Managing fuel quantities, fuel quality and safe switching
Updated 5 Sep 2019
Managing fuel on board a ship is a complicated business even if the same type fuel is used for both main and auxiliary engines. When multiple fuel types are used the factors needing consideration also increase. Managing fuel involves calculating what quantities and types of fuel are needed and also the much more specialist skill of switching between different fuel types as the ship progresses on its voyage and passes between areas where different regulations apply.
It might be thought that once 2020 arrives things will become easier with regards to fuel management but unless a ship is running on LNG or high-quality marine distillates, then the plethora of new products will pose new problems – at least until experience has shown how miscible and compatible different products actually are.
The first principle of managing fuel is to ensure that there is sufficient on board. For liner vessels making regular voyages between the same series of ports, this is quite a simple matter, especially as few liner vessels are ever loaded to full deadweight capacity and their earning capacity is not likely to be affected by bunker quantity.
For vessels in both the wet and dry spot sector, calculating the tonnage of bunkers needed to permit maximum cargo intake and whether bunkering can be spread over more ports than the load port is a skill that shipbrokers and operational staff need to learn early. For both liner and spot vessels, time spent in ECAs and ports where local regulations apply need to be accounted for. As well as ensuring sufficient fuel for the expected voyage, a safety margin also needs to be built in for when a vessel is unavoidably detained by weather, congestion or other reasons.
Some fuel management is regulated by MARPOL requirements and this will become increasingly important with the new 2020 regulations. Under the rules, every time a vessel takes bunkers it must receive a Bunker Delivery Note (BDN) that is retained on board for three years. The BDN must be accompanied by a representative sample of the fuel delivered. The sample must be sealed and signed by both the suppliers and the officer in charge of bunkering and retained on board for at least 12 months.
The BDN and sample system is designed to both protect the ship in case the fuel subsequently proves to be off spec and not in conformity with MARPOL rules and for port states to take action against rogue ships and fraudulent activities by bunker suppliers. When, after 2020, the ban on carrying non-compliant fuel for use on board ships comes into effect, the BDN and fuel sampling will take on even greater significance.
The next element in managing fuel on board is ensuring that sufficient tank space is available to keep supplies taken at different times separate and to ensure that mixing of fuels does not take place unless and until compatibility has been established.
For ships with scrubbers, the ability to continue using traditional HFO whether in the open oceans, in ECAs and in ports where more stringent rules apply will mean that the management of fuels on board will change little. However, as some port state authorities are refusing to permit the use of open-loop scrubbers or hybrid scrubbers operating in open-loop mode in their waters, some scrubber equipped ships will need to switch to compliant fuel on occasions.
The varying characteristics of new types of fuels will mean that separation and management may change considerably and new skills and techniques will need to be learned by engineering staff. The learning must be an ongoing process as the varying properties of each new fuel type used will need to be fully understood. Sharing of information between operators, class societies, P&I clubs and other specialists will be essential in the early days of using these new fuels.
The fuels can be very different in characteristics from conventional fuel oil and this has led numerous organisations to issue guidance to operators on their use. Lloyd’s Register issued the following advice in its publication Using hybrid fuels for ECA-SOx compliance.
Most of the new hybrid fuels are blended products and have some characteristics of distillate products. This means they may exert a ‘cleaning’ action, mobilising previously deposited asphaltenic material, potentially leading to increased filter loading and other operational issues. It is therefore recommended that fuel tanks which will carry these new fuel types are cleaned or at least cleared of the ‘unpumpables’ at the tank bottom.
Despite their distillate characteristics, most of these hybrid fuels are particularly waxy in nature, as exhibited by their pour point (the lowest temperature at which a fuel will continue to flow). The exact pour point may vary from product to product, but the usual rule is to maintain any fuel oil no lower than 7°C above its tested pour point. These fuels therefore need to be stored and handled in systems with heating arrangements.
These types of fuels should not be stored in tanks which are subject to low external temperatures, such as a ship’s side tanks. Even in tanks with heating coils that maintain the bulk of the fuel as liquid, the formation and then breakaway of material at the cold interface could result in operational problems.
These fuels will also need to be purified, taking into account their density (gravity disc selection) and viscosity for optimised preheat. Based on the tested viscosity and density of the fuels, the purifier manufacturer’s recommendations should be followed for the correct operational adjustments.
Making the switch
Whenever a vessel transits between an unregulated area and an ECA or an area subject to local rules over fuel sulphur content, the fuel supply to the ship’s engine(s) needs to be altered to reflect the requirements of the new area. This does not need to be done if the fuel being used is already of a type that meets the most stringent requirement or if a scrubber is in use that provides an equivalent exhaust emission.
Switching between fuels is rarely a matter of just closing one valve and opening another as the sudden change of fuel type can have highly damaging or even dangerous effects. For example, a fuel line that has been heated to allow HFO to have the required viscosity to flow, will be too hot for fuel such as MGO causing it to vaporise or even ignite.
A further hazardous situation could arise if the switch over causes the engine to cut out or lose power. Most often the switching will be taking place either when entering an ECA or in the cases where local regulations exist when entering a port. The heavy traffic in such areas makes the consequences of a power loss much more risky.
If the switch being made is on a low-speed two-stroke engine, Then as well as the fuel itself the upper cylinder lubricant needs to be changed to one which is more suited to the new fuel type. With oil fuels, the lubricant chosen should have a BN best suited to the sulphur content of the fuel.
After 2020 none of the new compliant fuel types or current distillates should have a sulphur content exceeding 0.5% for global use or 0.1% for use in ECAs. Ordinarily a single low BN lubricant would be able to deal with the acidity caused by the sulphur content. However, some compliant fuels derived from plant and animal matter may not contain sulphur but may still have a high acidic content since the fuel is derived from fatty acids. It would be hoped that the producers of these fuels and the engine makers co-operate on recommending appropriate lubricants.
Even under current regulations, the switching procedure is always ship specific taking into account the available fuel, desired engine operating parameters and the time needed to effect the take over so as to be burning compliant fuel immediately the ship crosses boundary lines or before.
Many ships not subject to mandatory low-sulphur fuel limits already switch from HFO to marine diesel oil (MDO) during port operations, so the procedure is not entirely alien, but in emission control areas there is a burden of proof upon the ship to show that at the point of entry into the controlled zone it was burning only the permitted fuels.
That involves a gradual changeover and if the switch is from ordinary HFO to low-sulphur fuel there will be a need for the temperature of the low-sulphur product to be managed so that a smooth switch can be achieved. Even so, the time taken to achieve compliance will depend upon the amount of fuel contained in the system and the tank space available for service fuel.
Class societies and industry organisations have offered a lot of advice on this subject, even going to the extent of preparing comprehensive manuals and guidelines for operators. The complexity of the subject can perhaps be judged by the fact that an Advisory Note on switching fuels distributed by classification society ABS when the US ECAs were being established ran to a full 36 pages. It also listed some 14 sources of further information.
There are alternatives to managing the switch between fuels manually with at least four automatic systems available. For a manual change, the largest designer of two-stroke engines MAN Energy Solutions recommends crew to reduce engine loads to 25-40% before changing fuel type, the automatic switching systems then enable a controlled and safe changeover independent of engine load. They do this through continuously checking temperature versus time and use software to operate coolers to adjust the MGO temperature. With the automatic switching full details of the switch between fuels including time and location (taken from the ship’s GPS) is recorded and can even be transmitted ashore using the ship’s communication system.
WinGD also favours automatic switching and in guidance notes relating to its engines lists advantages of automatic fuel change-over units. These include allowing a change-over between fuels even at 100% CMCR engine load rather than having to reduce to between 30 and 70% if managed manually; a significantly shorter change-over period saving in the more expensive fuel types and better management of temperature gradients. Furthermore, safeguard functions in the units reduce the risk of damage by abrupt temperature changes.