Permanent magnet motors and hybrid energy generation and storage reduces the size and emissions from tug propulsion systems
Propulsion systems have become one of the most important aspects of tug design as owners need more power from their workboats for manoeuvring larger ships in confined spaces. With this in mind, new propulsion options have been developed that deliver greater power, better manoeuvrability and improved energy efficiency.
Traditional propellers still have a role to play in tug design, construction and operations, but thrusters have become one of the main elements of many modern designs. With two or three thrusters, tugs are able to turn a full 360 degrees in ever-more tight circles, which is increasingly important for ship handling operations in harbours and terminals.
Veth Propulsion has developed an integrated L-Drive compact and efficient electric propulsion device with a permanent magnet (PM) motor designed in collaboration with Visedo. The unit does not have geared transmission, meaning that it generates less noise, while the electronically-controlled drive and water-cooled PM also contribute to noise reduction, since there is no gear transmission in the tug.
Veth told Riviera Maritime Media that the unit is designed for workboats and is available with nozzle or counter-rotating propellers delivering power ranging from 300kW to 1,325kW. The first vessels to use the integrated L-Drive is due to be delivered in October 2017.
The PM motor is 40 to 60 per cent more compact than an asynchronous motor. This is integrated into the thruster and housed inside the vessel, making it much less vulnerable than if it were located under water. Veth has made adjustments to the control box and aligned the headset to improve the compact design further.
These are important design requirements for workboats that have little spare capacity for equipment and shallow draughts, said Veth chief executive Erik Veth. “The minimal mounting height allows the thruster to be fitted below deck height, so that few vulnerable capital assets are underwater,” he said.
“The minimal mounting height allows the thruster to be fitted below deck height, so that few vulnerable capital assets are underwater”
Using a PM motor improves the propulsion unit’s efficiency, particular during part-load conditions when compared with an asynchronous motor. Mr Veth said they are 5.2 per cent more efficient when the thruster is operating at 25 per cent load.The design of the propeller, especially the shark tail on the counter-rotating propeller optimises the flow of water, improving the thruster performance further.
Wärtsilä has also developed new hybrid propulsion units. The Wärtsilä HY, as the concept is called, combines the engines, an energy storage unit and power electronics to drive propulsion thrusters on tugs.
Wärtsilä said this product was a “benchmark in marine hybrid propulsion” because of the wide range of benefits to tug operators, such as increased operational efficiency and flexibility, lower fuel consumption, reduced emissions and improved vessel performance. It said some operations would not generate any emissions at all, depending on load points and operating mode.
Another benefit is the reduction in engine maintenance required because of its fewer operating hours. Lloyd’s Register has issued an approval in principle certificate for the Wärtsilä HY, which has enabled Wärtsilä to deploy the first examples of these hybrid propulsion units on a tug under construction for harbour operations in the Mediterranean region.
They will give this tug 80 tonnes of bollard pull and is likely to be deployed in an Italian port by its owner Rimorchiatori Riuniti. The hybrid propulsion contract was signed during the Nor-Shipping exhibition, near Oslo in Norway, at the end of May this year.
Wärtsilä’s equipment is scheduled to be delivered during the second half of 2018 and the new tug is expected to be in service by the beginning of 2019. Rimorchiatori Riuniti group technical director Raffaello Corradi said the tug owner was focused on embracing advanced hybrid technology for its new tugs. He added: “Wärtsilä HY will provide operational flexibility, added safety and environmental sustainability.”
Specific operational features will be embedded in the control logic of Wärtsilä HY to enhance its safety and environmental performance when the tug is operating in waters adjacent to heavily populated areas. Wärtsilä HY will also feature an integrated energy optimisation system that has been designed for hybrid applications to improve reliability and predictability of the power generation.
Rolls-Royce has also studied the use of hybrid propulsion systems and various motors for a notional stern-drive harbour tug with 60 tonnes of bollard pull. The studies considered various diesel-electric systems driving two US205FP azimuth thrusters. The analysis considered 1,460kW engines and two small (215 kWh) batteries, or 920kW engines supplemented by two 500 kWh batteries. The baseline involved two 1,920kW engines and a small auxiliary generator set covering hotel loads.
These studies demonstrated that energy storage systems can significantly reduce emissions during most tug operations but will need to be combined with diesel-based power generation until the price and efficiency of batteries has fallen.
Another conclusion was the efficiency of using PM motors and induction motors with batteries was similar at full loads. However, the PM motor maintains high efficiency down to very low loads whereas an induction motor’s efficiency drops away. Therefore, a PM solution can save on battery capacity as tugs only need small amounts of time at their maximum load rating.
Veth Integrated L-Drive benefits
- Compact design: extremely low mounting space requirements
- High efficiency
- Minimal noise production
- Low weight
- Outstanding manoeuvrability due to the 360-degree thrust
- Electric motor inside the ship means few vulnerable components underwater
- Simplified installation
- No need for a slip ring cabinet
- No gear transmission
- Optimal flow of water due to a shark tail on counter-rotating propeller