Advanced automation and new design techniques are giving ship operators more room to manoeuvre in thruster selection
As automation systems on cruise ships and ferries become more advanced, so are the control strategies for thrusters and other manoeuvring equipment. Viking Line’s imminent newbuild, currently under construction by Xiamen Shipbuilding Industry, is a good example.
The new vessel, due for delivery next year, is the sister ship of Viking Grace, built by STX in Turku before Meyer Werft acquired the shipyard. That innovative vessel features dual-fuel engines driving twin shaftlines along with Flettner rotors to harness wind power for propulsion. The China-built sister ship will take a different approach.
The 2,800-passenger vessel will still feature LNG-burning engines. But this time the engines will provide electricity for podded propulsion. This will be the first application of twin Azipod XO 2100 propulsion on a cruise ferry. According to Viking Line president and chief executive Jan Hanses, electric propulsion is a better choice for the rapid speed variations and tight manoeuvring required for a vessel that will make fast turnaround trips through the archipelago between Stockholm and Turku.
“Electric propulsion is a better choice for the tight manoeuvring required for a vessel making fast-turnaround trips through the archipelago”
“Our expectations for this vessel is that it will be the most efficient cruise ferry operating in the Baltic, if not the world,” he said.
The XO 2100 propulsion pods each have a power output of up to 18 MW, enough to provide for the ship’s 1A Super ice class notation. And after an extension of Azipod supplier ABB’s contract late last year, the inbuilt control and condition monitoring systems of the XO 2100 units will be integrated into the company’s vessel-wide automation system.
ABB’s Ability 800xA system combines power, propulsion and vessel management systems. This allows for dashboards presenting the critical information from all three systems, helping crew and onshore staff to operate the vessel safely and efficiently. Adding marine automation to ABB’s package therefore improves vessel efficiency and digitalisation and offers project management synergies, said Mr Hanses.
“Extending our arrangement made perfect sense. ABB has the dedicated local automation team and access to global engineering resources that will ensure delivery of a vessel benefiting from the highest levels of systems integration.”
Control improvements were also a factor in Scandlines’ decision to upgrade thrusters on its four hybrid ferries running between Puttgarden in Germany and Rødby in Denmark. The company will spend more than €13M (US$14.7M) replacing CTZ35 thrusters provided by Rolls-Royce Commercial Marine with new AZP120 pulling azimuth thrusters.
The pulling thrusters – four per vessel – provide a more homogenous flow of water into the propeller because the stream is not interrupted by the gearbox casing. The result is a more efficient propulsion arrangement and, critically for Scandlines, less noise and vibrations.
The control and automation system relating to the power and propulsion will also be upgraded, offering further efficiencies. According to Rolls-Royce vice president service sales Kjetil Nilsen, Scandlines is taking “a smart approach towards a greener fleet”.
“Our tests of the chosen solution have shown a significant efficiency improvement, which again will deliver a reduction of CO2 emissions,” he said.
Meanwhile, Rolls-Royce's investment in the systems behind thruster technologies is continuing. It recently filed two patents that may eventually find a better way for monitoring the condition of azimuthing thrusters.
One of the difficulties of monitoring azimuth thrusters has been how to reliably power sensors in the thruster body and how to transfer data from the rotating to the static part of the thruster. One current method is using mechanical slip rings to convey data and power, but this has two major limitations. The first is cost, as slip rings need to be modified for each size and configuration of the thruster. The second is the wear this arrangement causes to the slip ring and shaftline, meaning regular service is needed.
Rolls-Royce energy and health management specialists Werner Schiffers and Lars Saarinen believe the answer lies in resonant magnetic induction. Two inductors – one generating a magnetic field, the other generating an electrical current from that field – are configured so they can be rotated fully while still creating the electrical current. This enables power and data to be transferred wirelessly.
“This would simplify wireless data and power transfer during health monitoring of marine thrusters,” said Mr Schiffers. “A major advantage of our idea is that the power and data transferring parts of the condition monitoring unit can be retrofitted without removing the thruster from the ship’s hull.”
There are other benefits too, stemming from the more robust and efficient transfer of power. The sensor in the thruster body would not require a battery, for example, and maintenance intervals could be extended as thruster designs are adapted to accommodate bigger, longer-lasting inductors. The commercialisation of the concept is not yet in sight, but it provides an intriguing example of how control and automation technology can influence the development and performance of thrusters.
It is not only new control technologies that are affecting thruster development; new design technologies are also being deployed to improve efficiency. According to Wärtsilä segment sales general manager cruise and ferry Mikael Arts the company’s biggest thruster for cruise ships, the Wärtsilä Transverse Thruster 40 (WTT-40), owes its design to such cutting-edge technologies.
Development work for the thruster, which has a diameter of 3,400 mm and a maximum power output of 4 MW, started in 2015 at Wärtsilä’s test facility in Finland. The company took a two-pronged approach to the design. First, it disregarded its existing range of transverse thrusters and redesigned the WTT-40 from scratch. Computational fluid dynamics helped the designer reduce noise and vibrations, while finite element analysis was deployed to scale up the thruster while maintaining structural integrity.
Other design parameters were tested using existing products under heavy loads and simulating the dynamic movement of ships. It was found that increasing propeller diameter by 400 mm lowered power density by 7%. Tip speed was also reduced by a further 12%. Compared with the low power density, a slower tip speed into less noise and vibration will provide higher comfort for passengers.
After the modelling stage, attention was turned to making the thruster convenient for shipyards to install. As a result, the hydraulic, pumping, filtration and cooling systems on the WTT-40 are pre-installed at Wärtsilä’s factory in Wuxi in China and shipped as a complete unit. The propeller can be dismounted for a quick exchange of the complete shaft seal without needing to glue pieces together.
“Unplanned servicing is a nightmare scenario for a cruise line operator with several thousand people on board”
“Due to its smart design and more efficient use of components, the WTT-40 is more reliable and competitive than previous thruster models,” said Mr Arts. “Reliability is important as all this equipment is below the waterline. This makes unplanned servicing difficult and extremely expensive – a nightmare scenario for a cruise line operator with several thousand people on board.”
Wärtsilä has delivered six WTT-40 thrusters to cruise ships in operation or under construction, and Mr Arts noted there are several more on its orderbook. Even with the biggest and most powerful thrusters, every improvement helps in the hunt for efficiency and reliability.
An upgrade with a porpoise
Scandlines’ imminent thruster upgrade (see main article) is not just about making its passengers more comfortable. The company is also hoping to improve the environment for marine life. Reduced underwater noise is expected to have a positive impact on the harbour porpoises of the Fehmarn Belt area.
“The new thrusters are an important element on our way towards zero emissions and I am very pleased this can be combined with reducing impact on the underwater life in Fehmarn Belt,” said Scandlines chief executive Søren Poulsgaard Jensen.
The company is co-operating with the German Nature and Biodiversity Conservation Union in a research project to evaluate the noise reduction levels and to communicate ideas about technical best practice.
Scandlines is not the only company concerned about the impact of shipping on marine creatures. Like the Scandlines routes, the area around Vancouver port is host to a community of whales that can be distressed by underwater noise from ships. To encourage shipowners to do all they can to reduce noise, Vancouver Fraser Port Authority offers them discounts on port dues of varying degrees if they install noise-reducing technology.
This year the port has included Wärtsilä’s EnergoProFin, an energy saving propeller cap, in its EcoAction programme. The listing in the vessel quieting category means that owners receive a 23% discount in harbour dues for vessels installed with this technology.
The EnergoProFin incorporates fins that rotate together with the propeller. This reduces the cavitation caused by the hub vortex of the propeller. It also reduces energy loss caused by flow around and behind the propeller boss. It has been installed on more than 350 vessels.
Staying safe in the shallows
While thruster suppliers focused on the cruise market specialise in bigger thrusters, German propulsion specialist Schottel is aiming at the smaller and shallower end of the market with the launch of a new pump jet. Available with a power output of up to 150 kW, the azimuth thruster is optimised for maximum manoeuvrability in shallow waters.
The SPJ 30 is suitable for passenger vessels as well as work vessels and freighters. Its compact design means that installation offers a minimal displacement loss. The above-water gearbox is available in Z or L configurations and the thruster is set up to receive power from diesel engines or electric motors.
Because the pump jet is installed flush with the hull of the vessel, hull resistance is not increased and the risk of collisions with flotsam is greatly reduced. Further protection against damage is provided by a protective grille at the inlet. The thruster is available with elastic mounting, further reducing noise levels and vibrations.
The new thruster will be available from the end of 2019 and joins Schottel’s established range of pump jets, with power output up to 3,500 kW and minimum immersion of as little as 150 mm.