Lubrication and environmental concerns
Any ship will have lubrication needs beyond that of the engine. There is the transmission system itself, multiple pumps and compressors, valves in piping systems for fuel, water and more. Engine lubrication is a subject in itself but all of the other mechanical systems will also need lubrication and things such as crane wires and lifeboat falls will need to be kept supple and protected against corrosion using greases.
Many machines will have unique requirements and in such cases there are normally special formulations provided by the leading lubricant manufacturers.
Many machine oils and greases are labelled as ‘general purpose’ but this should not be taken to mean that they are suitable for all purposes and in some instances very specific properties are needed. The wrong choice of lubricant can mean the failure of essential systems and this is frequently found to be the main or contributory cause of accidents. Closely allied to lubricants are the hydraulic fluids that can be found in cranes, winches, pumps and many other items of machinery and equipment such as watertight doors, cargo hatches and ro-ro ramps and lifts.
Most lubricants and fluids are used in closed applications but in deck machinery and in equipment such as controllable pitch propellers, podded propulsion systems, stern tube seals thrusters and steering gear there is a high risk that they can leak into the waters the ship is operating in. For regulatory purposes such as MARPOL or under local law, any escape into the environment of these products will be considered as an act of pollution and the operator is therefore at risk of prosecution.
Leaks from deck machinery should be dealt with as soon as is practically possible and especially before arrival into port where rain could cause the spill to be washed overboard.
If the leak is of hydraulic fluid and did not result from an accidental spill, there is a high possibility that the system is compromised. The risk of pollution is therefore not the only problem as the system itself may fail resulting in an inoperable winch or a hatch cover that cannot be opened or closed.
Failure to lubricate systems is also a cause in many PSC detentions especially if the system is one considered an essential safety device. Seized dampers and air pipe covers are notorious in this regard as are seized valves in many different systems.
Environmental factors to be considered
The pollution risk from deck machinery and stern tube seals can be reduced by making use of biodegradable lubricants. Such products are generally made from vegetable- or animal-sourced material. Biodegradable products are still in their infancy as far as marine lubricants go, as it was only in 2002 that Vickers Oils became the first company to make biodegradable lubricants commercially available to the global marine market. In addition to improving environmental credentials, the opportunity to reduce operating costs is just as important to the marine industry as any other business. The use of biodegradable lubricants is one way that vessel operators can achieve cost saving benefits if their makers’ claims as to performance are true.
It may be thought that the most demanding application for lubricants is in the engine where high temperatures will be experienced but exterior uses are probably even more so. Winches and cranes as well as underwater equipment will experience very wide operational temperature ranges. For some ships, there will be times when such equipment is exposed to sub-zero temperatures and others when it could be exposed to hot tropical sun. Wind, rain frost and snow will also affect performance. It is however important for the ship that the equipment operates at all these extremes so the choice of appropriate products is something thought must be given to.
In many areas, use of synthetic or biodegradable lubricants is a voluntary choice for operators but in the US the requirements of the Vessel General Permit (VGP) make lubricant choice more restricted. Under the VGP introduced in 2013, the list of permitted substances and the quantity each ship above 300gt will be allowed to discharge was reduced – quite dramatically in some cases.
Since December 2013, lubricants in any equipment or system that has an oil-to-sea interface (essentially all propulsion systems and deck machinery where run-off over the ship’s side could occur) must be environmentally acceptable lubricants (EALs) unless doing so would be ‘technically unfeasible’. EALs are defined as biodegradable, which rules out all mineral-based lubricants and even some synthetic alternatives.
The exact definition of an EAL is contained in the US Environmental Protection Agency’s document, EPA 800-R-11-002 November 2011. Operators have to apply for a VGP before a vessel enters US waters and to do so they need to identify all oil-to-sea interfaces and lubricants involved.
Among the most obvious systems are the stern tube, rudder bearings, CP propellers, thrusters, and fin stabilisers. In addition, winches, cranes, hatch covers and even crane wires and the like must be considered. The ship will be required to document all lubricants and any reason why the use of an EAL would be technically unfeasible. Most major oil companies and some specialist suppliers have formulated compliant products, but these are not necessarily compatible with some makes of seals, especially conventional rubber seals. This is a known problem and most combinations of lubricants and seals have been tested for compatibility over normal drydocking cycles of two to three years.
In selecting an EAL, operators must therefore seek advice from the seal manufacturer and great care must be exercised if the vessel makes use of enhanced or extended drydocking strategies. Inspections with regard to EALs would involve visual sheen tests and inspections of deck runoff. The ‘unless technically infeasible’ proviso can allow some temporary relief if the ship has seals that are incompatible with any EALs, in which case it can continue to use mineral oil until the next planned docking, when the seals are to be replaced, or if the equipment manufacturer has no recommended seal-EAL combination for its product. Some pre- lubricated wire ropes are also included in the exemption.
If the use of an EAL in an oil-to-sea interface is claimed to be technically infeasible, the ship must carry documentation to that effect. Supporting documentation written by the manufacturer or owner must not be more than one year old and must confirm the factual situation.
Of all the potential pollution sources covered by the VGP, stern tubes are arguably the main contributors. Manufacturers of stern tube seals and bearings have invested considerable sums in developing reliable and robust products, but damages can still occur for a variety of reasons and leakage is an inevitable result unless a seawater lubricated bearing is used. Most lubricant makers have developed EALs for stern tube bearings and while some seal makers have not sanctioned their use, most have.
In early 2018, DNV GL launched a new joint development project (JDP) in cooperation with marine insurers The Swedish Club, Norwegian Hull Club, Gard and Skuld to test the potential influence of EALs on failures in stern tube bearings. The JDP was prompted by an apparent increase in stern tube bearing failures over the last few years. This coincides with the increased uptake of EALs after the 2013 VGP rules, but another possible cause is the introduction of new propulsion system designs, such as single stern tube bearing installations and larger and heavier propellers operating at lower RPM. Aside from the JDP, there has been no other question mark posed on the use of EALs.
Another factor that could be causing the increase in failures is not the EALs themselves, but the manner of their being put into service in individual ships. Seal makers have advised of the necessity when switching to EALs to completely remove any traces of previously-used mineral oil lubricants. This is because in some cases a mixture of EALs and mineral oil will cause a chemical reaction, resulting in a gummy mixture that is not able to perform the requirements of a lubricant.
When removing old lubricant, all of the tanks, piping, stern tube and the seal boxes have to be cleaned or flushed thoroughly to prevent future problems. Linked to this is the practice of some shipyards to carryout sea trials of newbuildings with mineral oil lubricated shaft bearings but deliver the ships with EALs if requested by the owner.
It could be that such a practice will leave even new vessels vulnerable to shaft failure if the oil change process did not include proper cleaning. On older ships, the problem could be attributable to changes made during drydockings or by the crew, especially if incompatible products were added at some point in the case of normal service operations.
In June DNV GL announced that it is updating its shaft alignment design rules to differentiate between EALs and mineral oils in stern tubes. The update, effective 1 July 2019, is based on the phase 1 findings from the JDP DNV GL has been running in cooperation with marine insurers The Swedish Club, Norwegian Hull Club, Gard and Skuld to test the potential influence of EALs on failures in stern tube bearings.
Test results have proven that, in particular, the pressure- and temperature viscosity properties of EALs are different to those of an equal grade mineral oil. The findings show that while EALs provide safety margins that are equal to mineral oils in most operating modes, there are transient conditions where the EALs can have a reduced load carrying capacity.
In light of the results, DNV GL has added a viscosity influence parameter to the existing lubrication rule criteria for the aft most propeller shaft bearings. The JDP is now moving into the next stage, where oil film forming capabilities, mixed/boundary lubrication behaviour and lubricant degradation will be further scrutinised. The JDP complements DNV GL continuing focus on shaft alignment that has also resulted in revisions to the DNV GL rules for single bearing installations as well as the introduction of the shaft align class notations in January 2018.
Quality and quantity
Choosing the right grease for each application is important and although there are multi-purpose greases, these are unsuitable for very high-pressure use and other specialist applications. Reference should always be made to the equipment manuals and to the grease specification and technical data sheets.
Even when the correct grease is chosen, under- or over-lubrication can create problems. Both can cause component failure or heat build-up but for different reasons. Under-greasing obviously increases friction but too much grease in a bearing cavity for example, will cause the rotating bearing elements to begin churning the grease, pushing it out of the way, resulting in energy loss and rising temperatures. This leads to rapid oxidation of the grease as well as an accelerated rate of oil bleed. The heat that has been generated over time along with the oil bleed eventually will cook the grease thickener into a hard, crusty build-up that can impair proper lubrication and even block new grease from reaching the core of the bearing. This can result in accelerated wear of the rolling elements and then component failure.