Settling Tank Full of Sludge Caused a Ship to Drift Outside the ECA!


From 1st January 2015, ECA came into force and this was followed by new Chinese ECZs where fuel sulphur % are limited to 0.1% and 0.5% respectively – This has been a game changer in the marine fuel industry.  There has been many cases where ships lost propulsion or power due to fuel change over and many industry leaders have offered guidance for smooth low sulphur fuel operation.

Some of these guidelines can be accessed under:

  1. Exxon Mobil – 5 Tips for fuel change over
  2. ECA Legislation – Exxon Mobil
  3. Alfa Laval & Viswa Lab – Handling the ECA fuels – Purification related Questions
  4. Viswa Lab – 8 Important Points to consider – ECA fuels

Exxon Mobil HDME 50 – Best practices

Shell ULSFO & Exxon Mobil HDME 50 has reportedly obtained ‘NO OBJECTION’ certificate from the engine makers.  We asked Viswa Lab for a feedback and quality of ULSFO where Mr. Bhavnani (Vice-President) commented that the quality of the ULSFO’s in general are good and further added that it had better combustion/Ignition properties.

There could be many other suppliers offering ULSFO & provide guidance for safe usage of such fuels.  Despite several such guidances, problems do occur – which mostly are attributed to human error (operational error) & bad ship operational standards.  

Here is one interesting case – where the vessel bunkered ULSFO as the vessel was scheduled to transit through the ECA.  The vessel changed over to the low sulphur fuel and used it without any problem.  The ULSFO was stored in one of the two dedicated settling/service tanks.  The vessel, on exiting the ECA, changed over to normal fuel (sulphur > 0.1%) and the remaining ULSFO was stored for next ECA usage.

Due to commercial/operational pressures from various sources, and to stem maximum bunkers to the next port, the vessel mixed two different fuels in one tank.  (Name of the tanks, quantities and other details removed).

After a month, the vessel entered ECA again and the ship staff changed over to the ULSFO which was stored in the dedicated settling/service tank (Note: It is suspected that some different grade of fuel has been added on top of the ULSFO and stored for several weeks).


The ship’s purifier filters, purifier and all other system filters gradually clogged.  Both the main and auxiliary engines were unable to maintain fuel pressure.  When attempted to drain the service and settling tanks, no oil was coming out of the drains.

The vessel stopped and the crew changed over the diesel generators on to a distillate fuel.

What Happened?


The tank level was lowered by high suction or reportedly by other means.  Upon opening up the tank, the ship crew landed in a surprise where they found loads of sludge inside the tank.


Wilden pumps – unable to suck the sludge.


Manual cleaning of the tank – tank entry – was adopted and shovels were used to remove the sludge.





The ship’s crew tried to dissolve sludge with gas or diesel oil and the additive which was available onboard.  It could be possible that the sludge has already formed and predominant that the gas oil and additives had less or no effect on it.


Could this be used in the engines?


When it comes to internal oil transfer or bunkering of fresh fuel, it is inadvertent that some mixing of two different fuels will happen as a part of ship’s operation.  If the vessel’s endurance is not adequate, at times ship’s staffs are forced to co-mingle two different fuels in order to maximize bunker in-take.

Thus, this technical write-up would help to understand critical points to consider before co-mingling two different fuels.

What is Stability?

  • Stability is the propensity of the fuel to stay stable under long storage conditions. Typically fuel oils can be stored for at least a year without significant changes.  
  • ISO 8217 specifically states that “The fuel shall be a homogeneous blend of hydrocarbons derived from petroleum refining.”  This means that if the fuel is not stable, it is not meeting the specification.

How To Check a fuel for its Stability?

  • A simple ASTM D4740 test gives a good idea both for compatibility between two fuels to be blended and also the stability of an already blended fuel.  These tests can be carried out onboard.  You can also equip a compatibility test kit to keep onboard.  Ships are equipped with cooling water and boiler water test kits and it is recommended to consider equipping ships with a simple on board stability/compatibility test kits.  Write to MFAME ( to know more about the on board compatibility/stability test kit.  Ship staff can use this to assess the compatibility.
  • Lack of stability occurs when there is not enough aromatics to support the asphaltenes in the fuel.  This throws the asphaltenes out of solution and the precipitation of this substance leads to filter and purifier choking with possible risk of machinery coming to a halt.
  • From the supplier’s side, the co-mingling or blending of bunker fuels is a must.  The problem occurs if he uses a low quality and low cost cutter stock.  Problems also occur when the supplier tries to do line blending when delivering the fuel to the ship.  This can lead to wrong sampling, stratification and other problems.
  • From the ship’s point of view, it is ideal if the storage tank is empty to receive fresh bunkers.  Often the necessity to blend with an existing fuel in the tank leads to problems particularly if the two fuels are not compatible.  
  • Blending onboard is not advisable.  If it is unavoidable, first check for compatibility.   Secondly, the blending has to be carried out in a certain order such as a lower density fuel should be pumped on top of a higher density fuel, circulate the fuel as much as possible, heat the fuel to achieve mingling through thermal convection currents.  Keep in mind that two perfectly stable fuels can become unstable when blended.
  • The stability of the fuel can be categorized into blending stability (which is already covered above) and secondly, storage stability.  When the fuel is heated for a long time, degradation of stability may take place with thickening and asphaltene precipitation.
  • Please keep in mind that waste lube oil, contaminants in the fuel, excessive quantities of low quality cutter stock such as shale oil etc. will contribute to instability in the fuel.
  • Two other tests that can be employed to assess the potential of the fuel to become unstable are Xylene Equivalent and Reserve Stability Number.  It is possible to identify fuels which are fully stable or which are on the verge of becoming unstable.
  • Adding good fuel to an unstable fuel which has already precipitated asphaltene may not help.  It is possible that some additives may help.
  • When switching from HFO to distillate fuel while approaching port or similar conditions, there is a risk of incompatibility between the two fuels.  The asphaltenes could precipitate leading to filter clogging.
  • In addition to compatibility test which can be carried out onboard, a mini blending test can be carried out with half to one liter of fuel samples.  In an emergency, the best way is to take the settling tank level to the bottom and then put the second fuel into the settling tank.  A certain amount of incompatibility may occur but it will be restricted to the small quantity in the settling tank.
  • Please keep in mind that all stability/compatibility tests are carried out in a 50:50 ratio of the two fuels.  This is the worst condition.  If they are compatible at this ratio, they will be compatible at all ratios such as 80:20, 65:35 etc.

Viswa Lab has come out with an onboard compatibility/stability test kit where the ships need not send any samples just to check compatibility/stability.  A simple procedure with the test kit can save time, money and potential fuel problem and thus related loss!

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Share your experience for better awareness and understanding!

Disclaimer: The above case is for information and awareness purpose only. It is published in good faith to create awareness and reinforce the safety of ship at sea. Our intentions are not to comment on any particular fuel/supplier/fuel user/ship manager and the engineers out at sea. This case study is modified from the original incident and all names of ships, persons are fictitious. 

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