In many cases, there are inherent challenges and/or gaps in detecting the early signs of shaft aft bearing damage (hereafter referred to as the aft bearing) with the aim to minimise the risk and severity of damage:
- Older vessels using EALs which exhibit inferior load bearing capability in some extreme transient operating conditions with accompanying high local oil film pressure in the bearing.
- Aft sealing system lubricant quality which is not monitored to ensure suitability for continued use, in particular where an EAL is used. Degradation of lubricant quality may negatively affect the inner shaft sealing ring condition, consequentially risking contamination of the stern tube lubricant system.
- Continuous operation with incomplete propeller immersion leads to downward bending of the shaft because of eccentric propeller thrust forces. This runs the risk of edge loading in the aft part of the bearing and consequential local overheating.
- Abrupt increase in aft bearing temperature before the high temperature alarm is activated, particularly at lower operating temperatures.
How to reduce the risk of aft bearing damage
Some suggestive measures to enhance the design and aft bearing monitoring, and hence to reduce the risk of damage:
- Consider the use of a double-sloped aft bearing – to further optimize the load distribution and enhance the design and operating margins in continuous and extreme transient conditions.
- Provide means of warning for the high rate of rise of aft bearing temperature and incomplete propeller immersion – to be used as early signs for operators to undertake proactive actions.
- As a proactive measure to safeguard the inner shaft sealing ring(s) and stern tube lubricant condition, monitor and regulate the lubricant (EAL) quality in the aft sealing system to fulfil the oil makers’ and seal makers’ recommendations for continued use.
- Increase the EAL viscosity to the next higher grade above design specification for EAL application on older installations (for vessels with applicable DNV Rules older than July 2019). Vessels with applicable rules as of July 2019 or later will need a new approval if design oil type or viscosity grade is changed (ref DNV EAL news Oct 2019).
- Investigate and undertake prompt actions if main stern tube system lubricant analysis reveals elevated levels or an increasing trend of water content, Total Acid Number (TAN), bearing and/or shaft material wear elements, etc.
Recommendations
According to DNV, to reduce the risk of propeller shaft aft bearing damage, DNV recommends that customers review and consider the recommendations in this news. By opting for the TMON(Oil lubricated, +) notation, operators have the possibility to enhance the design as well as pick up early warning signs to prevent issues and minimize the risk and severity of bearing damage.
