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Article: IMO 2020 - Bunker Tank Cleaning & Preparation To Receive Compliant Fuel

News & Insights 17 July 2019

Members who have chosen to achieve compliance with the regulations by using low sulphur fuel oil, will need to start preparatory and precautionary steps several months ahead of the regulatory enforcement date.


To achieve compliance with the sulphur cap, simply switching to a low sulphur fuel at January 2020 will not be enough. High sulphur fuel oil (HSFO) is primarily based on residual fuel and these tend to stick to the inside of fuel tanks and pipelines forming layers of sludge and sediments.

The accumulated layers of existing sludge in the fuel tanks and pipelines may contaminate the new fuel loaded, rendering the fuel non-compliant and pushing emissions above the 0.50% sulphur cap. Cleaning this out will require exhaustive efforts - either manually which is risky, expensive and time-consuming, or with specialised fuel treatments (chemical additives).

As such, members who have chosen to achieve compliance with MARPOL Annex VI regulations by using low sulphur fuel oil, will need to start preparatory and precautionary steps several months ahead of the regulatory enforcement date.

This process is also an important measure to avoid compatibility and stability issues while transitioning to 0.50% m/m compliant fuel from high sulphur fuel oil (HSFO).


Options for preparing fuel oil tanks for compliance include flushing through fuel systems, manual cleaning of tanks during dry docking, manual cleaning during service or cleaning tanks in service with specialised additives.

The process of fully cleaning and flushing fuel oil tanks and piping systems requires advance planning; and at a minimum, the plan should provide guidance on:

  • fuel oil tank capacity and segregation capability for different fuel grades
  • fuel oil system modifications (if needed)
  • assessment of the new fuels on engines and machinery (compatibility testing)
  • fuel oil changeover plan
  • risk assessment and mitigation plan. 

It is also prudent to consult the engine maker on the appropriate method of preparation of the bunker fuel oil tanks and flushing of fuel oil piping system.

The overall fuel oil management plan, including tank cleaning/preparation procedures, should form a part of the ship implementation plan (SIP), taking into consideration the ship’s trading pattern, docking etc.

While the supply of fuel is the charterer’s responsibility, the responsibility of managing it onboard will be for the owner. As such, shipowners are recommended to reach an agreement with their charterers on when to undertake tank cleaning and purging of the fuel oil lines because the vessel may have to be taken out of service for some days.

Such agreement should cover scenarios when the ship is supplied with a non-compliant fuel oil – as post-2020, not all ports will provide discharge facilities for the reception of the high sulphur fuel oil. Even if discharge facilities are available, costs may need to be incurred for cleaning of the fuel oil tank before loading compliant fuel oil to avoid contamination.

Cleaning methods

Manual cleaning during dry-docking

The advantage with this method is that a ship will undergo cleaning of all its fuel oil tanks and fuel service systems and will be able to start loading compliant fuels and expect to be fully compliant right away.

The time required for manual cleaning of fuel oil tanks in shipyards may vary depending on the advance preparation carried out, tank condition (amount of remaining sludge) and efficiency of the chosen shipyard. In addition to the cleaning of tanks, all the pipework in the fuel oil transfer and service system would need to be flushed through thoroughly.

Manual cleaning while the ship is in service

If tanks are to be cleaned manually during service, risk assessment and safety measures for entering enclosed spaces would need to be specially considered and managed. The risk assessment and mitigation plan will also need to consider the handling of tank cleaning equipment and associated hazards.

The factors affecting the time required are similar to the manual cleaning in shipyards, except in this case a lot would depend on the number of crew available to perform the task. The tanks need to be empty before they can be cleaned. While the ship is in service, this would mean transferring the tank contents to a designated tank for final consumption or temporary storage and subsequent removal. The residues from tank cleaning will need to be retained on board until they can be properly disposed to shore reception facilities.

Cleaning by specialised fuel treatments (chemical additives)

Cleaning the sediments and sludge from bunker tanks and fuel systems by chemical additive doses could be considered as an alternative to manual cleaning. Use of such additives usually involves a gradual clean-up period conducted over a few bunker cycles prior to loading compliant fuel. This ensures a smooth system clean-up by preventing excessive sludge blockages. As such products would be dosed directly into the fuel tank, it would clean the whole fuel oil service system, including settling and service tanks. However, depending on the amount of accumulated sludge inside the tanks and potency of the chemical additives, it may take several months for the tank to be clean.

Members are advised to take proper consultation with chemical additive manufacturers and engine makers to ensure that the chemical additives have no adverse impact on the ship’s machineries.


With the 2020 deadline approaching, members need to be mindful that preparations for accepting any new compliant fuel would need to start well in advance.

Ship implementation plans need to include all such considerations and calculations for the time needed for ships to be fully flushed of all fuel oils exceeding the applicable sulphur content prior to entry into force of the regulation.

Categories: Alternative Fuels, Pollution

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