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. Most shipowners will want to change over onto compliant fuel well before the 1 January 2020. They will want to make sure all the tanks are clean, systems are flushed and there is little risk of contaminating subsequently bunkered fuel by residues remaining in the system.
Tank cleaning is not a simple or quick task. It needs planning. How long will cleaning take and how will it be done? It is unlikely that simply bunkering MGO into a tank that previously held heavy fuel oil, and then flushing through, will achieve compliance. Rather, it is much more likely that manual cleaning within each tank will be required or there may be cases where specialist chemical additives could be used.
It is important to do it right. If the system is not properly cleaned, it could contaminate several hundreds of tons of subsequently bunkered fuel. There also may be a need to clean tanks again after 1 January 2020. If compliant fuel is unavailable in certain geographical areas, a vessel may be left with no choice but to bunker non-compliant heavier fuels. Therefore, these tanks will need to be in a suitable clean condition before returning to low-sulphur service.
Record keeping considerations are absolutely necessary to be kept. Record keeping requirements are not specifically addressed within the IMO guidance on Ship Implementation Plans. Planned maintenance records should of course be updated, but official documentation such as the Oil Record Book must be kept up to date. The vessel must be able to account for the removal of any tank residues resulting from manual tank cleaning. source – https://maritimecyprus.com/2019/11/19/marpol-annex-vi-sulphur-cap-2020-records-for-the-cleaning-of-fuel-oil-tanks-guidance/