Introduction

The Marine Environment Protection Committee (MEPC) of International Maritime Organization (IMO) approved the Guidance on the development of a ship implementation plan (MEPC.1/Circ.878) at its seventy-third session.

Member Government administrations should encourage ships flying their flag to get prepared for compliance with the required 0.50% sulphur content limit under MARPOL Annex VI by 1 January 2020. A specific guidance on tank cleaning was included in the appendix for this purpose.

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.

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 including cleaning tanks during service manually and with additives may need to be considered.

Manual cleaning during dry docking

Time required varies from vessel to vessel but on average 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 is important to take another 2 to 5 days to flush through the pipework to ensure full compliance.

Manual cleaning during service

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.

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 either by the ship's crew or by employing a riding crew. 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 on board 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.

Advice to members

The time consumed in above alternatives are estimated according to shipping practices and are not applicable to all vessels. As IMO admits that 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 cleaning of the tanks;

   -- the condition of the tank coating and the effectiveness of the cleaning process itself.

The estimates may err on the side of caution and Members are suggested to make early arrangements to guarantee compliance:

1.  plan for dry dock in advance and be aware of potential overstock at the port during a busy period;

2.  make thorough plans if involving the existing crew for tank cleaning with full considerations to the number of personnel available, the appropriate schedule, ventilation and other safety issues;

3.  work with your charter early on to decide the period for employing a riding crew;

4.  order specialized additives beforehand and make sure the crew designated for the cleaning operation know how to dose them properly.

This text is based on IMO guidance on the development of Ship Implementation Plan (MEPC. 1/Circ.878). For further consultation, please contact your manager at the Club.