DNV’s Guide To Minimizing Propeller Shaft Bearing Damage

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In January 2022, DNV brought attention to an increasing trend of propeller shaft aft bearing damage on vessels aged 10 years or older. This trend persists, especially in vessels using environmentally acceptable lubricants (EALs) or those with a history of contaminated lubricants.

This guide focuses on mitigating the risk of shaft aft bearing damage, addressing key challenges and offering practical recommendations, says an article published on dnv website.

Key Challenges In Safeguarding Aft Bearing

  1. Inferior Load Bearing Capability: Older vessels using EALs may experience diminished load-bearing capability, particularly in extreme transient operating conditions.
  2. Sealing System Lubricant Quality: Lubricant degradation can affect the inner shaft sealing ring, risking contamination of the stern tube lubricant system.
  3. Incomplete Propeller Immersion: Continuous operation with incomplete propeller immersion can lead to shaft bending, causing edge loading and local overheating.

How To Reduce the Risk?

  1. Double-Sloped Aft Bearing: Consider implementing a double-sloped aft bearing to optimize load distribution and enhance design and operating margins.
  2. Early Warning Systems: Provide means for early detection of high rates of aft bearing temperature rise and incomplete propeller immersion, allowing operators to take proactive actions.
  3. Monitor Lubricant Quality: Regularly monitor and regulate the lubricant (EAL) quality in the aft sealing system to meet recommended standards.
  4. Adjust EAL Viscosity: Increase EAL viscosity for older installations, considering a grade higher than the design specification.
  5. Prompt Actions on Analysis Results: Undertake prompt actions based on stern tube system lubricant analysis results, addressing elevated water content, Total Acid Number (TAN), and wear elements.

TMON(Oil Lubricated, +) Notation by DNV

Introduction of TMON(Oil Lubricated, +): DNV introduced the TMON(Oil Lubricated, +) notation in July 2022 to minimize propulsion safety risk and prevent equipment failure.

Prerequisites: TMON(Oil Lubricated) and Shaft Align class notations are prerequisites, incorporating measures to minimize the risk of damage.

Benefits: Enhanced aft bearing performance, early detection of operational issues, proactive monitoring of the aft sealing system, and positive implications on commercial operations.

Recommendations

Consideration of Recommendations: DNV recommends stakeholders review and consider the provided recommendations to reduce the risk of propeller shaft aft bearing damage.

TMON(Oil Lubricated, +) Notation: Operators can enhance design and receive early warnings by opting for the TMON(Oil Lubricated, +) notation, reducing the risk and severity of bearing damage.

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Source:dnv