Typical fuel system layouts
The fuel systems for oil fuels whether HFO, MDO or MGO are all very similar but decidedly different from that for gaseous fuels. It could be said that until the IMO developed the IGF Code covering gaseous fuels, the main impediment to the take up of LNG as a fuel was that each flag state could formulate its own rules and those rules may not have been acceptable to port states. For LNG carriers running on boil off gas from the cargo there was no problem as the gas containment systems were already covered by SOLAS regulations.
There are rules relating to the oil fuel systems on ships, but in essence these are limited to the requirement for save-alls around bunkering manifolds and procedures to prevent pollution, the location of fuel tanks to prevent pollution in cases of damage to the ship, double walls on high pressure fuel lines and flash point levels for fire prevention reasons. Class society rules may require the capacity of fuel service tanks to be sufficient for a specified running time for the main engines.
Except when unavoidable ships rarely use bunkers immediately after they have been delivered. This is primarily to allow the fuel to settle and also to give time for tests to be done to ensure that the bunkers are as ordered and not contaminated. Unfortunately poor quality bunkers have become a major issue in recent years either because of deliberate acts where chemicals and other substances have been illegally disposed of into the fuel or because of unsuccessful blending or products to meet a set specification.
Heavy fuels often contain contaminants from the crude oil itself or from the refining process in the form of ash, heavy metals, sludge and catfines. The latter are the most damaging contaminants of all and have the potential to completely destroy an engine if they are not removed by the ship’s fuel management and treatment systems.
A typical fuel system on board a ship will comprise multiple tanks for various purposes and fuel grades, pumps, filters, heaters and coolers, separators and potentially emulsifiers and micronizers. In addition, there will be flow meters and temperature sensors to aid in managing the treatment systems.
The various SOx regulations and establishment of ECAs has affected the configuration of the bunker tanks on ships and will continue to do so in the future. Typically ships had a number of bunker tanks commensurate with ship size, additional tanks for settling and service use and also tanks for other fuels for auxiliary engines or alternative use in the main engine.
Since the advent of ECAs ships tend to have more bunker tanks allowing separation of fuels with different sulphur levels for use in and out of restricted areas. With the 2020 global cap on sulphur approaching, ships will likely have to ensure that the new 2020 compliant fuels are kept separate from each other because of concerns over compatibility between different fuel types. Even if the fuel is said to meet an ISO standard, (ISO 8217 will need to be amended to accommodate new fuel types) there are no guarantees that it will mix with other fuels.
Ships that do not have scrubbers fitted will not be permitted to load a fuel with a sulphur content above 0.5% after 2020 except in circumstances where it can be proved that no compliant fuels were available. In the early days after the deadline it would be appropriate for a ship to maintain at least one tank for such fuel until availability of compliant fuel has proved not to be an issue.
On bunkering, the fuel is pumped to an appropriate bunker tank. If tests are needed or considered desirable these will be done now and the fuel left until results confirm its suitability for use. Then it will be heated and pumped to a settling tank. In this tank, entrapped air will be allowed to escape and any sludge will settle to the bottom. Most of any water in the fuel will also sink to the bottom as even heavy oil is lighter than water.
The water and sludge that settle at the bottom of the tank will be drained and passed through a separator. Any water that is separated and which complies with the 15ppm MARPOL limit can be disposed of at sea and the sludge moved to a sludge tank for later disposal ashore.