LNG, hybrid or alternative fuels should be considered for powering escort tugs to cut emissions in terminals and ports
Alternative fuelled, hybrid- and electric-powered tugs could be considered for escort duties as more port authorities and terminal operators are pressured by public concern to reduce emissions.
The vast majority of escort tugs are diesel-powered tractor or azimuth stern drive vessels, but some are LNG-fuelled. For example, three Østensjø Rederi-owned azimuth reverse tractor tugs introduced 2016 and 2017 at Equinor’s Hammerfest LNG plant have dual-fuel engines. These were developed to Robert Allan’s RAstar 4000-DF tug design and Bureau Veritas class.
LNG fuel for tugs has taken off in Singapore, where Keppel Smit Towage (KST Liberty) and Maju Maritime (Maju Loyalty) were the first to operate dual-fuel tugs in 2018.
PSA Marine followed this with two LNG-fuelled tugs – PSA Aspen and PSA Oak – in 2019-2020. Their introduction is in parallel to the developing LNG bunkering facilities in Singapore.
These PSA 447-gt tugs were built by PaxOcean Shipyard to Robert Allan RAmparts 2800-DF design and Bureau Veritas class. During sea trials, PSA Aspen achieved bollard pull of more than 56 tonnes and free running speed ahead of 13 knots.
It is driven by a pair of Niigata medium-speed 6L28AHX dual-fuel main engines, each generating 1,618 kW of power at 750 rpm. They each drive a steel shaft connected to a Niigata ZP-31 Z-Peller propulsion unit, with a 220-cm diameter fixed-pitch propeller, and zero-to-idle slipping clutch.
In January 2021, Sembcorp Marine announced what will be the largest fleet of LNG-powered tugs worldwide. It is building a harbour tug for Jurong Marine Services in Singapore with two Rolls-Royce-supplied gas engines and a hybrid propulsion system.
“There are no differences in LNG or diesel mode performance”
Sembcorp Marine plans to build up to 12 tugs with MTU engines and hybrid propulsion to replace diesel-powered tugs by 2025.
PSA Marine has tackled challenges in operating LNG-fuelled tugs and seen benefits in lower emissions and increased crew comfort. PSA Marine fleet management’ s senior manager for fleet engineering Jeffrey Sim says when its LNG-powered tugs are burning gas, emissions of SOx, NOx and particulates are negligible, while there are significant reductions in greenhouse gases.
“We have seen a 22% reduction in CO2 and 15% lower noise levels for better crew comfort, compared with conventional diesel-fuelled tugs,” he says. “Gas mode is quiet and has increased the overall crew comfort onboard.”
By operating both LNG and diesel-powered tugs, Mr Sim can compare their performance and operating expenditures. “There are no differences in LNG or diesel mode performance,” he says. “We achieved similar bollard pull performance. There is also no difference in the maintenance and repair cost for an LNG and diesel tug,” Mr Sim adds.
However, challenges in operating LNG tugs include additional safety and manning for truck-to-tug bunkering.
“Bunkering takes three times longer than with a conventional diesel tug, so we need to book this up to seven days in advance,” says Mr Sim.
He said these operations need additional crew deployment for monitoring bunkering, manning the bunker station and controls in the engineroom. There are also additional safety protocols, such as quick connect/disconnect cryogenic couplings and hoses for fuel transfer, a water curtain and drip tray and nitrogen gas supply for purging.
Crew also need training to manage bunkering operations, dual-fuel engine maintenance and to maintain onboard LNG storage and handling systems.
Mr Sim said financial and regulatory support is important to help cover the additional capital costs for building a dual-fuel tug compared with a conventional diesel vessel.
“Financial support for an LNG tug could cover as much as 30% of the additional newbuild capex cost,” he says.
Tug designers think owners, faced with the high costs and operational challenges, will consider alternatives to diesel and LNG. There are plans to test methanol, ammonia and hydrogen for tugboats. Some owners are gaining experience with hybrid propulsion and the first mainly electric tugs are being tested.
“Green technology to reduce the emission of CO2, NOx and SOx will continue to lead the development of power generation and energy storage on board,” says OSD-IMT technical manager Herm Jan de Vries.
“The wheels have been put in motion and there will be no way back”
“However, it still remains extremely difficult for owners to find a good business case to finally actually buy tugs that incorporate this technology,” he says, “because costs are still high and revenues are not increasing. But it looks like the wheels have been put in motion and there will be no way back.”
Even when owners decline to invest in alternative fuels or hybrid propulsion, many will opt to ensure their tugs will remain compliant with changing environmental regulations throughout their lifespan.
“Remember that an investment in a tug is done for the next 25-30 years,” explains Mr Jan de Vries. “In an attempt to adapt to changing market conditions, owners are asking to future-proof tugs by designing them to accept technology such as selective catalytic reduction (SCR) equipment, alternative fuels and other engines somewhere in the future, without too much of a cost and size impact in the present day,” he explains.
OSD-IMT is working on designs to incorporate fully electric and hydrogen fuel-cell propulsion into tugs.
Damen Shipyards has producing its first all-electric harbour tug, for Port of Auckland, New Zealand. This reverse stern drive (RSD) tug is based on its own RSD-E Tug 2513 design and should be available later in 2021.
Damen product manager for tugs Erik van Schaik said this tug is equipped with two 1.5 MWh battery packs, two Caterpillar C32 1-MW generator sets and two 1.8 MW electrically driven rudder propellers. “Batteries are charged with shore power and deliver all required energy during normal (un)berthing operations in the harbour, with a maximum bollard pull of 70 tons,” says Mr van Schaik.
“The generators are only started to perform fire-fighting operations or where an extremely big autonomy is required with a maximum continuous bollard pull of 40 tonnes,” he says.
Damen uses Echandia’s air cooled battery system, consisting of Toshiba SCIB battery modules, based on lithium titanium oxide (LTO) technology. “The battery systems are situated in an isolated, temperature-controlled room,” says Mr van Schaik.
He thinks these batteries can last 30,000 cycles, possibly throughout the life of the tug.
“Tugs can perform two to three average (un)berthing jobs in the harbour on a full battery,” he explains. “In a normal port the vessel will be charged once a day and in an extremely busy port the vessel might be charged three times each day. This means 1,000 cycles a year and battery lifetime of at least 30 years.”
Mr van Schaik thinks all-electric tugs have sufficient power to perform escort duties. “RSD-E Tug 2513 can deliver the maximum bollard pull of 70 tons for at least 30 minutes only on batteries, or for at least one hour on batteries plus generator sets,” he says,
“During escort tug operations and other operations where bigger autonomy is required the IMO Tier III compliant back-up generator sets run continuously to generate basic power supply up to 40 tonnes of bollard pull and batteries only deliver additional power to boost up to 70-tonnes bollard pull,” he explains.
Wärtsilä Marine Power director of business development Giulio Tirelli thinks there should be no hurdles with applying hybrid technologies to escort tugs.
“The tug design principle for which a fully electric tug can be designed is, in principle, the same as for a hybrid tug or a conventional tug,” he explains. “The maximum bollard pull will be achieved and, independent from the maximum power, the propulsion system can create and the thrusters can absorb.
“This is nothing different from any other tug,” says Mr Tirelli. “At the same time, having batteries producing an output power similar to an engine is rather difficult with current batteries technologies,” he adds.