With less than six months remaining for the global implementation of the unprecedented 2020 sulphur cap, ship operators tend to switch to alternative fuels, unless they have scrubbers installed. However, compliance is much more than simply switching to a low sulphur fuel. High sulphur fuel oil (HSFO), the fuel that was used so far but now will be inappropriate, is primarily based on residual fuel and these tend to adhere to the inside of fuel tanks and pipelines forming sludge. As such, a proper tank cleaning becomes necessary for adoption of the new fuels.
IMO’s MEPC 73 in October 2018 agreed that Administrations should encourage ships flying their flag to develop implementation plans, although this is not a mandatory requirement.
The ship implementation plan for 2020 could cover various items relevant for the specific ship, including, as appropriate, but not limited to:
- risk assessment and mitigation plan (impact of new fuels);
- fuel oil system modifications and tank cleaning (if needed);
- fuel oil capacity and segregation capability;
- procurement of compliant fuel;
- fuel oil changeover plan (conventional residual fuel oils to 0.50% sulphur compliant fuel oil); and
- documentation and reporting.
Why tank cleaning
Due to lower cost, most ships will have been using so far high viscosity high sulphur fuel oil (HSFO) based primarily on residual fuel oils. Such fuels tend to adhere to the inside of fuel tanks forming layers of semi-solid substances containing sediments and asphaltenic sludge.
As such, the ship operator may choose to clean the fuel oil tanks of these residues before loading compliant fuel prior to 1st January 2020, as this fuel is also expected to have a high content of distillate components.
If such fuels are loaded into HSFO fuel tanks that have not been cleaned, there is a possibility that they could dissolve and dislodge sediments and asphaltenic sludge in storage tanks, settling tanks and pipelines, potentially leading to purifier and filter operational issues and in extreme cases fuel starvation resulting in loss of power,
Tank cleaning methods
Should the ship operator determine it is appropriate to clean the ship’s fuel oil tanks and system, the following considerations may need to be taken into account:
-Manual tank cleaning during dry docking
- Time required varies; it can be done in 2 to 4 days per tank. In addition to cleaning tanks, all of the pipework in the fuel oil service system needs to be flushed through. Overall, it may take 1 to 2 weeks.
- A ship that has had all its fuel oil tanks and fuel system cleaned can start loading compliant fuels and expect to be fully compliant right away.
- However, if only the tanks have been cleaned in dry dock, it could take 2 to 5 days to flush through the pipework in the fuel oil service system to ensure full compliance with the 2020 sulphur cap.
-Manual tank cleaning during service
- If tanks are to be cleaned manually during service, risk assessment and safety measures are vital, as per IMO resolution A.1050(27) on Revised recommendations for entering enclosed spaces aboard ships.
- Time required will vary depending on tank size and the number of tanks, how long it has been since the last tank cleaning and the number of crew available to perform safe and complete tank cleaning operations.
- Tank cleaning can be performed by the ship’s crew and/or by employing a riding crew for this purpose.
It is always good practice to inspect the tank once cleaned to check its condition and to inspect heating coils, conduct pressure tests and undertake repairs as necessary.
- If the cleaning is done by the ship’s existing crew, it would likely take a minimum of 4 days per tank. For an average tank, a week should be allowed. If employing a riding crew to clean the tanks, if working in shifts, it would likely take a minimum of 2 days to clean a tank, but 4 days per tank should be allowed.
- Tanks need to be empty before they can be cleaned, hence the time needed to drain tanks needs to be taken into account when estimating the overall time required.
- In addition to cleaning tanks, all of the pipework in the fuel oil service system needs to be flushed. Flushing the remaining pipework and fuel oil service system after all tanks have been cleaned could take another 1 to 2 days.
- The residues from tank cleaning should be retained onboard until they can be disposed of correctly or disposed to shore reception facilities.
-Cleaning tanks in service with specialized additives
- As an alternative to manual cleaning, consideration can be given to gradually cleaning the sediments and asphaltenic sludge from HSFO tanks and fuel systems by dosing additives.
There are successful examples of this approach for ships that needed to reallocate HSFO tanks to fuels complying with the 0.10% sulphur limit that took effect in ECAs in 2015.
For further information, refer to the following IMO circular: