Storage related issues

Updated 11 Oct 2019

Fuel Line

The advent of ECAs not only means that a shipowner without a scrubber is obliged to burn more expensive fuels but also those fuels need to be segregated. Therefore, it is necessary to have different storage, settling and service tanks for each grade of fuel.

This can be provided for in newbuildings easily enough but for existing vessels the choice may come down to running all the time on more expensive fuels or having existing tanks divided during a drydocking to allow for the extra number of tanks needed. Even where ships have a number of bunker tanks available, using two or more different grades of fuel may make management extremely difficult.

This is particularly true when the size of individual tanks means that only relatively large volumes can be allocated to any particular fuel type.

After 2020, when only ships equipped with scrubbers will be able to burn cheaper HFO, older ships which do not plan to fit scrubbers will need to clean and prepare tanks for compliant fuel. If operating mainly in the tropics, such ships can probably remove any tank heating equipment but for ships that sail regularly in other latitudes, it may be a wise precaution to retain the heaters to avoid waxing of MDO.

Blended fuels

As demand for low sulphur fuels has increased, the availability of inherently low-sulphur crude oil stock has not kept pace. As a consequence, much of the low-sulphur fuel on the market is a result of blending fuels with different sulphur contents to make one with the appropriate level of sulphur. Often the viscosities and other characteristics of the fuels blended are very different and this can lead to problems further down the line.

It is not unknown for blended fuels to separate out in a ship’s tanks or to become unstable when mixed with other fuel during subsequent bunkering operations. If a blended fuel separates it is possible that the ship will at some point be burning fuel that exceeds the sulphur limit permitted by MARPOL. This could lead to a ship being detained or penalised by an unsympathetic port state.

Connected with this issue is the interpretation of MARPOL regulations where limits on sulphur are set to a single decimal point and the question of whether rounding down is permitted where a fuel may have a sulphur content very slightly above that shown in MARPOL.

Blended fuels may also have very poor ignition characteristics causing fouling of cylinders, turbochargers and exhaust systems and in worst case scenarios engine failure and power blackout. Such problems are most likely to occur during switch over between standard and low-sulphur fuels which usually takes place as vessels are entering areas of heavy traffic and thus increasing the risk of collision or worse.

As fuel suppliers develop new products to meet the 2020 reduction to 0.5% sulphur levels outside of ECAs, the problems associated with compatibility are likely to grow. Ships fitted with scrubbers may suffer less as the standard HFO fuels are less likely to be affected but if the availability of HFO is affected by lower sales levels resulting in lower stocking, then such ships may find that they are obliged to accept other fuel grades from time to time and that these may not mix easily if at all with standard HFO.

In addition, a small number of ports and states have banned the use of open loop scrubbers. This ban is already in operation in Norway’s Heritage Fiords and one ship has been penalised for contravening it.

Thus, ships which could comply with the 2020 sulphur cap by using scrubbers must also ensure a supply of compliant fuel on board for use when operating in waters affected by the ban. The bans have been introduced because the wash water from open-loop scrubbers is considered by some to be a pollutant.

Switch over issues

As of May 2019 there are four emission control areas (ECAs) established under MARPOL and a handful of regional regimes – notably all of the EU and parts of China – where there are regulations on sulphur levels allowed in marine fuels. To meet the requirements, any ship without a scrubber is obliged to switch fuels unless it is already running on ultra-low sulphur fuel.

There are two issues relating to a switch to a lower sulphur fuel. The first is related to the treatment of the fuel itself and the second affects only two-stroke, low-speed engines and involves matching the upper cylinder lubricant to the different fuels involved.

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 ECAs 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 HFO 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.

Because heavy residual fuels will be used at temperatures above the flashpoint of low-sulphur distillates, when the switch involves these types of product, the temperature in the fuel lines must be monitored and managed with great care. This is especially true if the engine makers require the residual fuel to have a particular viscosity which may involve cooling it below ambient temperature. 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 need to switch lube oils is to ensure a match between the base number of the lube and the sulphur level of the fuel and is discussed further in the chapter on engine lubricants.

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 while the automatic switching systems 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.

As with some of the other devices, 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.

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