Norway-based classification society DNV GL will update its shaft alignment design rules to differentiate between environmentally acceptable lubricants and mineral oils in stern tubes effective July 1, adding a viscosity influence parameter to existing lubrication rule criteria for the aft most propeller shaft bearings.
The society said in a June 6 news release that it based the update on findings from the first phase of a joint development project DNV GL has run in cooperation with marine insurers The Swedish Club, Norwegian Hull Club, Gard and Skuld to test the potential influence of EALs on failures. The project was prompted by an upsurge in stern tube bearing failures that coincided with the increased adoption of EALs after the introduction of regulations requiring their use in commercial vessels trading in U.S. waters in late 2013. The first phase focused on mapping out differences in load carrying capacity between EALs and mineral oils.
Phase 1 has looked into how pressure, temperature and shear rate influence the viscosity of the lubricants, thereby affecting the oil film thickness and the load-carrying capacity, Oystein Asheim Alnes, principal engineer at DNV GL, said in the release. Test results have proven that in particular, the pressure and temperature viscosity properties of EALs are different to those of an equal grade mineral oil. The findings show that while EALs provide safety margins that are equal to mineral oils in most operating modes, there are transient conditions where the EALs can have a reduced load carrying capacity.
DNV GL added the viscosity influence parameter to the existing lubrication rule criteria for the aft most propeller shaft bearings. The DNV GL lubrication criteria provides [ship]yards and designers with a strong tool for optimizing stern tube bearing design, including both the lubricant viscosity and now the lubricant type, Alnes stated.
The joint development project is now moving into its next stage, where it will further scrutinize oil film-forming capabilities, mixed and boundary lubrication behavior and lubricant degradation.
DNV GL oversaw detailed laboratory testing for the project by Leonardo Testing Services Ltd. at the University of Sheffield, United Kingdom, and by Insavalor at INSA Lyon in France.
According to its website, DNV GL provides risk management and quality assurance services to the maritime, oil and gas, and power and renewables industries, with more than 100,000 customers in more than 100 countries.
The U.S. Vessel General Permit policy regulates discharges caused by normal operations of commercial vessels in U.S. territorial waters and the Great Lakes. It includes specific requirements for products used to lubricate stern tubes and other equipment that may be immersed in or otherwise in direct contact with seawater, referred to as oil-to-water or oil-to-sea interfaces. Commercial vessels longer than 79 feet must use EALs in all oil-to-sea interfaces, such as controllable pitch propellers, azimuth thrusters and paddle wheel propulsion systems. The U.S. Environmental Protection Agency defined EALs as biodegradable and minimally-toxic and as not bio-accumulative.