Environmentally Acceptable Lubricants allow a stern tube arrangement to comply with the US Vessel General Permit 2013, but lubricants need to selected carefully to ensure satisfactory operation
A spate of stern tube failures has been linked to the introduction of Environmentally Acceptable Lubricants (EALs), but these failures can be due to a variety of circumstances that may have little or nothing to do with EAL.
EALs allow shipowners to comply with US Vessel General Permit regulations, first enacted in 2013 and later extended under the Vessel Incidental Discharge Act (VIDA) in 2018. US Vessel General Permit 2013 regulates discharges that are incidental to a vessel’s operation, among which are lubricating oils used in equipment and machinery on deck and underwater. Oil leakage from oil-lubricated stern tubes has been identified as a significant source of oil discharge.
Most stern tube failures occur during sea trials, according to Shell Marine technical services manager Rob Harrison*. This is when a vessel is under the harshest conditions it will ever experience. When a ship is put through its paces during sea trials, it may perform hard maneuvering at high speeds or operate with a partly submerged propeller. These conditions involve hydrodynamic lubrication but could also involve mixed lubrication, where there can be some metal-to-metal contact.
Initially at start-up, there is metal-to-metal contact between the stern tube and bearing. This is called the boundary phase. As the rotational speed increases in the mixed phase, a thin film begins to form with occasional metal-to-metal contact. As the speed increases further, a thick oil film forms in the hydraulic phase, resulting in no metal-on-metal contact.
Besides film thickness, the pressure viscosity co-efficient must also be considered. The viscosity between the two surfaces will increase with increased load. “You don’t want to break that oil film down,” said Mr Harrison. He noted that bearing design, clearances, and materials, along with vessel design, can all influence the service lifetime of a stern tube.
The required EAL must be ‘readily’ biodegradable, have a minimum eco-toxicity to fish and marine life, be non-bioaccumulating and ideally bio-sourced. This last feature is not mandatory at the moment, as Croda Europe market applications specialist energy technologies Kevin Duncan said.
“EALs are a viable and effective option for stern tube lubrication,” said Mr Duncan, but he cautioned that all EALs are not all the same: “Picking the right EAL with high film-forming properties, high oxidation and hydraulic stability will eliminate engineering issues, reduce oil degradation and minimise the risk of stern tube failures.”
In addition, a stern tube lubricant should last the five-year drydock interval. “If your lubricant breaks down in any way, shape or form, you will be back in dry dock incurring costs,” said Mr Harrison, adding the lubricant should also remove water and resist hydrolysis. “By reducing hydrolysis, you will reduce acid formation,” explained Mr Harrison. “By reducing acid formation, you will extend seal life. If you maintain a lubricant with no water you will retain your viscosity and prevent equipment corrosion, which is the ideal scenario.”
The EAL needs also to accommodate a range of viscosities to suit various OEM requirements. Panolin International’s head of tec centre Dr Patrick Galda noted that EALs vary significantly. Dr Galda said the reason EALs failed was oil deterioration; this could be caused by high temperatures, such as oxidation, which leaves sludge formation in the system, or polymerisation, which could lead to an increase in viscosity.
“If your lubricant breaks down, you will be back in dry dock incurring costs”
At the other extreme, said Dr Galda, low temperatures could lead to crystallisation. Furthermore, shear instability could lead to a decrease in viscosity, or water ingress, which could result in the base oil decomposing or emulsification causing equipment corrosion. Incompatibility issues with seals, paints and even previous oils used could also cause problems, Dr Galda said. Such failures lead to damaged equipment, high drydocking costs, vessel downtime and client dissatisfaction, Dr Galda noted.
Dr Galda also linked design changes to EAL failures. “On one side it’s an issue of design changes. Think about slow steaming (lower shaft rpm), the use of heavier propellers, or the reduction of oil volumes which leads to overworked oil under high temperatures.”
It is also about transient conditions, he said. Newbuilding sea trials, new shaft and bearing roughness and hard turns, or perhaps heavy seas slapping the propeller of an unladen vessel, can impact the shaft and the bearing.
There is an alternative to the use of EALs but this requires a redesign of the stern tube. To comply with US Vessel General Permit 2013, shipowners can also use a void space seal option for a stern tube application, where there is no oil-to-sea interface. Such bearings still require a lubricant, but it does not have to be an EAL. Dr Galda noted that the void space seal is a “very environmentally friendly” sealing system, but these seals require trained personnel to properly operate and monitor them.
One such option is the Wärtsilä Airguard which works with compressed air applied to the void space between the seal rings. This constant flow of compressed air into the void space is automatically set higher than the seawater pressure, resulting in a small amount of air forced out into the seawater.
The void space is connected to an inboard drain collection system. Any seawater or oil that infiltrates the void space is automatically drained inboard, preventing oil leaking outboard or seawater entering the stern tube. As the Wärtsilä Airguard seal removes the oil-to-sea interface, it can be operated with mineral oil in accordance with the 2013 Vessel General Permit. Wärtsilä also produces water-lubricated seals, which comply with the Vessel General Permit 2013 and are said to accommodate large and repeated axial and radial shaft movements, as well as vibrations.
*This article is based on the webinar: Environmentally Acceptable Lubricants: safe or a safety hazard, which was produced by Riviera Maritime Media and the Marine Lubricants Webinar Week supported by Castrol.