Introduction
1 Most ships will have been using 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; such
residues will also typically have solidified and settled in various parts of the fuel oil
service system including pipelines, settling and service tanks.
2 The ship operator may choose to clean the fuel oil tanks of these residues before loading
compliant fuel prior to 1 January 2020 based on the following considerations.
3 Some of the fuels complying with the 0.50% sulphur limit are expected to be very
paraffinic due to crude sources of blending components and also 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.
4 Alternatively, ships have been using ship specific changeover procedures to effectively
and safely load on top of existing fuel oil and gradually flushing through the fuel system
until the sulphur content in the fuel oil is at a compliant level.
5 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 when making
arrangements for tank cleaning.
Options for tank cleaning, approximate timelines and considerations
6 Fuel oil tanks are normally cleaned on a regular basis on ships to remove built-up
sediments and sludge, usually during dry docking and whenever inspections of the fuel tanks
are due. However, leading up to 1 January 2020, it would not be practicable for the
majority of the global fleet that has been running on HSFO and decided to opt for tank
cleaning to undergo dry docking during a very short period. Hence, other options for
cleaning tanks and fuel oil systems during service may need to be considered.
7 The time and work involved in cleaning HSFO tanks cannot be defined precisely, as it will
vary depending on how long it has been since the last time the tanks were cleaned, the
condition of the tank coating and the effectiveness of the cleaning process itself. The
estimates in this document may err on the side of caution as it is almost impossible to
pinpoint at what stage the ship's fuel oil system is sufficiently clean to guarantee
compliance.
Manual cleaning during dry docking
8 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.
9 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.
10 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 0.50% sulphur limit.
Manual cleaning during service
11 If tanks are to be cleaned manually during service, risk assessment and safety measures
are paramount; refer to IMO
resolution A.1050(27) on Revised recommendations for entering enclosed spaces
aboard ships.
12 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.
13 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.
14 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.
15 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.
16 The residues from tank cleaning should be retained on board until they can be disposed
of correctly or disposed to shore reception facilities.
Cleaning tanks in service with specialized additives
17 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.