Shipowners need to carefully select the right EAL to eliminate engineering issues, reduce oil degradation and minimise the risk of stern tube failures
Stern tube failures can occur in a variety of circumstances and be impacted by factors including the condition of the propeller, rudder, stern tube and main engine. Such failures can lead to costly vessel repairs and unanticipated downtime.
Panellists scrutinised the causes behind recent oil-lubricated stern tube failures and what could be done to prevent them during the Environmentally Acceptable Lubricants: safe or a safety hazard webinar, produced by Riviera Maritime Media and supported by Castrol. The event was part of a series held in July during Marine Lubricants Webinar Week.
Focusing on stern tube failures, Shell Marine technical services manager Rob Harrison said most have occurred during sea trials, when a vessel is “under the harshest conditions that it will ever see.” When a ship is put through its paces during sea trials, it may perform hard manoeuvring 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.
Mr Harrison used a stribeck curve diagram to illustrate the various rotational speeds that create the lubrication film between the bearing and the stern tube. Initially at start-up, Mr Harrison explained 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, Mr Harrison made the point that the pressure viscosity co-efficient must also be taken into account. The viscosity between the two surfaces will increase with increased load. “You don’t want to break that oil film down,” he said. He also said bearing design, clearances and materials, along with vessel design, can all influence the service lifetime of a stern tube.
Croda Europe market applications specialist energy technologies Kevin Duncan described the various environmentally acceptable lubricants (EALs) and their use in stern tube lubrication. EALs are defined as ‘readily’ biodegradable, have a minimum eco-toxicity to fish and marine life, are non-bioaccumulating and are ideally bio-sourced – although this is not mandatory at the moment, Mr Duncan noted.
“EALs are a viable and effective option for stern tube lubrication,” said Mr Duncan, but 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.”
Using EALs onboard a vessel allows shipowners to comply with US Vessel General Permit (VGP) regulations, first enacted in 2013 and later extended under the Vessel Incidental Discharge Act (VIDA) in 2018. VGP 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.
“If your lubricant breaks down in any way, shape or form, you will be back in dry dock incurring costs”
Other discharges regulated by US Environmental Protection Agency (EPA) and the US Coast Guard under VGP/VIDA are ballast water, bilge water, greywater (ie, water from sinks, showers), and deck washdown and runoff.
To comply with VGP, 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.
Another alternative shipowners can use to comply with VGP is a water-lubricated stern tube bearing.
What to look for in an EAL
If a shipowner is going to use an EAL, Mr Harrison enumerated the characteristics they should look for in a stern tube lubricant. Firstly, shipowners should look for a stern tube lubricant that will 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.
Secondly, you need a lubricant that removes water and resists 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.”
Thirdly, the EAL needs to accommodate a range of viscosities to suit various OEM requirements.
Like his fellow panellists, Panolin International head of tec centre Dr Patrick Galda said 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.
At the other extreme, said Dr Galda, low temperatures could lead to crystallisation effects. Other factors include shear instability, which 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.”
On the other side 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.
He pointed to a study performed by DNV GL in 2019 that examined these effects on pressure and temperature on the lube viscosity.
While Dr Galda called the void space seal a “very environmentally friendly” sealing system, he noted these seals require trained personnel to properly operate and monitor them. There is also a possibility of potential damage to the system, which could result in a discharge of mineral oil, leading to a violation of the VGP.
EAL failures are often linked to weak base oil performance. The right combination of base fluids and additives can form a good lubricant, he concluded.