Battery-hybrid propulsion delivers redundancy for whole tugboat life

More battery-electric and hybrid-propulsion tugs are under construction and in operation as the towage industry explores ways to reduce greenhouse gas (GHG) emissions and operating expenditure. Owners seeking to order them from shipyards need to analyse the propulsion options before investing.

Operational profiles are generally considered a key starting point when designing electric tugs, says Caterpillar Marine global steward for marine technology Marinus Jansen.  

He says there are several parameters to consider when selecting a whole propulsion system for newbuild tugs, including anticipated towage and services, estimated total cost of ownership, total energy source mix, fuel availability and power recharging, and capital and operational expenditure.

“With battery and hybrid systems, charging and discharge, do owners feel lucky in next 10 years on energy and fuels?” Mr Jansen asks. Implying owners should consider long-term requirements from low-emissions tugs and whether there is fuel and charge availability throughout the operational lifetime of a tug.

A decision should include “total cost of ownership, energy flexibility and GHG emissions reductions and operations through time. We need to remember our industry is about towage services and there will be an impact on tug designs from batteries and hybrid systems.”

Electric tugs may be required to handle and escort ships, provide standby services in case of high winds, secure ships at quaysides, and provide fire-fighting services – with the ability to throw water to 120 m for 24 hours. “This is the most power-intensive tug operation,” says Mr Jansen. “Fire-fighting can define power requirements on board and require a separate genset. If there is an engine on an electric hybrid tug, will it be able to undertake fire-fighting?”

Other questions to answer are the probability of success and failure, and whether this tug will have sufficient power to return to port and cope with issues. “There are many variables, such as battery capacity and charging station location, and the operational profile of tugs and load on energy sources,” Mr Jansen explains.

Battery-powered tugs can usually do two or three towage jobs a day on average, but there would be a spread with some tugs expected to achieve five to seven jobs in one day. “Some work will take just a few hours and others half a day,” he continues. “Complicated jobs could take several hours.”

Therefore, there is a need to know the expected number, time and energy per job, and the probability of these times. “Owners need to ensure there is enough energy for jobs, and know the charging opportunities, ie the time between jobs.”

They must also understand how long it takes to charge onboard batteries on a local grid and whether it is cost effective to recharge.

Other parameters to consider involve energy consumption per towage job which may depend on the size of the assisted ship, whether the ship is loaded or on ballast, its time in port, the anticipated weather, season of operations and the skill of the tug master.

“Tugs need to be ready for a windy day when a container ship arrives,” says Mr Jansen. “There needs to be energy available in the system. So, know the charging and control strategy, distribution of energy in the batteries and the amount of energy available at end of the job.”

Owners should simulate tug operations and operational profiles to “get distribution curves and run the numbers”, look at service levels, the success of operations and potential for issues.

“What is the probability of failure? It will be slim, but consider if there are 1,000 jobs a year, this accumulates to 10,000 jobs over 10 years,” says Mr Jansen. “If owners want maximum electric use with just batteries, they should accept some failure rates. But with gensets on board, these can be engaged for operations and charging.”

Mr Jansen says tugs should have gensets on board for redundancy and be able to run on sustainable fuels that would be widely available in ports, alongside batteries in a hybrid propulsion architecture.