From 1 January, 2015, the maximum sulfur content of fuel oil used within the MARPOL Annex VI Emission Control Areas (ECAs) will be 0.10%, unless using an approved alternative means for controlling sulfur emissions. According to LR, this means that in most cases, the fuel used will be low-sulfur distillate oil (LSDO). Many ships operating both inside and outside ECAs will change-over between LSDO and residual fuel oil (RFO) when entering and exiting ECAs.

The(LR rule requirements for fuel oil systems contain design requirements for systems which are to operate on either LDSO or RFO grades, or both. LR also offers an optional descriptive note, DIST, for shipowners and operators applying industry best practice to the use of LSDO. The ShipRight Procedure for Assigning DIST Descriptive Note contains further guidance on measures to be considered by owners and operators.

In cases where existing fuel systems, engines or other equipment on LR classed ships are not suitable for operation on LSDO in addition to RFO, or where manufacturers have recommended equipment modifications, any proposed modifications must be submitted to LR for approval.

Alternating satisfactorily between LSDO and RFO depends on owners and operators applying established change-over procedures and giving attention to maintenance and operating requirements. Some of the main points to consider are as follows:  Each ship should have an established and documented fuel change-over procedure which limits the maximum rate of temperature change when changing between RFO and LSDO; crews should receive training and instruction on applying the fuel change-over procedure; fuel oil spill returns from engines and other equipment should be routed and managed to avoid contamination of LSDO tanks with other fuels;  and the compatibility of the RFO and LSDO should be checked. If instability occurs, it will generally be at the interface of the two fuels, necessitating more frequent filter change-over and cleaning to avoid blockage.

As for maintenance, LR stated that LSDO is less tolerant than RFO of worn fuel pumps and excessive barrel clearances will result in high leakage rates and failure to generate the required pressures, and may even prevent the engine starting. Excessive fuel supply and booster pump clearances will result in reduced delivery rates and pressures.

Other potential issues include the cleaning and searching nature of LSDO relative to RFO, and the temperature differential between the two fuels, which can result in seepage from pipe flanges, joints, seams and instrument connections.

Heating of LSDO will lower its viscosity below the minimal allowable limit of fuel equipment, requiring heating systems to be shut-down and meaning that indirect heating must be avoided. Cooling or even chilling of LSDO may be required. Fuel with a higher minimum viscosity (e.g., ISO 8217 DMZ) should be considered as an alternative, according to LR.

The base number of the engine lubricants (particularly cylinder oil) must be suitable for the sulfur content of the fuel being used.

The sulfur content of LSDO is expected to be close to the limit; marginal contamination with higher sulfur fuels will cause it to be non-compliant.