Passenger comfort, fuel efficiency, less downtime, reduced maintenance – drivetrain optimisation can result in significant gains across the vessel
Shipping is at a stage now where the discomforts of old, once accepted as part and parcel of the mariner’s lot, are simply no longer tolerable. Today’s crew have minimum expectations as regards comfort and operators must factor these expectations into their equations if they are to hire and maintain the best personnel. Similarly, customers expect hotel-room conditions on board: noise and vibration are no longer part of the equation.
There are obvious ways to increase onboard comfort, with improved connectivity for example, but from a mechanical perspective, a key area of focus is the drivetrain. Innovation in this area can mitigate vibrations and noise, improving both the experience for those on board and the environmental impact on marine species. Further, optimisation in the drivetrain area naturally feeds through into greater efficiencies across the vessel, resulting in cost gains and reduced maintenance patterns.
The topic was discussed in detail during a Riviera Maritime Media webinar, Drivetrain innovation – what’s new on the engine block, held in October in association with premier partner Geislinger.
During the webinar, AVL manager multi-body and NVHG simulation Christoph Priestner focused on the key triumvirate of noise-vibration-harshness (NVH), highlighting developments in this area that are having a significant impact on vessel design.
“If you are not familiar with how NVH can impact the onboard experience, imagine you are sitting in your car,” he said. “But rather than a smooth ride, everything is shaking, vibrating a little and making you rather uncomfortable. There is a lot of wind noise, coupled with engine noise, and all together the experience is not that pleasant.”
Mr Priestner explained that in reality, this does not happen very often because car designers put a lot of effort into mitigating or eliminating these factors, and it is these goals that vessel designers, and in particular drivetrain designers, are now also targeting.
He noted that these issues often come about as the result of a trade-off between
cost and performance. “Higher performance normally results in more noise and vibrations, as does a lightweight construction and a more robust platform,” he said.
It is no surprise then that these attributes have traditionally combined to place working vessels on the ‘utilitarian’ side of the comfort spectrum.
But thanks to the work of companies like AVL, that is no longer necessarily the case. Said Mr Priestner “Whether we are discussing conventional internal combustion engines or hybrid electric units with direct drives, to alleviate NVH we must take into consideration the full system, whether that is the crank train or the inverter or the battery.”
Mr Priestner explained that AVL identifies two ways of approaching the issue of NVH: the first is to address the source, be that the crank train, the gear drive or the valve train for example. “Here we can employ torsional vibration dampers and with the right clearance settings, we can make significant improvements.” He likened this to a ‘free’ win, “because we are just tailoring what is already there.”
The second approach is what Mr Priestner describes as the transfer path, “where we focus on everything from the source to the surface. Here, optimisation procedures are available that can soak up higher frequencies to bring real benefit,” he said.
He explained that once all the potential benefits have been identified, the designers and owners can determine what is necessary based on their cost structures. “With our very powerful models we can do optimisation simulations and bring everything into balance at a very early stage in the development.” Added Mr Priestner: “We can hand over a first design that is already at a very high level of maturity.”
Focusing the discussion on acoustics and their impact on the vessel and those on board, Geislinger managing director Torsten Philipp explained that when addressing acoustics and noise, it is important to consider the transmission of vibrations (excitation) from the engine to the ship structure and the hull. This occurs in two ways, said Mr Philipp. The first is the primary path, which transmits through the engine mounts into the ship structure, resulting in vibrations and noise that can impact passengers and, noted Mr Philipp, also creates underwater noise which is a concern for Navy applications.
“There is an increasing focus on acoustic development and optimisation from almost all sectors of the maritime industry”
More important though, explained Mr Philipp, is the secondary path, where the acoustic waves travel through the rotating parts of the vessel’s mechanism, the drive train and the gearbox for example, into the ship’s structure.
Mr Philipp said there is an increasing focus on acoustic development and optimisation from almost all sectors of the maritime industry, from yachts and passenger vessels to commercial shipping and Navy applications.
“It is the source of the noise, the subsystem, which is really responsible for creating noise and vibration inside an engine,” explained Mr Philipp. “To address this, we developed a new product to reduce noise and vibrations coming from the engine gear train; we call it the ‘gear-integrated coupling’,” he said.
Mr Philipp said this device adds elasticity and damping to the gear train, reducing gear excitation, bearing forces and noise excitation across the vessel structure.
When implemented into a complete system, the gear coupling is located in the front end, driving the camshaft, fuel injection pumps or similar. “We create a torsional model with each application together with our customers,” said Mr Philipp. “The gear-integrated coupling introduces additional elasticity and damping into the gear train, which results in very low vibratory torque and therefore very low acoustic excitation from the gear train.”
Mr Philipp noted that Geislinger had also developed a special acoustic coupling which stays soft and offers very low vibratory amplitudes. “It does not show any stiffening effect, allowing us to reach vibratory levels below 2% or nominal torque.”
Geislinger has also focused its developments on higher frequency ranges, from 300 hertz up to 2 kilohertz. “For these frequencies, we have developed a special misalignment coupling, which consists of acoustically optimised membranes,” said Mr Philipp.
He explained this modular concept can be adjusted to meet the requirements of specific applications; important, said Mr Philipp, as each application has different boundary conditions. He noted that during sound pressure tests on a luxury yacht, the coupling resulted in a clear reduction of sound level, in the range of 20 dB.
Addressing the source
Mr Philipp said this type of coupling has now been installed in a high-speed integrated electric marine drive, a unit that already offered low excitation due to the absence of combustion engine-related components.
“This drive has been installed in a 110-m mega-yacht application and our measurements highlighted gains in the sound level on the engine and ship structure side; the primary and secondary paths combined,” said Mr Philipp. “In this example, we achieved a noise reduction in the range of 20 to 30 dB over the whole frequency range,” he said.
Mr Philipp reiterated that reduced vibrations are key to less engine and power train noise. He emphasised it was far better to address the problem at source, rather than trying to implement measures to mitigate the problem further down the line. “[You must] focus on the source to get the best vibration levels on a system level,” he said. “That is why we model digital twins for all the propulsion systems we work with, to really optimise the systems and help us focus on integrated solutions.” Added Mr Philipp: “Geislinger has more than 60 years’ experience in the marine business and we really understand what the boundary conditions are.”
Moving on to discuss shaft alignment, Wärtsilä area sales manager Europe and Africa – shaft line repair services, Marco Gordon, explained why regular alignment checks are critical to ensuring optimal performance and to reduce costly repairs.
He explained that stresses and vibrations caused by a misaligned shaft can eventually lead to a breakdown in the engine shaft assembly, resulting in damage to both struts and hulls. Because vessel engine drive systems are in permanent motion, they do not remain in alignment throughout the vessel’s lifetime. Therefore, said Mr Gordon, systems need to be periodically checked and realigned to reduce wear and damage. “It’s all about avoiding a chain reaction, leading to unexpected dockings, exactly when you don’t need them,” warned Mr Gordon.
Elaborating on the physics of lubrication, he cited the occurrence of a hypothetical
vibration somewhere along the shaft line. “It doesn’t have to be significant,” said Mr Gordon. The vibrating shaft can cause a suppression in the lubrication film, lessening the damping effect between the static and the rotating part. “At that point, extensive bearing wear begins almost immediately and this can ruin the bearing within minutes,” explained Mr Gordon.
Now the temperature of the bearing starts to rise rapidly, due to abrasion, which is usually when the crew detect the issue. “This is really too late,” said Mr Gordon, “because the abrasion and heat are already having a wiping effect on the bearing sliding material, something that we call a ‘complete bearing failure’.”
Within a matter of minutes a minor vibration may have resulted in expensive and time-consuming repairs. ”And it can all start with just a misaligned coupling,” said Mr Gordon.
To reduce and potentially eliminate this risk, Wärtsilä has developed a 3D condition-based monitoring system which detects shaft behavior in real time and identifies potential problems before they result in real-world damage. “By installing sensors on the shaft line we can analyse its behavior while you’re operating across all conditions,” said Mr Gordon. “We even simulate in a 3D model how the shaft line behaves under particular conditions and we are able to provide recommendations if special attention is required.”
Another topical issue impacting drivetrains involves environmentally acceptable lubricants, (EALs), some of which are under investigation for causing stern tube failures. Mr Gordon pointed to Wärtsilä’s Air Guard Sealing System – “a continuously pressure-balanced system” – that allows vessels to continue using mineral oil “because the seal drains potentially leaking lubricants to the inside tank.”
He explained “Installation is possible without tailshaft removal; it is an efficient option considering the cost comparison between EALs and mineral oils.”
Mr Gordon concluded by reiterating the importance of regular maintenance and monitoring. “Many factors can influence shaft alignment and the tolerances within the bearings are becoming smaller and smaller – the tolerance in modern stern tube bearings can be as little as a few tenths of a millimetre,” he said, urging operators “Please be aware, the more modern your equipment gets, the more importance you have to place on a perfect alignment.”