new ultra-low-sulphur fuel types have been introduced to the bunker market. The term “hybrid fuel” refers to a blended product with specifications similar to HFO, and/or to certain refinery product streams that till now have not been used as marine fuels. Therefore, they also do not necessarily fit into the traditional specifications for MGO, MDO or residual fuel oil (RFO).

Although the sulphur content is below 0.10% like ECA compliant MGOs, certain other properties of these so-called hybrid fuels seem – based on manufacturer information – to resemble types of HFO e.g.:

  • Viscosity in the range 15–50 cSt at 50°C (or even higher) requires some heating to enable pumping, but will at the same time reduce the temperature difference compared to HFO (often pre-heated to 130°C) and subsequently the risk of thermal shock on fuel system components, including fuel pumps and injectors, during changeover.
  • The aliphatic nature of these hybrid fuels indicates that ignition and combustion properties are good. This has been verified by engine manufacturers during continuous operation of such fuels on board ships.
  • The flash point is generally higher than for MGOs, hence it will not conflict with ISO and SOLAS marine fuel specification requirements (FP > 60°C).
  • However, due to the mainly aliphatic nature, these hybrid fuels may have a tendency to wax formation which could clog up filters. These fuels should therefore not be stored in tanks in direct contact with the (cold) sea to ensure the temperature is kept well (at least 10°C) above the actual pour point (PP) of the fuel in question. Heating of the storage tanks will usually be required.
  • It has further been noted that the hybrid fuels may have a cleaning effect on, for instance, storage tanks which have not been thoroughly cleaned. The result may be that deposits are released from the tanks and will follow the fuel flow with the potential to clog filters, etc. For a similar reason, it is recommended to use separate bunker and transfer lines, or, if that is not possible to ensure the lines are flushed before bunkering or transferring fuel.
  • Due to the low sulphur content of hybrid fuels, the cylinder lubrication of 2-stroke engines needs to be monitored and a switch to lubricants with a lower base number should be considered to match the lack of acidity and thereby avoid excessive engine wear or even damage. Alternatively, the adjustment of lubricant feed rates according to engine manufacturer may be an option.
  • Similar to the use of MGO for ECA compliance, the on-board use of these new hybrid fuels shall be carefully considered and planned in order to avoid unsafe engine and vessel operation. In particular, the following actions should be followed:
  • Familiarisation with the oil company’s or the fuel supplier’s information about the fuel characteristics and properties, and the following of the recommendations or best practices given with regards to storage, treatment and operation on board.
  • Familiarisation with recommendations provided by the manufacturer or designer(s) of the engines installed on board (main and auxiliaries) with regard to the suitability of the fuel and of possible restrictions. Most likely, there will also be guidance given for the fuel changeover procedure. The same applies to boilers if also intended to use the hybrid fuel.
  • Components of the fuel system, such as pumps, should be checked for suitability in operation with the new fuel type. Since some fuel suppliers recommend that these hybrid fuels are treated in the same way as common HFO with respect to heating and separation, the availability of fuel treatment systems should be checked and confirmed. If available, separator settings such as gravity discs may need to be adjusted accordingly.

Incopatibility issues during changeover

Although hybrid fuels have successfully been tested during continuous engine operation, some uncertainty still remains with regard to the changeover from conventional HFOs.

Operational feedback from practical changeover processes from HFO to hybrid fuels is currently almost non-existent, and the main concern is related to potential incompatibility issues.

Based on previous experience, fuels with very different chemical compositions have a tendency to be incompatible when mixed in certain blending ratios.

Whilst the hybrid fuels are known to be basically aliphatic in nature, conventional HFOs generally consist of highly complex aromatic structures.

All rules of thumb would indicate incompatibility between these types of fuel with the likely precipitation of sludge and clogging of the fuel/engine system as a result.

Laboratory testing of the individual HFO and hybrid fuel components prior to use may provide some guidance, but since these fuels will be mixed in various ratios in the fuel/engine system, an exact prediction of the behaviour will be difficult in practice.

Careful monitoring of the fuel performance during the changeover process is therefore advisable. Starting this process well in advance of entering the ECA area is also advisable to ensure the changeover process is completed in a safe manner.

Detailed procedures for the changeover process should be established for the specific ship in question. Such procedures should include recommended actions to handle any potential emergency situations that could occur in connection with changeover operations.

 You can view the technical update by clicking below:


Source: DNV GL

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