The variety of stern tube-related injury claims has elevated significantly over the previous few years, and that is inflicting marine insurer Gard some concern.
Previously, injury to a stern tube was typically thought of a comparatively easy casualty by surveyors. The trigger was typically apparent – contact with a floating object resembling ice, fishing nets or ropes, for instance.
These causes nonetheless exist, however the vital enhance in frequency over the previous years, with none obvious rationalization, raises concern, says Gard.
Over the past decade, the variety of shaft / stern tube-related claims Gard has dealt with, both as claims’ lead or on observe accounts, exceeds 700. The rise doesn’t merely mirror a rise within the variety of vessels insured. Reasonably, the numerous enhance in claims appears to have adopted the introduction of the revised Vessel Common Allow (VGP) necessities in 2013 by the U.S. Surroundings Safety Company (EPA).
These require all vessels over 79 toes buying and selling in U.S. waters to make use of environmentally acceptable lubricants (EALs) in all oil-to-water interfaces.
Based on a research carried out in 2019 by DNV with the help of Scandinavian Insurers (Gard, Skuld, Swedish Membership & Norwegian Hull Membership) it was found that two key options of EALs make them completely different from the normal mineral oils extensively used earlier than VGP necessities got here into pressure:
Strain / viscosity coefficient => below excessive load operations (i.e. arduous turns at excessive speeds) the EALs function with a decrease security margin of the minimal oil movie between the tailshaft and the bearings
Viscosity index (EALs considerably larger than mineral oils) => EALs function with decrease viscosity below decrease temperatures (i.e. mooring trials and chilly start-up).
One other notable attribute of the EALs is poor hydrolysis stability when interacting with sea water as soon as bypassing the aft seals. When contaminated with sea water, lubricant decomposes, and carboxylic acid is generated which may injury the seals leading to even higher sea water ingress, says Gard. In sum, it has been confirmed that the efficiency capability of the normal mineral oil – the sturdy lubricating oil movie, consistency on the presence of water within the oil and glorious seal compatibility – can’t be matched by the EALs. Because of this shipowners have a tendency to return to the “conventional” mineral oils mixed with air-type seals, for each newbuildings and present vessels.
Gard’s evaluation of bulk carriers and container ships confirmed that as much as 80% of the incidents investigated concerned stern tube seal failure with EAL oil in use while the common age of the vessels was 12 years.
“Design fault” was additionally recognized as a trigger worthy of additional exploration.