MARPOL annex VI - Sulphur Cap 2020 - Tank cleaning guidance


Published: 25 July 2019

Credit to: Andrey Polivanov /

The majority of ships are expected to use distillate fuel 0.50% sulphur limit taking effect on 1 January 2020. Most of these ships will have been using high viscosity High Sulphur Fuel Oil (HSFO) based primarily on residual fuel oils.

Such fuels tend to stick 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.

Chemicals additives are used by some shipping companies to clean their tanks prior to change in use. It can prove to be very efficient and a cost-effective way to reduce the tank cleaning. We all know of the imposing 1 January 2020 date for tank preparation.

Fuel oil tanks should be cleaned on regular basis to remove build-up of sediments and sludge, usually during dry docking or 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 to undergo dry docking during a very short period. Therefore, other options for cleaning tanks and fuel oil systems during service need to be considered.

Unfortunately, the time and work involved in preparing HSFO tanks to fuel complying with the 0.50% sulphur limit cannot be defined precisely as it will vary depending on:

  • the length of time since the last cleaning of the tanks;
  • the condition of the tank coating and the effectiveness of the cleaning process itself.

The estimates in this article 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.

Then again, timings vary from vessel to vessel but on average cleaning can be done in two to four days per tank.

In addition to cleaning the tanks, all of the pipework in the fuel oil service system needs to be flushed thoroughly. Overall it may take up to two weeks per vessel.

A vessel 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 the tanks have been cleaned in dry dock, it could take two to five days to flush through the pipework and this step should not be underestimated.

If tanks are to be cleaned manually during service a risk assessment and adoption of safety measures are paramount.

Time required will vary depending and the number of crew available to perform safe and complete tank cleaning operations. Tank cleaning can be performed by the ship's crew or by employing a riding crew. If the cleaning is done by the ship's existing crew, it would be likely to take a minimum of 4 days per tank. 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.

The residues from tank cleaning should be retained on board until they can be disposed of correctly, for instance at shore reception facilities.

An alternative to manual cleaning is to gradually clean the sediments and asphaltenic sludge from HSFO tanks and fuel system by dosing specialised additives resulting in less sludge. This is a soft and easy way to clean the tanks.

Sulphur 2020 cap will impact the fuel supply chain, procurement and operational safeguards. The impact is happening now, so act on any identified gaps, e.g. crew training in fuel management, preparation for efficient and cost saving tanks cleaning during service and preparing all HSFO tanks to load compliant fuel.

Note: This text is based on IMO guidance on the development of Ship Implementation Plan (MEPC. 1/Circ.878)