Over the past few years, there has been an increase in reports of stern tube bearing failures and a perception that environmentally acceptable lubricants (EALs) may be to blame. While the uptick coincides with the increased use of EALs due to 2013 regulations requiring them in all ships sailing in U.S. waters, classification society DNV GL says changes in vessel operation and design — as well as changes in the design of propulsion systems — complicate the situation.
DNV GL does not want to rely on assumptions and has launched a joint development project (JDP) with marine insurers The Swedish Club, Norwegian Hull Club, Gard and Skuld to test the potential influence of EALs on failures in stern tube bearings. DNV GL is overseeing detailed laboratory testing of EALs by Leonardo Testing Services Ltd. at the University of Sheffield in the United Kingdom.
“Today’s DNV GL design rules on shaft alignment and stern tube bearings are developed based on our experiences with mineral oil as lubricants, where we have considered only the kinematic viscosity as a parameter influencing the lubrication performance,” said Oystein Alnes, principal engineer of maritime transport at DNV GL. “There are now some studies emerging that indicate performance variations within EALs for lubricants of similar specified viscosity. The available data and information is however very limited, so we have seen a need for DNV GL to build firsthand knowledge on the topic. We are, through our study, hoping to gain a better understanding of factors influencing the lubrication performance of EALs.”
Ben Bryant, marine market manager at Kluber Lubrication North America, said the testing is needed and explained that different chemistries can be used in the production of biodegradable stern tube lubricants.
“Some of those chemistries lead to a reduced film thickness under load or shear under stress,” he said, “meaning that the lubricant can reduce its viscosity and you’re not getting the protection you need or that the manufacturer of the stern tube system is expecting out of the lubricant.”
That said, Bryant believes there are EALs on the market that can perform as well as or better than the mineral oil products they replace, and that their performance will improve over time as manufacturers develop new and better products. An understanding of the different EALs on the market and their chemistry will help shipowners choose the right product, he said.
Phil Cumberlidge, marine lubricant business development manager at Panolin International, is sympathetic to the plight of shipowners as well.
“The quality and performance of any lubricant (depends) not only on the selection of the best base oil, but just as important is the knowledge of the additives that can be used with ‘bio’ base oils,” he said. “Many additives that are normally used in mineral-based oils cannot be used in EALs due to their toxicity. The way that the environmentally friendly base oil and additives work for hydrodynamic oil film formation — and shear thinning effects — that this JDP is investigating will certainly indicate those EALs that work and those that don’t. This information will certainly be of benefit to the shipowners and OEMs (original equipment manufacturers).”
However, Cumberlidge has broader concerns. It is not only the stern tube bearings that have been failing; stern tube seals have been suffering premature wear and blistering with some EAL types, leading to seawater ingress and oil loss.
“Some lubricants are only lasting for around half the expected time to the next five-year dry-docking,” he said. “The last six months have seen many owners around the globe really concerned about the reliability and availability of their vessels to continue to service their contractual obligations to deliver goods. These owners are now having to plan to dry-dock to change the failing EALs to those superior-performance EALs that have shown to have no such poor performance issues.”
The first phase of the JDP testing focused on synthetic esters and was scheduled to be completed in the first quarter of 2018. DNV GL expects to have the results published later this year.