A combination of operational factors lie behind many bearing failures, Wärtsilä reports
Stern tube bearing failures are on the increase and some of the reasons behind the rise appear to be unintended consequences of operational trends. In an exclusive interview with Marine Propulsion, Wärtsilä UK OEM sales manager, seals and bearings, Simon Wiles, highlighted a few of them.
Environmentally acceptable lubes (EALs) have been blamed for some bearing failures, he said, prompting Wärtsilä to conduct an in-depth study into EAL performance in stern tube bearings. That work was in progress in Japan at the time of Marine Propulsion’s interview in mid-July but Mr Wiles said that early indications suggested that EALs may not perform as well as traditional mineral oils, especially during the early sea trial period.
A dynamic shaft alignment check with the shaft in motion (credit: Wärtsilä)
Another factor is a by-product of the drive for energy efficiency that has led some operators to fit larger propellers than originally installed. These are heavier and slower, which has an impact on bearing loads. Their replacement might be accompanied by hull modifications – making them asymmetric, for example – which can affect shaft alignment.
Overheating caused by unfavourable operating conditions can also result in bearing failures he said, giving as one example the forces generated by partially-submerged propellers running at high speeds.
Single-bearing stern tube designs are another regular feature among bearing failures, he said, a choice often made to reduce cost. But such systems are sensitive to some operational situations, such as sharp turns. For those installations, it is critical that detailed calculations are done at the design stage “to make sure you are operating with the correct alignment criteria,” he said. “That is perhaps not always done,” he added.
Wärtsilä is able to make additional calculations to support customers, he noted. “When we see a general arrangement that we think is of concern we try to work through it with the customer.”
When it comes to preventing stern tube bearing damage, how a ship is managed is at least as important as how it is maintained, judging by Mr Wiles’ comments.
"Make sure you are doing your due diligence in the calculations to ensure that you are aligning the shaft line correctly to operate under [new] conditions”
Asked what ship operators can do to reduce their bearing problems, his first advice was to follow class requirements closely, take more oil samples and filter the lubricating oil. But he also mentioned operational and management factors.
“Not all ships are the same,” he said. In particular, older ships can have more problems because “the analysis and the calculation depth we can provide today is far more advanced than it was five years ago.” Because of that, vessels built earlier than that “perhaps were not designed according to everything that we know today.”
Whatever the age of a ship, however, operators should “ensure that they are operating under the conditions that the system was designed for,” which is not always the case, he said. “Far too many operators and owners want to save money through fuel savings and make modifications to the [propulsion] system without updating or changing the alignment criteria.”
His advice was that if a ship is modified, “make sure you are doing your due diligence in the calculations to ensure that you are aligning the shaft line correctly to operate under those [new] conditions.”
As for maintenance, Mr Wiles said: “Using OEM parts and following the OEM’s recommendations for each of those components is very important. Use the correct lubricants and follow the right maintenance procedures. Simple as that, I would say.”
Dynamic calculations give accurate alignment checks
Shaft alignment checks are often unrealistic, believes Wärtsilä UK OEM sales manager, seals and bearings, Simon Wiles, because they are based on measurements that are taken in a static condition.
Historically, alignment calculations “have effectively been theoretical,” he told Marine Propulsion, based on data provided by equipment suppliers and on the vessel’s expected operating conditions. These are checked against measurements taken in a drydock and afloat.
A Wärtsilä technician conducting a laser shaft alignment check (credit: Wärtsilä)
Better, he suggested, is to take dynamic readings when the propeller shaft is operating. Those will not be theoretical, he said, “and will provide a true interpretation of what is happening on board.”
To achieve that, it uses its portable condition-based monitoring (PCBM) equipment to make these checks and measurements. It was developed by Wärtsilä’s Denmark-based shaft-alignment specialists to be a suitcase-sized set of test equipment that can be taken on board a ship to make measurements. It costs about €5,000 (US$5,700) to have the equipment for a week. “The value is in the data analysis that we perform afterwards,” said Mr Wiles.
These tests are carried out when a ship operator has concerns, he explained. For example, the shaft might be vibrating or bearing temperatures are high or a seal might be leaking. In those cases, “it would be very sensible to install a PCBM.”
This can be done by the ship’s crew or local yard workers. The equipment would be delivered to the ship with instructions on how to install it, which Mr Wiles said is a relatively simple job. “It is just mounting sensors,” he said. Some framework might be needed but the pre-installation work can be done without delaying the ship, he said. As an alternative, Wärtsilä could send an engineer with the equipment who would install it in less than a day.
He did not rule out providing the equipment for shipowners to install permanently to provide predictive analysis, although that is not a priority. “We prefer to work on a rental basis because then it allows us to follow up with work that might be required to realign the shaft line, for example.”
Measurements would be taken over the course of a sailing, or over about a week for a ship such as a ferry, after which the ship’s crew would pack up the equipment and the data it has logged and return it for analysis. Results are available within a week; possibly within a day, Mr Wiles said.
Asked what this dynamic measurement system is revealing, Mr Wiles spoke of “a multitude of different issues” being involved. “Anything along the shaft line can be misaligned”, he said, linked to “design, installation, operation and lubricants”. There can also be operational factors, such as temperature changes as the ship moves from region to region, that cause hull deflections.
DNV GL addresses early stern tube bearing failures
Increasing numbers of stern tube bearings are failing early in a ship’s life, according to a webinar hosted by class society DNV GL in April.
Its principal engineer, group technology and research, maritime, Øystein Alnes, said that the early failures are being seen during mooring trials, sea trials and within the first few weeks or months in service.
These failures usually involve ‘wiping’, in which the white metal bearing surface is rubbed off, particularly on stern tubes with single bearings. This can happen when a vessel is making hard manoeuvres during its sea trials, creating transient propeller loads and a large downwards bending moment on the propeller shaft.
This puts a larger share of the load at the aft end of the bearing, concentrating the load on a small area of the bearing surface. “If the load-carrying area becomes too small the pressure will get too high, the temperature will increase and you might end up in a situation where you have a wiped bearing,” he said.
Øystein Alnes (DNV GL): “The risk of ending up with a critical situation has increased” (credit: DNV GL)
This is in marked contrast to the traditional situation, in which stern tube damage tended to be caused by fatigue failures that were spotted during five- and 10-year dockings. There appear to be no obvious common factors behind these failures. “It is not possible to isolate single yards or single class societies – it is across the whole fleet, although some segments, such as bulk, tanker and container, are the most frequent segments experiencing problems,” he said.
Asked by Marine Propulsion why this has become a particular problem in recent years, Mr Alnes listed a number of factors that, in combination, might be relevant.
One was vessel design trends towards larger and slower propellers, along with propeller efficiency devices that might influence the propeller hydrodynamic loads. Another was the increased popularity of single-bearing stern tube configurations.
“Minimise propeller speed to limit the additional load on the aft stern tube bearing”
He also noted that there have been cases of sub-optimal operation with semi-submersed propellers running at high speeds. “Typically, mooring trials will be run quayside with propeller submersion of about 50%,” he said. And since many yards – especially in China – are located in shallow waters, “large vessels need to run for substantial distances at very light ballast draught before entering deep waters.”
Because “vessels and propellers are getting larger, the risk of ending up with a critical situation has increased,” he said. His advice in such cases is to minimise propeller speed to limit the additional load on the aft stern tube bearing.
He also commented on the use of environmentally acceptable lube oils (EALs) in stern tube bearings. “We suspect that there might be differences in the load-carrying capacity of these new lubricants compared to the traditional mineral oil stern tube lubricants,” he said.
“Some yards have opted to run sea trials with mineral oil in the stern tube and switching to EAL upon delivery,” he said. But DNV GL believes that sea trials are important in validating a propulsion system “and we therefore now require yards to test the system with the same lubricant as the vessel will be delivered.”
This is reflected in revised class rules for single stern tube bearing installations, which were published in January and came into effect on 1 July. That new requirement is included in Chapter 2 of Part 4, which covers rotating machinery.
That is not the only stern tube-related aspect of its rules that were updated in January. It also introduced two voluntary class notations: Shaft align(1) and Shaft align(2). They also came into effect on 1 July and in a statement in February, DNV GL said that they would “help customers better manage the risk of stern tube bearing failure.”
They were developed in response to demand from customers, Mr Alnes later said in his webinar and he reported a lot of interest in them.
They take account of “the bending moment induced by the propeller during operation”, the February statement said, specifically mentioning “turning manoeuvres at higher ship speeds [when] exaggerated propeller bending moments can occur, potentially resulting in a reduced shaft-bearing contact area and an exponential increase in local pressure and thermal loading.” Although DNV GL has not specifically said so, the rules appear to be aimed at the problems that Mr Alnes described as causing early bearing failure.
During the Webinar, Mr Alnes said that Shaft align(1) is a basic option for vessels with conventional hull forms while Shaft align(2) is an advanced option for more sophisticated propulsion systems, for example in ships with unconventional hullforms and requires designers to carry out computer-aided fluid dynamics calculations to find the hydrodynamic propeller loads and them apply them to a finite element analysis to calculate the bearing contact area and pressure during transient operations.
Causes for concern over shaft alignment
During DNV GL’s April webinar, those taking part were invited to nominate the most significant factors that cause them shaft-alignment concerns from a list of five options. They could select more than one topic.
The two most significant factors were installation and operational issues, but DNV GL principal engineer, group technology and research, maritime, Øystein Alnes remarked that the spread of results reflected its own views. “The failures we have seen are often caused by a combination of several factors,” he said.
Factors behind shaft-alignment concerns
Design criteria and industry standards
Source: DNV GL
Poor quality EALs ‘cause stern tube bearing problems’
Poor quality EALs can decompose to form a black sludge (credit: RM Propulsion)
Dutch stern tube maintenance specialist RM propulsion has no doubt what the cause of many stern tube bearing problems is: low quality biodegradable environmentally acceptable lubes (EALs) that do not match the quality of mineral oil.
In an exclusive interview with Marine Propulsion, its founder and general manager Robin Mulders said that many EALs offer film thicknesses of only 30% of mineral oils, although he named some manufacturers that he believes produce EALs that do not suffer from that problem.
“Another issue we see is that they deteriorate rapidly,” he said. They effectively rot within the stern tube seal, he said, producing a foul-smelling sludge that can cause poor lubrication of the seals. “We have seen this result in seal damage, which may compromise bearings,” he noted.
In his view, class societies should pay more attention to this problem, rather than issue new rules on stern tube bearings. “I think they might be looking in the wrong place,” he said. “There were not any issues when people were using exclusively mineral oil.” And if a good quality EAL is used, a stern tube bearing can be “absolutely as good as it has ever been.”
But there are additional costs associated with using EALs because components – in particular seals – must be made from EAL-resistant material, which makes them expensive, he said.
Asked for advice on how to select a good EAL, he recommended choosing one with a viscosity of at least 150 cSt to ensure adequate lubrication film strength. He also suggested that shipowners should check that their EAL is a fully-saturated synthetic ester. These are not cheap, he said, but they “will be stable enough to do this kind of job.”
He also recommended that shipowners should demand a guarantee from their supplier that the lube oil will remain usable for a long time. One manufacturer offers a five-year guarantee under certain conditions, he said.