Variable speed in action

Harbour tugs operate in extremely variable conditions: sometimes they have to tow ships at full throttle, and at other times they have to manoeuvre with precision. The “Harbor 712” project demonstrates how a new propulsion concept addresses precisely this issue. 

The break between two shifts in port operations is short. But for the tugboat “Harbor 712”, it is enough: in around an hour and a half, it charges its batteries from 30 to 90 per cent during its regular layover. And when work resumes in the Port of Shanghai, the vessel is ready. 

“Harbor 712” is a research project aimed at developing new tugboat propulsion systems, through which the Port of Shanghai is responding to national policy on environmentally friendly ports. 

The fast-charging capability is part of a new propulsion concept developed by the Shanghai International Port Group in collaboration with Rolls-Royce Power Systems. At its heart is a hybrid system featuring variable-speed mtu diesel engines. Until now, the technology has hardly been used on harbour tugs. Yet its advantages are particularly evident here. This is because, in port operations, efficiency is determined across several areas simultaneously: fuel consumption, fleet availability and emissions. 

The new concept is based on the interaction between the propulsion system and the energy system. The three 1,300 ekW 12-cylinder engines from the mtu Series 4000 M64 do not run at a constant speed. Instead, they continuously adapt to the actual power requirements. This is particularly important in port operations: tugs operate under complex and varied conditions, ranging from high power output to finely tuned manoeuvres. The variable speed ensures that the engines operate within the optimum range in every situation. 

The result: compared to conventional units with a constant speed, variable-speed units reduce fuel consumption by up to 20 per cent, extend the time between overhauls by up to 20 per cent and lower noise emissions. 

The system is complemented by a 2,000 kWh battery pack and a DC bus power distribution system. The battery balances out peak loads and stores energy when it is not currently needed. This creates a system that uses energy selectively rather than constantly holding it in reserve. 

In hybrid systems such as these, it is particularly important that the engine and energy management system work together seamlessly. “We chose mtu engines not only for their consistently high performance and reliability, but above all for their extensive track record in variable-speed on-board power generation,” says the operator of the tugboat at Shanghai International Port. “

The operator also intends to rely on mtu technology for future projects involving variable-speed hybrid solutions. This is because, in the face of increasing environmental requirements, hybrid propulsion systems open up new possibilities for port operations. Rolls-Royce Power Systems is working with partners to further develop these technologies and make them suitable for use in real-world conditions. The fact that this approach is already working today is demonstrated by ‘Harbor 712’.