Survey of Oil-Lubricated Stern Bearing and Shaft

(a) Survey Procedure

The survey of an oil-lubricated stern tube shaft (tailshaft) is normally carried out at five-year intervals, although extensions may be granted when supported by satisfactory condition monitoring. The objective of the survey is to assess wear, detect defects, and ensure the continued reliability of the shafting system.

1. Wear-Down Measurement

Before any dismantling work begins, the vertical wear-down (clearance) of the stern bearing is measured using a poker gauge.

• This measurement indicates the extent of bearing wear.
• The obtained value is compared with:
  • Original (as-built) clearances
  • Previous survey readings
• This comparison helps in determining the wear rate and whether it is within acceptable limits.

2. Oil Sample Analysis

Samples of stern tube lubricating oil are periodically analyzed to assess internal condition without dismantling. The analysis includes checking for:

• Water contamination
• Metallic particles such as iron, copper, and white metal
• Changes in viscosity and acid number

Consistently satisfactory results indicate good internal condition and may support extension of the survey interval.

3. Visual Inspection of the Shaft

Once the shaft is withdrawn (or exposed during a partial survey), a detailed visual examination is carried out.

• The shaft surface is checked for:
  • Corrosion
  • Pitting
  • Scoring or surface damage
• Special attention is given to areas in contact with seals, as these are more prone to wear and damage.

4. Non-Destructive Testing (NDT)

Critical regions of the shaft are subjected to NDT methods such as:

• Magnetic Particle Inspection (MPI)
• Dye Penetrant Testing (DPT)

These tests are focused on:

• The tapered end
• Keyway
• Threaded portions

The purpose is to detect fatigue cracks or hidden defects that may not be visible to the naked eye.

5. Bearing Inspection

The stern bearing, usually lined with white metal, is carefully examined for:

• Wiping (indicative of overheating or lubrication failure)
• Pitting
• Fatigue cracking

In addition, the bond between the white metal lining and the backing shell is checked to ensure structural integrity.

6. Seal Examination

The sealing arrangement, typically consisting of rubber lip seals, is inspected for:

• Wear and tear
• Loss of elasticity
• Hardening or cracking

Even if no obvious defects are visible, these seals are generally renewed during major surveys to ensure reliability.

(b) Proving Integrity of Outboard Seal Before Dock Flooding

The outboard seal is the final barrier that prevents:

• Oil leakage from the stern tube into the sea
• Seawater ingress into the stern tube

Therefore, its integrity must be confirmed before the dry dock is flooded.

Methods of Testing

1. Static Pressure Test

This is the most commonly used method.

• The stern tube is completely filled with oil.
• The header tank level is raised to create a pressure head slightly higher than the expected draft pressure when the vessel is afloat.
• The aft seal area (near the rope guard) is observed over a period (typically 6–12 hours).
• Any oil seepage indicates leakage, while no leakage confirms proper sealing.

2. Air Pressure Test

• Low-pressure compressed air is introduced into the space between the sealing rings.
• The pressure is monitored using a pressure gauge over a specified duration.
• If the pressure remains constant, the seal is considered:
  • Airtight
  • Therefore, effectively watertight

  3. Vacuum Test

  • A vacuum is applied to the seal arrangement.
  • Stability of the vacuum over time indicates that:
    • The seal lips are maintaining proper contact with the shaft liner
    • No leakage paths are present

    4. Interspace Drain Check

    • In systems with an interspace (void) between seals, the drain from this space is opened during testing.
    • Observation of oil or air escaping from this drain indicates:
      • Leakage past one or more sealing rings
      • Failure of seal integrity