Main Engine Breakdown and Cargo Claims

Cargo Damage Through Delays – Slow Speed Main Engine Damage

High value cargo claims and charter party disputes caused by engine damage delays during a voyage continue to occur. Time sensitive cargoes may spoil before reaching the discharge port and the vessel’s seaworthiness may be questioned by charterers and cargo interests. A notable proportion of these delays result from damage to slow speed main engines, caused by accelerated liner and piston ring wear.

If you’re unfortunate enough to work on, own or manage a vessel which has been struck by major liner and piston ring wear, you will recognise a scenario where the vessel may be disabled for some time with associated costly repairs. Sourcing, supplying and shipping main engine liners, pistons and piston rings are both costly and time consuming. Long lead times on the supply of replacement parts can extend the period the vessel is out of service. This may result in further losses for the shipowner.

In this article we focus on some of the causes of large two stroke main engine liner damage, with a particular focus on the concerns of using the incorrect cylinder oil and feed rate and its relationship with the fuel used. 

The Relationship Between Low Sulphur Fuels and Liner Damage

Since 2010, the allowable sulphur content of marine fuels has been subject to reduction measures. These restrictions on sulphur content have become very stringent in designated emission control areas (ECAs). The global sulphur cap of 0.5% in 2020 will mean all vessels around the world need to find a way of controlling their emissions, and in many cases this will involve burning ultra-low sulphur fuels. 

Acid Attack – The Role of Cylinder Oil

Most modern two stroke engines have been designed for operation on high sulphur residual fuel. Engines operated using high sulphur fuel normally use cylinder oil with a high total base number (TBN) or base number (BN). The BN of cylinder oil is the measure of the cylinder oil’s ability to neutralise acid. The higher the BN the more alkaline the oil.

During the combustion cycle, sulphur in the fuel is released forming SO₂ (sulphur dioxide) and some of this forms SO₃ (sulphur trioxide).  Water contained within the scavenging air and from the combustion process reacts to form sulphuric acid.

Additives, for example calcium carbonate, are added to cylinder oil to increase the BN and neutralise the sulphuric acid. If the acid is not neutralised, corrosion of iron will occur and this is the primary cause of corrosive wear for liners and piston rings.

Low Sulphur Fuel: Less Sulphuric Acid

 A vessel using low and ultra-low sulphur fuel will produce less sulphuric acid during the combustion process. If the BN of your cylinder oil is too high this may mean that alkalinity in the cylinder oil is not neutralised and compounds can be formed which cause damage. Some slow speed engines suffer from alkaline deposits building up on the piston crown which can damage the oil film between the piston ring and liner. This can lead to scuffing and seizures. Deposits may also form between the piston rings and pistons, preventing free movement of piston rings and increased liner wear.

If a vessel is operating in an emission control area where low sulphur or ultra-low sulphur fuel (less than 0.1% m/m sulphur) is used  the correct choice of cylinder oil, with an appropriate base number, is vital.

Feed Rate

Another factor to consider is the cylinder oil feed rate.  This is important because engines operate at different loads, revolutions and power. Whilst some engines have automated adjustment for feed rate based on revolutions and sulphur content, there are still many manually adjusted systems in use.

 Incorrect cylinder oil feed rate may also lead to liner and piston ring wear. The feed rate should be adjusted in accordance with the BN of cylinder oil and sulphur content of the fuel.

In addition to this there may be concerns over the liner surface. There is a need for controlled corrosion in order to form small pockets in the cylinder lining running surface. This allows hydrodynamic lubrication from oil within the pockets to occur. This is known as a lubricating oil film. Maintaining a good lubricating oil film is important in preventing liner wear as well so this is another reason why the optimal feed rate is essential.

Summary:

If a vessel is trading in an area where low sulphur fuel is mandatory, it is very important that the type and feed rate of the cylinder oil is carefully considered to prevent damage to main engine liner and piston rings.

If you’re operating an engine without an appropriate balance between sulphur content and BN of cylinder oil it may lead to either scuffing or corrosion wear. Both of these will have an impact on the lifespan of the engine components.

Cylinder Oil – Loss Prevention

• Ensure that main engine and cylinder oil lubricators are operated as per maker’s guidance. Specific advice may be given for cylinder oil feed rate and cylinder oil BN. Lubricating oil suppliers may also provide suitable guidance.
• If your vessel is undertaking voyages where there are different sulphur content fuels in use then check to make sure that the cylinder oils on board are fit for purpose for all fuel used.
• Consider increasing the frequency of scavenge inspections of piston rings and liners when changes are being made to cylinder oil feed rate or the sulphur content of the fuel being used.
• Record the cylinder oil feed rate with a greater focus during periods of change and take pictures of liner and piston ring condition where possible, in port and after changes in cylinder oil feed rate or cylinder oil BN. Ensure this information is shared with fellow engineers on rotation and the ship management office.
• During piston overhauls, record and analyse piston ring gaps, ring wear and liner wear. This should be carried out as per routine planned maintenance. However extreme wear should be flagged up and properly investigated.
• It is important to ensure that all planned maintenance records and monitoring are accurate and well recorded. If there is an allegation of unseaworthiness, this may be needed to defend a claim. Please refer to our loss prevention guide The Mariner’s Role in Collecting Evidence for guidance.
• If liner temperature monitoring is fitted, then monitor this closely during periods of change.
• Consider the use of scrape down analysis to monitor liner wear and adjust cylinder oil feed rates.

Please contact Mark Smith or Alvin Forster should you wish to discuss any of the issues raised in this article.