Conditions that Promote Microbial Growth:

• Presence of water
• Availability of nutrients
• Favourable temperature (25-40°C) and pH (8-9)
• Oxygen (depending on the type of microbes)

Indications of Microbial Degradation:

• Rotten egg-like smell due to gas production
• Slimy oil appearance, often with peeling paint inside the crankcase
• Black staining on white metal bearings, pins, and journals
• Excess water and sludge accumulation after purification
• Frequent filter plugging
• Corrosion on unprotected surfaces

Sources of Microbial Contamination:

• Distillate fuel
• Lube oil itself
• Cooling systems, bilge, retention tanks, and ballast tanks
• Contaminated bunkered oil

Effects of Microbial Degradation:

• Corrosive damage to bearings and journals due to acid production
• Increased water content in the oil, challenging to remove by purification
• Filter blockages and restricted flow
• Deterioration in oil properties, such as viscosity and pH
• Reduced heat transfer in coolers

Prevention of Microbial Degradation:

• Regular draining to avoid water accumulation
• Maintain ideal temperature conditions to inhibit microbial growth
• Avoid water contamination in the oil
• Regular testing and correct operation of purification systems
• Use biocides or fungicides, as recommended by oil suppliers

Correct Sampling for Shore-Based Testing:

1. Always use the same sampling location, ideally in the main supply line just before the entry to the main engine.
2. Drain a sufficient amount of oil before collecting a sample.
3. Rinse the new container with oil before collection.
4. Draw samples only after the engine has been running at normal operating conditions.
5. Fully seal and label the sample with the date, vessel name, running hours, oil grade, and sampling point identification.

(b) Abnormal Water Content: High water content indicates water ingress into the system, potentially due to leaks in piston cooling pipes, heat exchangers, or cylinder liners, or purifier malfunction.

Action:

• Locate and repair the source of the water ingress.
• Drain the water after allowing sufficient time for settling.
• Use the purifier to remove remaining water and contaminants.
• Consider batch purification for more thorough cleaning.

Abnormal TBN (Total Base Number): Low TBN suggests the oil's alkalinity is depleted. This can be caused by water ingress or microbial contamination.

Action:

• Remove contaminants through purification.
• Depending on the severity, replenish or completely renew the oil. Consider the potential need for a complete oil change if the contamination is severe.