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 seawater or that may otherwise come in to direct contact with it, referred to as oil-to-water or oil-to-sea interfaces. Commercial vessels longer than 79 feet must use environmentally acceptable lubricants 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 these lubes as biodegradable and minimally-toxic and as not bio-accumulative.

Kevin Duncan, market applications specialist – energy technologies for Croda Europe, noted that environmentally acceptable lubricants are a viable and effective option for lubrication, but he added that it is important to understand the differences between different products. Key ISO classifications include hydraulic environmental triglycerides, hydraulic environmental ester oil synthetic, polyakylene glycol base such as polyglycol, and hydraulic environmental polyalphaolefin and related products.

Key factors such as oxidation stability and hydrolytic stability can vary between each classification, and within different producers under each classification. These differences underscore the importance of careful selection, he said. Oxidation stability and hydrolytic stability help slow oil degradation. Another desired effect of the oil, film forming behavior, helps eliminate engineering issues. Together, these help minimize the risk of equipment failure, Duncan said.

Ian Nielsen, strategic account representative for RSC Bio Solutions, noted that each type of hydraulic fluid is characterized by the chemical composition of its base oil. For example, those made with triglyceride base oil offer superior lubricity but rank lower in resistance to oxidation and water. Those made with PAOs and synthetic hydrocarbons offer the strongest oxidation and water resistances, superior lubricity and broad seal compatibility.

Don Gregory, technical director for Gulf Oil Marine, said biobased lubricants may be more sustainable, and they meet requirements because they are biodegradable, non-toxic and not bio-accumulative. Their high polarity makes them excellent lubricants, he noted, and they typically offer a high viscosity index. He emphasized that when considering a changeover in stern tube lubricants, it is important to check suitability of the formulation for the application and to check the compatibility with the previous lubricant. Draining – and if possible purging or manually cleaning – the system is important, he said, to remove all residues of the previous lubricant. It’s also important to change lubricant filters prior to refilling and to ensure any potential sources of external contaminants are eliminated, Gregory said.