Naval architects have developed electric-powered tug designs, but more advances are required to improve their economic feasibility
Naval architects are tackling the tricky challenge of designing tugboats for net-zero greenhouse gas (GHG) emissions that are technically and economically feasible. The first electric-powered harbour tugs have been designed and architects are now looking at using alternative fuels.
Robert Allan Ltd (RAL) president and chief executive Mike Fitzpatrick says there has been a step-change in design philosophy brought about by a growing desire to minimise carbon emissions.
“The past couple of years have seen us move from the question, ‘How do we reduce greenhouse gas (GHG) emissions on vessels we design?’ to now asking, ‘What is the most feasible path to near net-zero emissions on these vessels?’,” he explains to International Tug & Salvage.
Incremental changes have been made to modern tugboat designs to improve performance, safety and efficiency, but the economic feasibility of a near-zero emissions tugboat has until 2020 seemed a long way off.
“We are now at a point where the technical feasibility of a near net-zero tugboat for at least some operating profiles has been established,” says Mr Fitzpatrick.
“We still have work to do on the economic feasibility of these designs, but there are a lot of very committed individuals and companies working on closing the price gap between conventionally fuelled tugs and low-emissions ones.”
More tug owners are looking to operate tugs with reduced GHG emissions, while others are already running tugs compliant with IMO Tier III and US Environmental Protection Agency (EPA) Tier 4 emissions requirements.
“Battery-electric tugs in various configurations are now a well-proven technical solution that can for some operations be justified economically, which is a big step forward,” says Mr Fitzpatrick, “but unfortunately this is not a one-size-fits-all solution.”
However, most tugboat operations worldwide are not suitable for pure electrification. “Alternative clean fuels present the only path, as far as we can see, to net zero for most vessels.”
There are few clean energy-carrier fuels available for marine applications and some are unlikely to be suitable for tugboats due to a lack of onboard storage space.
“Our conclusion is that green methanol made from renewable electricity or sustainable biomass is the most feasible for tugboats, although this might be better stated as the option with the fewest downsides,” says Mr Fitzpatrick.
“All these alternate fuels present significant practical challenges for incorporating into a highly powered compact vessel like a tugboat.”
RAL is busy providing designs for tugboats, workboats and other vessels, with more than 100 vessels under RAL’s design or construction in 27 shipyards in 14 different countries.
The vast majority are 20-50-m commercial workboats. RAL is also designing a 150-m articulated tug-barge unit and a 95-m Antarctic research and supply ship for the Brazilian Navy.
Among the tugs under construction are dual-fuel (/diesel) vessels and seven battery-electric tugboats, plus several more hybrid designs, with more than 20 tugs fitted with IMO Tier III or EPA Tier 4-compliant engines.
“By the end of this year, we will add several methanol-fuelled tugs under construction to the mix,” says Mr Fitzpatrick.
In total, RAL design vessels under construction have 250,000 kW of installed propulsive power.
“With this rate of propulsion power added to the global workboat fleet set to continue in the years ahead, from our designs, the associated potential increase in GHG emissions can become a very big number,” he comments.
“At RAL, we take this as positive motivation because if we can reduce the emissions of these vessels under our influence, we can make a significant difference.”
Operational experience
Navtek Naval Technologies has developed a series of electric tugboat designs with the first already accumulating more than 750 days of service. Gisas Power was the world’s first fully electric harbour tug, built to Navtek’s Zeetug30 design.
It has been operating in Tuzla Bay, Turkey since Q1 2020 with Corvus-supplied batteries on board and is recharged from a dedicated station at the quayside.
Navtek general manager Ferhat Acuner says during the first 750 days of service, Gisas Power completed 1,981 jobs with more than 2,500 motor running hours and accumulative charging of around 492 kWh.
“Its operating expenditure savings are really good, being 50% cheaper on fuel than a tug using marine diesel oil, and maintenance and repair costs are 79% less,” Mr Acuner says.
A key aspect of this project was deploying a quick-charging station to keep the zero-emissions electric tug in full operation. This infrastructure is available to charge electric-powered vessels in a port, including a fleet of Zeetugs under construction at Navtek’s new shipyard Turkey.
“Two Zeetug30s are under construction with conventional propulsion and one Zeetug45 is being constructed with azimuth stern drive propulsion,” says Mr Acuner. “Bigger Zeetugs of up to 80 tonnes are on the design desk.”
These tugs will have a bollard pull of 60-80 tonnes, whereas Gisas Power has 30 tonnes of bollard pull and the Zeetug45 will have 45 tonnes.
Mr Acuner says the tugs’ harbour operations can be fully electric, butalternative power sourceswill be needed for long-range operations. “It depends on the range and hours a tug operates. Electric tugs can operate up to 60 hours non-stop on batteries,” he explains.
Tugs could also be designed with swappable battery technologies, fuel cells or IMO Tier III diesel generators. “We can add range-extension devices, convert to hybrid tugs and go to another fuel system for long-range applications,” Mr Acuner says.
Electric and compact
Damen Shipyards has adjusted its reverse stern drive (RSD) design for electric power and delivered its first tug of this type, Sparky, to Ports of Auckland, New Zealand.
Sparky arrived 7 June after its voyage from Damen’s Song Cam shipyard in Vietnam. It is an RSD-E 2513 tug design with a top speed of 12 knots and a bollard pull of 70 tonnes. It has an overall length of 25 m and beam of around 13 m.
Charging takes two hours from empty to full using four high-power cables of 375 kW capacity each. Damen installed 2,240 Toshiba lithium-ion batteries and an Echandia Marine battery management system on Sparky in 2021.
This RSD tug has two Caterpillar C32 engines to provide back-up power through a generator, which can step in if the battery charge is low. These can provide a power boost, run fire-fighting systems and charge the batteries if required.
Sparky’s Damen E-Drive has enough battery capacity to perform at least three berthing operations on batteries alone.
During these operations it can push and pull with 70 tonnes of force for at least 30 minutes, after which it takes just two hours to fully recharge the batteries using a shore-charge station.
Most operations can be performed fully electric with zero-emissions.
Damen has also designed a compact azimuth stern drive (ASD) tugboat with IMO Tier III-compliant propulsion if required, and with better fuel efficiency. Its ASD tug 2111 design uses less fuel than larger harbour tugs at typical operating speed.
Damen says energy costs can be kept low with this vessel because its smooth hullform has a higher agility with less fuel consumption; indirect towage requires less fuel for higher line forces; its optimised size means less auxiliary power is needed and shaft generators can be used for accommodation loads.
