DNV GL offers advice on avoiding problems with EALs
Ever since environmentally acceptable lube oils (EALs) became mandatory in US waters in December 2013, there have been anecdotal reports of increased bearing wear in installations that use these non-mineral alternatives to conventional oils.
Some studies have been done to understand the phenomenon. In April last year, for example, ABS, Vickers Oil and University College London made a joint study to understand EALs’ properties compared with conventional oils and now DNV GL has completed Phase 1 (of three) of a joint development project (JDP) that began in January 2018 and involves four P&I clubs, the UK’s University of Sheffield and the Institut National des Sciences Appliquées of France.
DNV GL has not released the full report but used some of its findings in a webinar on 29 October called ‘Environmentally acceptable stern tube lubricants – How to avoid costly failures’. It circulated a link to the presentation material and a recording of the event this week (11 November).
One surprising part of the presentation was left unresolved at the time: in the first couple of years after the US Environmental Protection Agency issued its Vessel General Permit mandating the use of EALs, DNV GL’s data shows that the percentage of vessels using EALs in stern tube bearings rose from 6% in 2013 to 47% in 2015 but has since fallen each year and stood at 22% in 2018 (See Table 1).
These figures relate only to ships that have adopted the class society’s tailshaft monitoring notation TMON, so may not be representative of the world fleet.
They were mentioned during a presentation by Arun Sethumadhavan, DNV GL’s senior principal engineer for hull materials and machinery, who said the reversal was due to “various reasons we would not wish to comment on because this is customers’ choice.”
However, a later written response, included in the downloadable presentation material, offered a possible explanation for the reducing take-up of EALs. “It is reasonable to believe that this trend was a result of the uncertainty in the market (due to the high number of incidents on vessels with EALs in their stern tubes) and the void space seal option emerging as an accepted solution to stay compliant with the VGP requirements,” the post-webinar feedback suggested.
Also speaking during the webinar was Øystein Alnes, principal engineer in DNV GL’s technology and research team, who highlighted some common threads running through the incidents the study had reviewed. It was an industry-wide challenge, he said, but incidents “happened typically very early in the vessels’ lifetimes … [and] the dominating failure mode was what we call a wiping failure”. This is caused when the oil film is lost, resulting in metal-to-metal contact. This generates high temperatures that melt the white metal surface, ‘wiping’ it around the bearing assembly.
This frequently happens during hard manoeuvring, when large downwards-acting bending moments on the propeller affect how the load is distributed within the aft stern tube bearing.
With conventional oils, when the pressure is increased in an oil film, its viscosity will increase. As a result, “resistance towards a breakdown of the oil film will increase as you load your bearing,” Mr Alnes said. But the JDP found that EALs do not have the same relationship between pressure and viscosity as mineral oils and an EAL will have a lower viscosity for a given pressure than a mineral oil would in the same situation. In short, “the safety margin towards a failure will be lower. That is a key takeaway.”
Temperature also has a different effect on viscosity for the two types of oil, the research found. EALs have a higher viscosity index than mineral oils, which means their viscosity is less affected by temperature than mineral oils. In temperatures below 20°C, for example, EALs have a lower viscosity than would the equivalent mineral oil, giving them a lower safety margin.
This is particularly relevant for newbuilding yards conducting mooring trials, Mr Alnes said. They may be familiar with how systems behave in particular operations using mineral oils, but “if they do the same operation with an EAL, they might get into problems.” On the other hand, at temperatures above 40°C, an EAL will have a higher viscosity than the equivalent mineral oil, “so there are positives as well,” he said.
Researchers also looked at the shear stability of various lubricants because shear thinning had been mentioned in some studies as a potential problem, Mr Olnes said. This effect results in reduced viscosity when a fluid is subjected to shear strain but “we found this phenomenon only for one lubricant,” he said. And that oil was unusual in having viscosity improvers so, in general, the effect was very limited and not as significant as the pressure and temperature effects, he added.
Nonetheless, “it is something to be aware of” and he advised shipowners using EALs to monitor their viscosity on a regular basis to confirm that shear thinning is not giving them operational problems.
He gave two pieces of advice as a result of the work done in Phase 1 of this research. First, to reduce the peak pressure in stern tube bearings by optimising their shaft alignment and bearing slopes. This will “significantly reduce the risk of any problems,” he said. Second, when specifying an EAL, choose one with a viscosity that is one grade above that of the mineral oil specified for the bearing. By doing this, “you will build in a bigger safety margin,” he said.
As a result of the work, DNV GL revised its oil film lubrication criteria that are used to determine the minimum shaft speed required to establish and retain an oil film of acceptable thickness. The formula now includes a viscosity parameter that takes one of two values depending on whether mineral oils or EALs are being used, which has the effect of setting a higher minimum rpm when EALs are being used. This has been applied to all DNV GL newbuilding assessments since July this year.
But how widespread are these problems? A poll conducted during the webinar asked attendees which “key challenges have you experienced with EALs?” The results are shown in Table 2.
That final figure of 42% reporting no challenges from EALs sends “a very important message that if you handle your installation properly … and choose the correct combinations of design and EALs then there is no reason to say that it will not work,” Mr Alnes commented.
• The study did not consider water-lubricated bearings. During the Q&A at the end of the webinar Mr Olnes said this was because they are covered by “a separate part of our rules … and there is no problem in using those kinds of designs for DNV GL-classed ships.”