Very few owners have any input into the design and construction of the propeller shaft arrangements although they are able to stipulate the choice of bearing materials or lubricants even if their choice attracts a premium over the yard’s standards.
In recent years, a number of apparently unconnected factors have combined to affect the performance of propeller shafts, bearings and seals. Firstly ships have become larger taking the experience of classification societies, designers and ship operators into uncharted territory. The impact of the IMO’s EEDI regulations has seen more attention paid to the construction of ships and propulsion systems. This has resulted in a lower installed power while at the same time an increase in propeller diameter for many ship types.
Another factor is the trend to switch from fore and aft stern tube bearings to a single aft bearing and finally the need to use non-mineral oil lubricants to meet US VGP regulations. An aggravating factor for some ships, particularly container vessels is that while their size has increased quite dramatically the same might not be true of cargo volumes carried.
Other ship types have found that changing trade patterns often mean ships must make longer ballast voyages between cargoes. The ballast water regulations introduced to combat invasive species transfer now require ships without treatment systems to carry out ballast water exchange at sea; while some may do this by the flow through method many do not. Thus, ships are rarely in a fully laden state and the degree of propeller immersion is affected.
The change in vessel configuration over the years has seen the engine room move from amidships to aft locations and the installed power and heavier propellers has required a thicker shaft. The result is a much less flexible arrangement and stiffer shafts leading to increased bearing wear or worse if shaft alignment is not optimal under a wide range of operating conditions.
Classification societies have reacted to these developments by way of advice to operators and in a number of cases new rules for shafting arrangements. For example, in 2016 ABS added a new voluntary Enhanced Shaft Alignment (ESA) notation to address requests from owners and operators who wish to perform a more detailed shaft alignment analysis and installation assessment and this year, following a bulletin issued in late 2017, DNV-GL revised its class rules for single stern tube bearing installations and introduced two new class notations, Shaft align(1) and Shaft align(2). Most of the leading classification societies had earlier introduced new rules for water-lubricated stern tube bearings.
Taking practical action
Since class societies have not only become aware of but have taken steps within their rules to take account of the latest experience with propeller shaft related problems, it might be expected that future newbuildings will suffer less. However, even the most diligent calculations cannot take account of poor construction and fabrication techniques, later changes to equipment or switching to new lubricants.
The shipping industry is under pressure to cut its CO2 output and many owners are more than happy to explore ways to achieve this since it implies a lower fuel bill. One of the ways to achieve a significant saving on an existing ship is to swap the propeller to one more suited to the operating profile of the vessel and its installed engine. In some instances, this may have a beneficial effect on the propeller shaft functioning but in others it may result in additional weight, less clearance of the propeller or even significant periods when the propeller is not fully submerged.
A DNV GL bulletin issued in 2017 states that ‘operation with incomplete propeller immersion may induce an excessive eccentric thrust on the propeller and, consequently, a downward bending moment on the shaft. This may result in exaggerated localised loads (edge loading) and surface pressure on the aft bearing arising out of an increased relative slope and reduced bearing contact area.
Localised bearing loads acting on a diminished contact area, not catered for in the design criteria, lead to total or partial loss of an effective hydrodynamic oil film of minimum thickness. Hence, the risk of prospective consequential bearing damage co-exists under exceptional operating conditions with incomplete propeller immersion.
The additional bending moment generated is a function of the degree of lack of propeller immersion, RPM and the power. Elaborating this further, the bending moment is proportional to the thrust force, which is proportional to the square of the RPM. Consequently, increasing RPM introduces an exponential degree of risk in a partially submerged propeller condition.
The advice that is given by the organisation is similar to that given by other class societies and includes lowering the RPM/power whenever possible and limiting steering angles. Whether this is possible or not will depend upon where the ship is operating and what manoeuvres are necessary. Some of the problems may be alleviated by taking on more ballast if under keel clearance allows or even making use of tugs to aid manoeuvring.
The latter may seem an expensive way of avoiding an unknown risk but the consequences of a shaft or seal failure could be even more so. There is though little expense involved in making crew aware of the potential and relying on their own skills to mitigate the load placed on bearings. Even with a seawater lubricated bearing where the risk of pollution is nil, the damage to the bearing or the shaft must be a consideration.
If a ship does suffer any problem in operation, it goes without saying that the operator should inform other vessels in the fleet of the cause of the problem and any advice received as to avoidance of similar occurrences. By the same token, any problem that is experienced on a recent newbuilding should be fully investigated and appropriate measures taken to ensure that future vessels in the series can be modified as necessary.
Monitoring bearings and the shaft for vibration, temperature changes, flow rate of lubricant and bearing wear are all sensible measures that should be practiced. Several companies produce whole systems for this including for example Wärtsilä’s Sea-Master. Even without such systems, a change in vibration levels or loss of propulsion efficiency may be noticed indicating potential damage to the propeller or the shaft which may have become bent due to a sudden change in forces acting on it.
In oil lubricated bearings, regular analysis of the lubricant can give early warning of excess bearing wear and allow for corrective action. A change in the needed feed rate for lubricant will also be an immediate cause for concern that requires further investigation.
Any possible problems with the shafting arrangements should be investigated as early as possible so as to avoid catastrophic failure and the need for drydocking. There are many specialist engineering firms that can carry out inspections and make many repairs underwater using divers and equipment allowing work to be carried out on bearings and seals under ‘dry’ conditions.
Some work on the shaft is possible inside the ship including machining or building up of the shaft in case of misalignments or minor damage such as small cracks or pitting. In the case of a badly bent shaft, a replacement might be avoidable if the shaft can be cold straightened by a specialist engineering firm using hydraulic presses and sensors. Cold straightening does not affect the metallurgy of the shaft, as it actually releases stress from the material. This differs from other straightening methods, such as the hot-spot method which uses heat and puts stress on the material, affecting the metallurgy of the shaft.