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.
The next stages are the most important and involve treating and cleaning the fuel in preparation for use. From the settling tank, the fuel is pumped through a filter to remove large contaminants and the heated before passing to the fuel line separator where more water, small contaminants and light sludge are removed. This is also the point where the damaging catfines are removed.
Catfines are minute highly abrasive particles that have detached from the refinery catalytic cracker. They are easily removed by a separator but only if the flow rate is sufficiently high. Around 2006 leading separator makers along with class societies and engine makers developed a separation standard aimed at matching separator size to engine requirements. The standard is not mandatory but failure to apply it may result in a separator of the wrong size being installed.
After separation the fuel then moves on to the service tank in readiness to be pumped to the engine. As mentioned, the size of the service tanker – and their number – may be regulated by class society rules. Fuel in the service tank must be maintained at the correct viscosity as laid down by the engine makers. More heaters are needed at this stage to ensure the viscosity and booster pumps to ensure fuel pressure is maintained.
From the service tank, the fuel is pumped to the injectors through a fine filter and heating system to ensure the appropriate viscosity. This part of the fuel line will include flow meters, temperature and pressure sensors to ensure the fuel delivery is in line with the engine maker’s requirements. If the engine is to be switched from HFO to MDO, the lines may be too hot for the new fuel and to avoid problems it may be necessary to cool rather than heat the MDO to ensure correct viscosity and pressure. This is done using a heat exchanger unit which can be by passed once the fuel lines have cooled sufficiently. For the switch over process, a three-way valve allows HFO and MDO to be mixed and fed to the injectors.
Some systems may contain a homogeniser to reduce sludge and emulsify fuels to aid in NOx reduction. A homogeniser will grind long chains of carbon molecules that often occur in heavy fuels into smaller pieces that combust more easily reducing black smoke. If used to emulsify fuels some of the water present will mix with the fuel rather than being separated out. Emulsified fuel also combusts better and thus reduces fuel consumption.
Another possible modification could include a means of incorporating fuel additives. These are intended to reduce fouling or improve combustion. Some engineers swear by additives but others question their efficacy. Surprisingly few scientific studies have been done to resolve the issue either way.
Ships that run solely on distillate fuels or if with a dual-fuel engines only use distillates will have a similar tank layout comprising storage, settling and service tanks but the need for heating will be less and the better cleanliness of the fuel will mean that there will be less sludge and no catfines should be present. There will still be a need to filter fuel and remove excess water with a separator.
The fuel supply for gas-fuelled engines is very different from that for oil fuels. There are none of the contaminants associated with oil fuels and for keeping gases liquid, they must be handled at extremely low cryogenic temperatures and under high pressure.
Tanks for oil fuels are merely voids within the structure of the ship but the tanks for gas fuels are specialist constructions. There are three types of tanks approved under the IGF code the most common of which used currently is the insulated Type C tank. These comprise an inner vessel, which contains the liquid gas and the outer vessel, which is regarded as a second barrier. The space between the inner and outer vessels, which is filled with perlite, is vacuum evacuated. The tank is connected to the bunkering station by way of insulated pipes. As more vessels are built to run on LNG, tank technology may change. There is already one project running which intends to make use of lightweight composite materials used in the US space programme,
The pressure in the tank is sufficient to feed the fuel to the engine without the need for pumps. However, some process systems are necessary to ensure that the gas is delivered at the correct temperature and pressure. Heating is done by a vaporiser fed by low temperature water from the engine cooling system or some other appropriate source. Usual operating temperature of the gas is between 10 and 40°C. Pressure is managed by a pressure build up unit. The vaporiser and pressure build up systems can be attached to the tank or housed in a separate space as required. In either case, the location should be temperature controlled to reduce oil off.
Dual-fuel vessels will obviously require both types of fuel systems to be installed.