According to Gard, the number of stern tube-related damage claims has increased considerably over the last few years and such damages are now one of the more frequent claim types handled by Gard under the hull and machinery policies.
s John Balaouras and Svend Leo Larsen explain, the nature and severity of damage suffered by stern tube / shafting systems, vary greatly – ranging from a simple aft seals’ leakage that can be rectified over a few days whilst afloat, to loss of propulsion involving costly salvage operations and extended repairs in drydock.
In the past, damage to a stern tube was often considered a relatively straightforward casualty by surveyors. The cause was often obvious – contact with a floating object such as ice, fishing nets or ropes, for example.
These causes still exist of course, but the significant increase in frequency over the past years, without any apparent explanation, raises concern.
As informed, over the last decade, the number of shaft / stern tube-related claims Gard has handled, either as claims’ lead or on follow accounts, exceeds 700 with the below spread over the years:
In addition, another notable characteristic of the EALs is poor hydrolysis stability when interacting with sea water once bypassing the aft seals. When contaminated with sea water, lubricant decomposes, and carboxylic acid is generated which can damage the seals resulting in even greater sea water ingress.
The speed of hydrolysis depends on various factors, for example, chemistry of the oil, temperature and additives. The most important factor, however, is the quantity of sea water present in the lubricant. According to IACS, the upper water limit suggested is 1%, which is the same for both the traditional mineral oils and EALs. Experience has nevertheless shown that this limit is quite “relaxed” and somehow misguiding when it comes to EALs, as apparently even minor water quantities can trigger the hydrolysis reaction of the EAL and affect its characteristics including stability.
Advice to shipowners
To prevent or mitigate stern tube damage, we recommend that Owners take the following actions.
During Vessel’s operation
- Selection of proper EAL viscosity (one grade up) and thoroughly consider the various qualities of EAL being offered in the market to ensure minimum risk of failure (i.e. selection of EAL based on fully saturated synthetic esters, non-emulsifying)
- Frequent testing of representative oil samples from aft / fwd seals’ tanks and stern tube system
- Retrofit installation of filtering / surveillance systems available in the market
- Continuous monitoring of bearings’ temperature
- Close monitoring of aft seal oil circulation if such a system is fitted.
- Frequent monitoring of the drain tank in case air-type seals are used
- Change of aft seals’ oil once sea water presence has been confirmed even for minor quantities. Drain water if non-emulsifying EAL is used and water contamination was swiftly spotted.
- Keep a close eye of elevated levels or increasing trend of water content, TAN, bearing / shaft material elements (Tin, Lead etc.) in the lub oil analysis
- Avoid continuous sailing with partially immersed propeller (if unavoidable, propeller to be used with reduced rpm)
- Properly check condition of rope guard and net cutter during UW surveys
During routine repairs / new buildings
- Air-type seals are preferred together with use of traditional mineral oil
- Modify aft seal oil system to the continuous circulation type rather than the static / gravity type. Note some stern tube seals manufacturers have reduced the provided warranty from 5 to 2.5 years for single piping s/t lub oil arrangements for EAL oil
- Use of multi-sloped bearings’ modification at first opportunity in dry dock to ensure better hydrodynamic lubrication between bearings and tailshaft
- Verification of shaft alignment from M/E flange to aft end via laser & jack load tests
- If not already fitted, consider installing net pickup ring and rope cutter to avoid ropes/fishing net to enter the stern tube seal area.