In ‘Petroleum and its Combustion in Diesel Engines’, published in 1955, John Lamb demonstrated remarkable foresight when he warned that “imperfect combustion in any type of internal-combustion engine is always accompanied by greater wear and tear of vital parts” and that “the cost to make good the damage done in a week might well exceed a year’s increase in the fuel bill due to running an engine under such conditions”.

Lamb’s seminal book was written at the advent of residual fuel use by commercial ships, when many vessels were still operating on gas oil or diesel oil. His advice is especially apt now, given the concern over the potentially higher maintenance bills for ships embarking on slow-speed steaming, which may significantly offset the potential savings in fuel costs.

Fuel-efficiency strategies – such as systematic bunker management, enhancing engine monitoring and maintenance, improving the hull and propeller condition as well as slow-speed steaming – have all received market attention as a means to reduce fuel consumption and CO2 emissions. However, slow-speed steaming also implies that engines are not operated at their rated capacities, which could cause lower engine temperatures and pressures, a drop in efficiency, and a possible increase in particulate matter emissions. Compliance with NOx emission requirementes may also require to be assessed.

Poor quality fuels can cause engine problems whether a vessel is slow steaming or operating at normal speeds.

In the past couple of years, high bunker fuel prices and the blending of fuel with little attention paid to the origin and quality of the cutter stock have resulted in fuel quality becoming more suspect. Ironically, the blending is often carried out in order to meet certain quality and regulatory requirements.

Fuels that are unstable due to the incompatibility of blend components, poor ignition and combustion properties, excessive sedimentation and chemical contamination are becoming more common even as the fuels appear to have met the ISO 8217: 2005 specification. There has also been an increase in bunker fuel with elevated levels of abrasive fines and a low flash point.

Fuel quality issues can cause incomplete burning, in turn leading to increased deposition of carbonaceous materials in exhaust spaces, economisers and turbochargers, and the clogging of engine parts such as piston rings.

As slow-steaming vessels reduce their power, they may therefore be more vulnerable to the problems described.

The answer to such problems is – again ironically – to power up and boost the combustion efficiency so as to burn off as much of the fuel as possible and minimise the deposits.

Slow steaming may affect fuel injection and burning processes, bringing about poor fuel ignition and combustion. The scavenge pressures generated at lower loads may also not be optimal, further contributing to the deterioration of combustion efficiency.

In addition, the steam output from the economiser will be reduced and may not be sufficient to meet the plant’s heating requirements.

The cylinder oil feed rate is another area requiring attention during slow steaming. Under-lubrication can lead to acid formation and cold corrosion. On the other hand, excessive cylinder lubrication causes hard deposits to form on piston crowns and also reduces ‘controlled corrosion’, which in turn could cause cylinder liner bore polishing and scuffing.

Both under-lubrication and over-lubrication can wear down the cylinder liners rapidly. In the modern cylinder lubricators where the cylinder oil feed rate is proportional to both the engine load and the fuel sulphur content, the ship crew should adjust settings according to the sulphur content of the fuel as specified by the engine manufacturer. Ships operating on low sulphur fuels over long periods of time may require a switch to a cylinder oil with a lower ‘Total Base Number’, or TBN for short.

Excessive cylinder lubrication may cause cylinder oil to carry over into scavenge and exhaust spaces. If ignited, the cylinder oil could set off scavenge fires or explosions in the exhaust uptakes and turbochargers. Hence, close attention needs to be paid to the adjustment of cylinder oil feed rates during slow steaming.

As far as fuel quality trends go, DNV Petroleum Services is receiving more reports of increasing problems with fuel ignition and combustion quality.

Fuel oils with sub-par ignition and combustion properties can engender poor fuel economy, loss of power, the build-up of carbonaceous deposits, damaged piston rings, burnt piston crowns, ruined cylinder lubrication, and fatigue failure or the metal-to-metal contact of shock-loaded components. Even turbocharger explosions caused by poor fuel ignition and combustion have been reported.

A number of variables, including engine type, model and age, load profile and operational conditions, can influence how well an engine tolerates fuels with poor ignition and combustion properties. Medium and high speed diesel engines, and in general engines of older design, are more prone to operational problems when consuming such fuels, particularly at low and part load.

Ships may have some success in mitigating poor ignition and combustion by blending a distillate fuel or residual component with good ignition and combustion properties into the problem fuel, but again, onboard fuel blending may not be practicable.

This brings us back to the issue of slow steaming. Vessels operating on reduced speed should first determine the ignition and combustion quality of the fuel in use, since slow speed coupled with low load operation can be problematic if the fuel has poor ignition and combustion properties.

Some engine manufacturers are offering upgrade kits for their low speed engines to help shipowners and operators save on fuel costs while slow steaming their ships. They are also offering evaluation of compliance with the IMO NOx emission requirements during slow steaming.

When slow steaming their vessels, some shipping companies may not have adequately factored increased maintenance and repairs into the total operating costs, as such outlays become more apparent only during routine maintenance over time. Reducing these hidden costs successfully can therefore bring about substantial overall savings.

At the end of the day, good housekeeping and operational vigilance remain the keys to reducing maintenance and operating costs. Companies considering a slow-steaming strategy should not overlook this.