Comprehensive due diligence on choice of shaft line, bearings and seals is now an essential part of effective ship management, says Thomas Pauly, General Manager, Future Portfolio & Market Intelligence at Wärtsilä Shaft Line Solutions.
Talking to Seatrade Maritime News recently, Pauly insisted that failure to focus effectively on these components risks damage and downtime which, in the worst cases, can run into millions of dollars. In today’s healthy shipping markets, with many repair yards full and no short-term options, the risks are greater than ever, Pauly explains.
Worst case scenario
Citing a worst case, he recalls a Capesize laden with iron ore which got into difficulty in the Malacca Strait, a notoriously dangerous stretch of water. The ship’s engineers noted high temperatures indicating bearing damage. The vessel diverted, sailing slowly for Singapore where divers discovered a 17kg fishing net tangled around the shaft. The stern seal was leaking.
No shipyard in Singapore could accommodate the vessel for the two weeks required for repairs, and owners were advised to slow-steam to China. Meanwhile, new parts were dispatched by Wärtsilä. On arrival in China, the cargo was discharged and the bearing and shaft were repaired while the vessel was afloat in trimmed condition.
The ship was one of three in a profitable block charter and unless the repair could be completed within a short timeframe, the charterer had the right to cancel all three ships and benefit from lower day rates that were then prevailing. The repair was completed in time.
Pauly reveals that although owners and operators normally stipulate the type of key components such as engine, propeller, and navigation system for a new ship, shafts and bearings are usually chosen by the shipbuilder. It is no surprise, therefore, that they are often standard components, picked on the basis of availability and price.
Meanwhile, seals and bearings are only inspected at special surveys. Therefore for large tracts of time, they are out-of-sight and out-of-mind.
The scale of the problem is demonstrated by statistics from Wärtsilä which show that in an analysis, no fewer than 540 bearings needed urgent repair over a three-year period. In reality, there were probably many more.
The repairs did not always involve off-hire; some damage was discovered during dockings. However, the average frequency of 15 incidents a month demonstrates the scale of the problem.
The cost of these repairs, meanwhile, should also be considered a wake-up call. With a typical repair duration of 20 days, the average repair bill worked out at $535,000.
Risk assessment
Some ships are more vulnerable than others. Large bulk carriers are at particular risk because they are designed for efficient operation when fully laden but sail in ballast for up to half the time. In this condition, the ships may bend longitudinally and the propeller may not be completely submerged, generating huge stress and the propeller ‘hanging in the bearing’.
These ships usually have large slow-speed two-stroke engines and fixed-pitch propellers, a combination that generates the largest stresses and strains on stern tubes, bearings, and seals. Other types of vessel, with four-stroke engines and controllable-pitch propellers, for example, have constant shaft speeds and adjust ship speed with the propeller. They are far less prone to shaft damage.
Big bulk carriers may be the most vulnerable, but other vessels are potentially at risk too. They include smaller ships that operate in coastal waters where there is more sediment.
Feeder container ships, small general cargo vessels, ferries, and small bulkers are potentially at greater risk than large tankers, for example, which sail between terminals, often in deep water offshore. Container ships, meanwhile, never sail empty, do not have large draft variations, and spend much of their time in clear water.
All-in-one service
So how has Wärtsilä tackled these challenges? Pauly explains that the company’s strategy is based on a comprehensive service embracing every stage of development. R&D, design, manufacture and service of components is undertaken solely by company experts, even down to the particular formulation of rubber used for seals.
Recently, the company has launched Shaft Line Solutions’ (SLS) FuTube, an all-in-one package which comprises all of its shaft line technologies. And Pauly is keen to stress the importance of the test rig at the Wärtsilä SLS factory site in Spain. The rig, he says, has been designed to replicate real-life conditions in a marine environment.
A key component of the FuTube package is the Intellisafe Bearing with its advanced sensors and real-time analytics. Pauly explains that the bearing constantly monitors fluid film thickness. If there is not enough, more lubricant is automatically injected. Meanwhile, sensors monitor shaft vibration, temperature, whirling, and propeller movement that could cause damage.
Then there is the EvoTube System, launched in December. This is a simplified stern tube system, with the inboard seal mounted on the aft bearing housing and the forward bearing replaced by a standalone bearing in the engine room. It is suitable for both oil and water lubrication and can be converted from one to the other in the future, if required.
These developments have built on the 35-year history of the company’s ten-year Airguard Seal which guarantees ten years of operation without a requirement for inspection or seal replacement at a ship’s first special survey. This the company claims, can generate lifetime savings of close to 60% compared with conventional seals.
The FuTube package is available for new ships and, says Pauly, provides owners and operators with a degree of certainty over efficient future shaft line operation. The system is also available as a retrofit for existing vessels.
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