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Marine Fuel Oil Microbiological Infestation

Minute micro organisms in fuel oils
: Minute microorganisms, i.e. bacteria, can exist in lubricating oils and fuel oils. Under suitable conditions they can grow and multiply at phenomenal rates. Their presence leads to the formation of acids and sludge, metal staining, deposits and serious corrosion. The presence of slime and the smell of rotten eggs (hydrogen sulphide) indicates a contaminated system.

Water in a lubricating oil or fuel oil, oxygen and appropriate temperature conditions will result in the growth of bacteria and infestation of a system. The removal of water, or ensuring its presence is at a minimum, is the best method of infestation prevention. The higher the temperature in settling, service and drain tanks holding fuel or lubricating oils, the better.

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Test kits are available to detect the presence of bacteria, and biocides can be used to kill all bacteria present in a system. The system must then be thoroughly flushed out.

Fuel-water emulsification (FEW) – Invisible smoke technology is based on fuel-water emulsification (FEW), whereby the heavy fuel or diesel oil is homogenised with fresh water before injection into the engine. Apart from lowering smoke and soot emissions significantly, the system is claimed to reduce NOx levels to.

Fuel viscosity – A measure of resistance of the fuel to flow at a stated temperature. According to ISO, the kinematic viscosity of heavy fuels should be specified in centistokes (cSt = 10-6 m2/s) at 100°C.
When burning heavy fuel oil in a diesel engine, it is necessary to reduce the high viscosity of the fuel oil to a value at which correct atomisation and combustion can take place. This will allow for correct mixing and burning for efficient combustion.

Standard treatment: The recommended standard treatment of residual fuel to be used in large diesel engines is to first allow the fuel to settle in a settling tank where any water/sludge can be drained off. It is then purified and clarified to the service tank, the purifier further removing any water or sludge and the clarifier removing any solids. Before reaching the engine from the service tank, it will pass through a fine filter which will remove any particles larger than 50 microns, or even less for smaller engines.

Bunkering option : It is advisable to store ship's bunkers from different ports in separate bunker tanks as they may not be compatible. If this precaution is neglected, there is a risk of heavy sludge formations occurring due to incompatibility and not mixing successfully.

General Safety and good practice with regard to Fuel Oil Systems and pipelines

This procedure shall ensure that full and proper maintenance is carried out to accepted and safe standards. Leakage from fuel pressure systems are a serious fire hazard. The PMS is required to include the following checks:
  1. Fuel pump restraining bolts must be proven tight by testing with a torque spanner at frequent intervals. Please include such a test in your PM system with an interval of 3 months.


  2. The supports and retaining devices of the low pressure fuel system must be checked at regular intervals and be proved tight and providing adequate restraint. The lining of such devices must also be examined for wear and renewed if they provide insufficient support. Please include such a test in your PM system with an interval of 3 months.


  3. Flexible pipes must be closely examined and renewed if signs of material cracking or deterioration are evident. Extra care must be exercised in the tightening of these pipe connections to ensure that they are not twisted when re-installed.


  4. All gasket and seal ring materials, and any jointing compounds used must comply with the requirements of the engine manufacturer.


  5. Compression fittings are to be carefully examined and tightened (but not over-tightened), in accordance with the manufacturer’s specification. Copper piping must be heat treated (annealed) and sufficient supports fitted to prevent vibration


  6. All component locking devices, such as spring and tab washers, locking wires, etc. must be present and currently fitted.


  7. Spray or deflection plates and insulation must be correctly replaced after maintenance to reduce the possibility of fire in the event of fuel leakage.


  8. The installed system is to be inspected at regular intervals for the level of vibration present and evidence of fatigue stresses to welded or brazed pipes and connections.


De-Sludging & Cleaning of Fuel Tanks

It is essential that all fuel tanks are regularly "de-sludged" using the sludge cocks provided. They must never be operated by artificial means. The presence of water in appreciable quantities must be immediately brought to the attention of the Chief Engineer.

This is to be carried out at four hourly intervals on a conventional watchkeeping vessel. Prior entering a bad weather area and whilst in heavy weather conditions, the frequency of draining must be increased. On vessels operating for periods of time with unmanned engine room spaces, the interval between sludging during the manned period is to be four hours.

For the unmanned period, this is to be detailed by the Chief Engineer in the Night Order Book, however it is to be not less than three times during that period i.e. at commencement of the unmanned period, during the late evening inspection of the Engine Room spaces, and at the end of the unmanned period. All other double bottom and/or deep bunker tanks are subject to Classification inspection and would normally only be inspected on a 60 month frequency.

Each Service and Settling fuel tanks, on a rotational basis, are to be emptied at intervals not exceeding 36 months or at Dry Dock. This is for the purpose of sighting the internal condition, in particular any build up of sludge, solids and other contaminants. Manual cleaning is to be done if applicable, during a safe period at a safe location, with suitable notice to the Master. Testing of all float alarms where applicable should be done at this time.

It is strictly forbidden to enter a fuel tank unless the appropriate "enclosed space entry permit" conditions have been complied with and the permit issued. Known contamination with solids such as "Catalytic Fines" can only be dealt with by taking the tank out of service, isolation, and subsequent manual cleaning.

The opportunity must be taken when tanks are cleaned to test any heating coils fitted, and check/overhaul all the tank mountings including examination of vent pipes for obstruction etc.


Heating of sludge oil tanks, waste oil and oily water bilge tanks in order to extract water

Care must be taken when using internal heating coils. The heating of a water/oil mix from any such oil or sludge tank must be deemed as a critical operation . It should be kept in mind that any heating of the sludge/oil water mix will expand the volume of the liquid and may cause an overflow of the tanks. A maximum filling level for such tanks must be identified that leaves a safety margin for this expansion. This level should be recorded on the checklist.


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