The mega marine engines of ships burn tonnes of fuel every day in order to propel massively loaded ships. These engines are known to use low-grade fuel oil to lower ship operating costs as cost of fuel represents as much as 30-50% of total operating costs of a ship.
Unfortunately (or fortunately), it’s not always possible to use low-grade fuels such as heavy fuel oil or HFO in regulated areas known as ECAs or Emission Control Areas. Marine Gas Oil Fuel or MGO is one of the most prominently used clean fuel in such cases.
The shipping industry is amongst the “early-adapters” in the transportation sector to develop and follow stringent environmental laws in order to move cargo worldwide. Fighting air pollution from ships has been the top agenda for the Marine Environmental Protection Committee, and hence several regulations have been enforced to curtail the harmful emissions from ships such as Sox and NOx.
With the ever-changing technology on ships, the fuel that is used to run marine engines is also changing rapidly. Recently, LNG is being seen as the “future fuel” for the shipping industry, however, the industry as a whole is yet to adapt to this change, and thus Marine Gas Oil is still one of the most preferred clean fuel used on ships.
Sulfur Content in Marine Gas Oil :
Marine Gas Oil used on ships is a blend component of light cycle (gas) oil (LCGO) that contains about 60% aromatics. Due to the high aromatic nature, the density of a marine gas oil blended with LCGO will be higher than the gas oil from an atmospheric distillation refinery. The density of MGO will usually be closet to 860 kg/m3 (at 15°C).
Sulfur Content and Cetane Index :
The marine gas oil is also considered as low sulfur fuel oil or LSFO because it has sulphur content between approx. 0.10 and 1.50 m/m %.
Types of clean fuel oil for the maritime industry available in the market:
Residual marine or RM fuel oil is a type of diesel oil which needs heating for usage.
Distillate marine or DM fuel oil, which does not require any pre-heating for usage. It can further be classified as DMA fuel which is clear and brighter in appearance; DMB and DMC marine diesel oil grades which are not required to be clear and bright.
The RM and DM are types of marine oils, which can be further divided into groups, depending upon the sulphur content in the marine fuel oil.
– LSFO : Products that are above 0.10% but meeting a 0.50% sulphur limit.
– ULSFO : Product with maximum 0.10% sulphur content.
Nowadays, the new age ULSFOs in the market typically have only 10 to 15 parts per million (ppm) sulphur or 0.001% to 0.0015%.
Cetane index is one of the essential marine fuel oil properties only applicable to marine gas oil and distillate fuels. It defines the ignition quality during the combustion process in a diesel engine. The cetane index is calculated from the Cetane number of the fuel which is provided in the “property of marine fuel details” in bunker delivery note.
In general, the higher rpm engine requires high cetane index fuel. A low cloud point gas oil may only be stored onboard in drums because of its low flash point.
General Problems Associated with MGO
Microbial contamination, caused by bacteria and fungi, occurs in a fuel having water quantity. Due to bacterial presence, the fuel systems will generate problems such as chocked fuel filters and erratic engine operation. Possible microbiological contamination indicators are :
– Hazy appearance on the oil surface.
– Suspended impurities in the oil.
– Presence of the emulsion or a slimy interface layer between water and gas oil.
– Foul smell from the tank drain with slight sludge discharge during draining operation.
Fuel pumps of marine engines are designed for a minimum viscosity. The viscosity of Marine Gas Oil is very low as compared to the pump design factor which leads to inadequate hydrodynamic lubrication, causing wear and scuffing.
A decrease in fuel viscosity may cause an increase in fuel leakage between the pump plunger and barrel.
The leakage can lead to hot start, and low fuel setting starts difficulties, especially in worn fuel pumps.
Due to the low viscosity of the marine gas oil, the external and attached fuel pumps may not deliver the fuel at the required pressure, which will eventually hamper the designed power output of the engine.
Burning MGO in 4 stroke engine
The diesel generator installed on ships these days operate on both residual and distillate fuel. The valve seat deposits (on the inlet valve) is significantly less when using distillate fuel as compared to using residual fuel oil. This is because the distillate fuels such as Marine Gas Oil produces fewer combustion deposits.
The 4 stroke marine engine efficiency on the ship is measured on the basis of residual oil, and the design of the engine is done focusing on the use of residual fuel. Most of the 4 stroke engines are installed with water-cooled injection nozzles to reduce the injector tip temperature (for avoiding coking of the fuel which would cause deposits known as trumpets on the nozzle tip).
When using distillate fuel, the nozzle cooling arrangement will further reduce the temperature of the distillate fuel, already having very low viscosity. The additional cooling with water may also cause overcooling of nozzle, leading to falling of temperature below the dew point of the sulphuric acid in the combustion gas and cause corrosion of the nozzle. To tackle this, the engineer must ensure to turn off nozzle cooling during distillate marine fuel oil operation.
Another problem associated with usage of marine gas oil is leakage. As the viscosity of the fuel is much lower than the regular fuel of the engine, it accelerates the fuel leakage from pumps and also contaminates the lubrication. To tackle this problem, most of the 4 stroke engines comes with lubricating sealing oil at the fuel pump. This oil seals the passage of distillate fuel to minimise the leakage.
Most of the residual fuels are not compatible with sealing lubricating oil, hence the engineer must ensure to switch off the sealing oil to avoid compatibility issues, else it will lead to problems like fuel pump sticking etc.
During the burning of low sulphur fuel oil or LSFO, lacquering in liner may also be observed. Marine gas oil produces deposits which stick on the liner surface and disturbs the oil film lubrication in the liner. The engine design and use of aromatic fuels as the primary burning fuel are important factors that can contribute to increasing the lacquer formation.
The BN of the lube oil used in 4 stroke engines that operate majorly on distillate marine fuel oil is in the range of 10 to 16 mg KOH/g. When the engine is operated with residual fuels, the BN of the lubricating oil is kept between 30-55 mgKOH/g. When using distillate fuel for a more extended run (more than 1000 hours), it is always advisable to switch the lube oil with lower Total Base Number (TBN) with value as stated above. For shorter operation, it is not critical for the engine to keep using lubricating oil with BN of 30-55 mg KOH/g.
Burning MGO in 2 Stroke Engine :
The 2 stroke engines operate typically under heavy fuel oil outside the ECAs and before entering the Emission Control Areas they switch over fuel from HFO to LSFO.
During the switchover process, there is a mixing of heavy fuel oil with a low aromatic hydrocarbon distillate fuel. This increases the risk of two incompatible fuels burning inside the engine cylinder, causing the asphalt of the heavy fuel to precipitate as heavy sludge and leading to filter clogging.
As the names suggest, LSFO produces a negligible amount of sulphuric acid, and hence if the correct TBN lubricating oil is not used, the alkaline components produced in the cylinder will not be neutralised. This will potentially harm the liner and other parts of the combustion chamber. These alkaline deposits will lead to the removal of cylinder oil film causing contact of metal to metal parts between liner and piston rings and resulting in scuffing and seizure of the engine.
The engineer operating the marine engine must ensure to switch to lubricating oil of LOW TBN when switchingto LSFO and vice versa when using heavy fuel oil.
Leakages during the running operation of MGO or LSFO is another problem experienced in 2 stroke marine engines. This is because of the viscosity of MGO is lower than that of HFO.
For new marine engines running on heavy fuel oil, the engineer officers have to evaluate the cylinder conditions and report to the engine maker after changing the fuel to LSFO to check the deposits and scuffing on combustion chamber parts such as piston, crown, liner, and ring.
Responsibilities of ship staff While receiving Marine Gas Oil
While receiving the LSFO during bunker operation, check the bunker delivery note to ensure the quality of the oil meets the main and auxiliary engine manufacturers’ fuel oil specifications.
During the bunkering operation, the ship’s staff has to ensure to set the bunker line carefully so as not to mix the receiving marine gas oil with different grades present onboard.
It is always advisable to use an empty tank to receive marine gas oil or receive the MGO in a tank which is filled with a similar grade.
Ensure the ship management office works with bunker supply staff when ordering correct grade and ISO standard fuel with required sulphur content for ECA use.
A de-aerating arrangement during bunker operation will remove the airwaves entering the bunker tank which may affect the bunker quantity.
The contamination of fuel can happen at any part of the supply chain, i.e. at Fuel-producing companies, when the fuel is with dealers, or at the end- users. It’s a collective responsibility of all to avoid contamination.
Onboard Care :
Ensure the gas oil storage does not contaminate from water ingress. Thus, it is important to maintain good water draining and housekeeping.
Check filters of the fuel line when distillate fuel is in use.
Check the water content of the fuel and ensure the sample is sent to shore lab to check for lube oil and microbial contamination etc. at regular intervals of time.
Whenever necessary, drain the tanks which are filled with MGO.
When using distillate fuel in the marine generator, ensure to switch off nozzle cooling water to avoid overcooling of injector nozzle.
The crew should have excellent knowledge on the changing-over procedure from HFO to LSFO when entering ECA and vice versa with minimum possibility of mixing of fuels to avoid non-compatibility issues.
During the changeover process, care must be taken to monitor the temperature of the fuel system and when the HFO is changed to LSFO and vice versa, the viscosity within the system must not drop below 2 CST or exceed 20 CST.
The company has to ensure all the technical requirements are in place when using LSFO or marine gas oil. If the ship is plying the first time into the ECA, the company should revise fuel oil management procedures to ensure the crew has prior knowledge of HFO to LSFO changeover and low TBN lube oil for the main engine etc.
The bunker storage tanks used for marine Gas oil or low sulphur fuel oil should be cleaned regularly to keep them free from sludge, which cannot be drained during normal operation.
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