Recent operational experience reveals an increasing trend of reported propeller shaft bearing damages on older vessels, typically above 10 years of age, operating with environmentally acceptable lubricants (EALs). This technical news provides a summary of our experience with such damage cases and some considerations on how to minimize the propulsion safety risk and avoid costly damages.
Relevant for owners/managers, design offices, shipyards, suppliers, flag states, insurance, and onboard crew.
Probable causes of reported damages
The latest trend of bearing damages observed on older vessels is in contrast with the previous trend reported between 2013 and 2018, which involved damages on newer ships younger than 5 years of age.
Partial or complete wiping of aft bearings has been more prominent (see Figure 1), concurrently with leaking propeller shaft aft seals in many cases either as a contributory factor or a consequential event.
Figure 1 – Aft bearing wiping damage
Damages to propeller shaft bearings which have been performing well over a long period of time may typically arise out of one or a combination of:
- Continued operation with contaminated and/or degraded lubricant, typically arising out of a leaking aft seal
- Lowering of design and/or operating margins, i.e. shaft alignment sensitive installations operating with EALs (inferior EAL load bearing performance, see DNV GL study 2019)
- Operational factors, e.g. incomplete propeller immersion
- Life cycle fatigue
- Other events adversely influencing shaft alignment, e.g. grounding
Aft sealing arrangement – additional challenges with EALs
Figure 2 shows a typical shaft aft sealing arrangement where the proportion of seawater content in oil in the tiny barrier chamber space between sealing rings 2 and 3 and in the associated aft sealing system can be inherently high even during normal operation. When operating with EALs, this may increase the risk of probable biodegradation and accelerated deterioration of the lubricant quality in this system beyond the oil maker’s recommended limits for continued use.
Figure 2 – Typical shaft aft sealing arrangement
Typical signs of deterioration of EAL quality may also involve elevated levels of TAN (total acid number), most commonly resulting from hydrolysis of unsaturated esters.
Consequently, the compatibility of the sealing ring material with the deteriorated lubricant is challenged, which increases the risk of damage of sealing rings 2 and 3. Deterioration of ring 3 and the sealing function opens the gateway to water contamination of the main stern tube bearing lubricant system, risking bearing/shaft damage and/or leakage of oil to the sea.
The comparative risk of similar issues is lower for mineral oil installations, i.e. mineral oil in the chamber between rings 2 and 3 is not prone to a similar degree of deterioration under similar operating conditions. Besides, mineral oil properties also allow better water separation and drainage inboard.
Note that oil analysis and trending of bearing and shaft wear elements in a non-tight main stern tube lubricant system are not considered fully representative due to the exchange of contents between the system and the sea. There is a high probability that one of the key signs of underlying or developing bearing damage may go undetected under these conditions.
DNV rules include verification of tightness of the shaft sealing arrangement periodically as part of bottom and propeller shaft-related surveys.
Propeller shaft bearing damage handling
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Recommendations
Monitor and regulate the EAL quality in the aft sealing system to fulfil the oil makers’ and seal makers’ recommendations for continued use, in addition to the main stern tube bearing lubricant system.
Ensure that the samples are representative so that lubricant analysis results reflect the actual condition of the system.
Investigate and undertake prompt actions if main stern tube system lubricant analysis reveals elevated levels or an increasing trend of water content, TAN, bearing and/or shaft material wear elements, etc.
As a proactive measure for EAL applications on older vessels, consider the use of oil with the next higher viscosity grade above the design oil viscosity (see DNV T&R news 2019). Another alternative could be to optimize the aft bearing design to fulfil the latest main class DNV shaft alignment rules (see DNV rules Pt.4 Ch.2 Sec.4) or prescriptive Shaft align class notations with double sloped aft bearings.
References
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