The (nearly) new regime
From 1 January 2015, regulations reduced the maximum permissible content of Sulphur Oxide (SOx) and particulate matter (PM), to 0.10% within the Emission Control Areas of the Baltic Sea, North Sea, United States and Caribbean (as defined).
Five months into the tightened regulatory regime, it is apparent that the changeover from residual (heavy) to distillate (low-sulphur) fuel brings with it significant practical issues and safety concerns.
Cruise ships in particular should take extra precautions for the reasons discussed below.
The distillation process
The concentration of sulphur in marine fuel arises from a combination of the naturally occurring sulphur content of the crude oil and the blending that has been undertaken to prepare the fuel.
Distillate fuel is composed of petroleum fractions of crude oil separated at the refinery during the distillation (i.e. boil off) process. The fractions which do not boil, and are therefore left behind, are referred to as residual fuel, with a higher sulphur content.
Going with the flow
Unsurprisingly, the different chemical composition of each fuel type affects how it behaves in an engine, which is where problems can creep in.
The ‘leaner’ or less dense distillate fuels do not always serve the engine as well as their heavier, more lubricious counterparts. The latter, as well as providing a fuel source, protect some moving parts of fuel pumps and injectors from wear and tear.
This issue goes hand in hand with viscosity where, again, the leaner, less viscous fuels do not perform as well. When the viscosity in the fuel pump is too low, hydrodynamic lubrication of the pump can be inadequate, causing wear and scuffing. The viscosity of a fuel source must be within the limits prescribed by the engine manufacturer to obtain an optimal spray pattern and avoid fuel leakage (a potential fire risk), safeguarding against poor combustion, deposit formation and energy loss.
Quality standards and safeguards
Although most marine medium and slow speed engines are designed for residual fuel operation, they are also capable of operation on distillate diesel fuel, provided that the following specifics are considered:
- Viscosity of fuel – at engine inlet.
- Lubricating oil choice.
- Lubricity of the fuel.
- Changeover procedures to and from the residual fuel1.
With this in mind the current ISO 8217 quality standards, which define the requirement for petroleum fuels for use in marine diesel engines and boilers, include minimum viscosity limits and lubricity and oxidation stability requirements for all distillate grades. These standards are to guide manufacturers and users alike.
For all vessels transiting through Emission Control Areas, aside from the practical benefits arising from the properties of residual fuel, there is an obvious business need for frequent changeover between the less expensive and more easily sourced residual fuels and their low-sulphur and regulation compliant counterparts. Each changeover presents a risk of fire and even partial or total propulsion failure and/or power interruptions. Since Emission Control Areas tend to contain attractive ports for cruise ship passengers, and cruise ships often transit these areas at high speed (in comparison with many, although not all, other merchant vessels), cruise ships in particular are regularly exposed to these risks.
Prevention is better than cure
There is currently no indication of any requirement for legislative changes in relation to the sophisticated fire detection and power monitoring systems already in use onboard cruise ships. However, extra vigilance is obviously required at all stages from the initial purchase of low-sulphur fuel to changeover times and their aftermath, to ensure that the identified risks do not translate into an incident. Distillate fuels should be sourced with sufficient viscosity to achieve the equipment manufacturers’ minimum requirements. Fuel coolers or chillers can be used if necessary to help prevent the viscosity of the fuel from becoming too low. Additionally, fuel temperature can be reduced during a changeover by switching off the pipe work steam and trace heating systems early.
Conversely, when changing back to residual fuel from distillate, operators must take care to ensure that the temperature of the residual fuel is high enough to achieve the required viscosity at the fuel pump inlet, although heating of the distillate fuel is to be avoided.
A period of review
In addition to the more obvious issues of expense and sourcing appropriate fuel, this changeover process represents a further sulphur-related burden for cruise operators. At HFW, we already have experience of casualties involving main engine failure and the use of low sulphur fuel. As the frequency of changeover related issues are monitored and recorded, time will tell whether such changeovers could become causative in a greater number of significant casualties.