With a regulatory clampdown on diesel particulate emissions expected, tugboats are a highly visible target. But new technologies offer the prospect of dramatic reductions
Operating the world’s busiest container port is a source of pride for Shanghai International Port Group. But with scale come sizeable challenges. Last year the company’s facilities welcomed 60,000 vessels that required a tugboat escort. With large container ships requiring up to four tugboats to help them navigate in and out of port, the harbour workhorses could become a natural focus for regulators in a country taking big strides to tackle emissions.
The regulation of particulate matter is a question of when, not if. European Inland Waterway Stage V requirements, coming into force next year, are the first definite move. IMO is also expected to act, but ports are likely to act first. Coastal and port vessels – particularly the growing fleets of tugboats operating at ports like Shanghai – will be among the first to face regulatory demands to reduce particulate matter emissions.
“Tugboats may have become the main source of air pollutants at ports, with a potentially high impact on air quality,” says Tongji University School of Automotive Studies professor Dengguo Liu. “But while many researchers have examined emissions from large ships, only a few have studied the discharge characteristics of tugboats.”
“Tugboats may have become the main source of air pollutants at ports, with a potentially high impact on air quality”
Dr Liu has rectified that imbalance with a real ship test on a tugboat operating in Shanghai Waigaoqiao port. The study team used a portable emissions monitoring system to collect data on particulate matter emissions across the vessel’s operating profile, which comprised five distinct sectors: departing the harbour, cruising, fast pushing, slow pushing and docking.
Apart from during cruising operations, the concentration of both particulate matter (measured by mass) and the number of particles emitted fluctuated significantly. Emissions concentrations reached the highest values under fast pushing and the lowest under slow pushing. But factoring distance travelled as well as emissions concentrations yielded clearer results. The total emissions for both departure and docking were significantly higher than for cruising. Slow pushing generated higher emissions still and fast pushing was the biggest contributor to emissions.
Based on the results, Dr Liu recommended that further studies are made of the relationship between speed, fuel consumption and particulate matter emissions. He also suggested that tugs’ cruising speed could be optimised for fuel consumption and emissions.
But managing speed and fuel consumption is unlikely to be the only step required to reduce emissions. The forthcoming European inland waterways regulations are severe enough that vessels using conventional fuels will be unable to comply without installing diesel particulate filters. However, these filters are a relatively immature technology and as such are sensitive, requiring careful integration into existing aftertreatment systems if they are to be effective. For tugboats, there are perhaps better ways to reduce emissions.
Cutting NOx emissions
The Shanghai Marine Diesel Engine Research Institute (SMDERI) has joined the increasing number of designers deploying two-stage turbocharging as it reacts to new emissions demands from the Chinese Government.
On 1 July 2018, the first stage of China’s new emissions regulations governing marine engines used on rivers and around islands came into force. Known as China I, the regulations apply to the vast majority of vessels on the country’s domestic waterways that fall between existing domestic emissions legislation on small engines (with a power output of less than 37 kW per cylinder) and the engines of internationally trading vessels whose emissions are regulated by IMO.
The regulations cover carbon monoxide, unburnt hydrocarbons, NOx and particulate matter. Perhaps the most onerous levels for engines to reach are those relating to NOx. For a medium-speed engine like SMDERI’s six-cylinder 6CS21, with a total rated output of 1,320 kW, China I demands a 13% reduction in NOx emissions. That will become even more onerous from 1 July 2021 when a second stage of the regulations, China II, come into force. NOx emissions for the current 6CS21 – which is deployed on several tugboats operating at Chinese ports – will need to be cut by nearly a third.
SMDERI turned to a two-stage turbocharging process to help the engine meet the first round of China’s new emissions demands. The two-stage turbocharged engine, 6CS21TS, has 14% higher power than the 6CS21, with rated power increased to 1,500 kW. The emissions characteristics of the two-stage turbocharged engine are also improved. The weighted fuel consumption is reduced by 11.6 g/kWh, smoke emissions at all the operation loads are reduced and NOx emissions are reduced by 12% – satisfying not only the China I but also IMO’s Tier II NOx emissions regulations.
The two-stage turbocharged engine has a much higher pressure ratio than the original single-stage turbocharged version, up from 5.09 to 7.15. The average exhaust gas temperature was reduced by 23°C, reducing the heat load of the diesel engine.
SMDERI also studied the influence of timing on the 6CS21TS engine’s performance. It concluded that variable valve timing technology can be used to solve low-load performance problems as well as other challenges – including insufficient air intake and higher fuel consumption – that would be caused by a strong Miller cycle.
So much for China I, but how will SMDERI’s engine meet the even higher demands of China II in two years’ time? The company has conducted simulations deploying exhaust gas recirculation (EGR) on the 6CS21TS engine. It found that while the brake mean effective pressure remains constant at 2.71M Pa, emissions are further reduced, to below the limit required by China II.
Although there is an inevitable trade-off between NOx reduction and fuel efficiency, the engine simulation showed good economy with a minimum fuel consumption rate of 195.1 g/kWh at 75% load. SMDERI’s research also showed that the new 6CS21TS diesel engine could use larger EGR rates (recirculating more than 25% of exhaust gas) to satisfy the IMO Tier III emissions regulation.
The institute believes its research so far shows the technical route to bring the 6CS21 diesel engine up to the China II regulations is to use a high-pressure common rail fuel system, a two-stage turbocharging system, a strong Miller cycle and EGR. So far it has completed the design of the 6CS21TS with the EGR diesel engine. According to the project plan, the prototype engine is expected to be bench tested in October.
Battery-powered tugboats built in Turkey
For even deeper emissions reductions, operators are looking towards hybrid and now fully electric propulsion options. Turkish ship designer and builder Navtek Naval Technologies, a subsidiary company of TK Tuzla Shipyard, is building the world’s first battery-powered, all-electric tugboat. The zero-emissions electric tugs – four newbuildings are planned – will provide a towing capacity of around 30-tonnes bollard pull and will operate in Marmara Sea in the port of Istanbul. The first will be delivered in Q3 2019.
Hybrid tugboats are already in operation, but the nature of the Navtek vessels’ operations meant that full electric propulsion was an option, explains Navtek vice chairman Orkun Özek. “The tug is small and will work in a very narrow and tight environment, which is why we opted for a fully electric, battery-powered propulsion system rather than a hybrid solution,” he says.
The Navtek NV-712 ZeeTug, which stands for Zero Emissions Electric Tug, will be delivered to GISAS Shipbuilding Industry Co. It will house a Corvus Orca energy storage system (ESS), with the 1,500-kWh battery providing power for two 968-kWe Siemens propulsion motors to drive a conventional propulsion system that includes ABB thrusters and drive systems. The propulsion and management systems will be integrated by BMA Technology.
According to BMA research and development manager Akin Artun, ZeeTug will carry out harbour towing duties during the day and be recharged overnight. One hour-long charging per day will be enough to keep the tug powered up, although this could be shortened to half an hour if shore-power capacity is added at further locations.
Although it is easy to predict charging periods and operational ranges for ferries and passenger vessels on fixed, regular routes, the flexible and varied nature of tug work makes it harder to establish an accurate estimate of the required charge times and energy usage. For this reason, Navtek is developing another tool for GİSAŞ to manage power on the Zeetugs. The Smart Tugboat Energy Management System (STEMS) will enable communication with all Zeetugs from shore and will monitor battery status and shore connection availability, predicting time and energy requirements for the next ship towing operation.
Navtek NV-712 Zeetug particulars
Length: 18.7 m
Beam: 6.7 m
Draught: 3.5 m
Gross tonnage: 112
Speed: 10 knots
Bollard pull: 31 tonnes
Hybrid is a stepping stone to full electric tugs
The weight of batteries means significantly higher pulling capacities on bigger vessels remain beyond the range of full electrification, but developments in hybrid technology continue apace. Wärtsilä’s hybrid power module, the Wärtsilä HY, is now fully operational on Vilja, an escort tug owned by the Swedish port of Luleå.
The hybrid solution enables significant benefits in terms of environmental performance and a big reduction in fuel and maintenance costs. During trials, the ice-breaking tug achieved a 100-tonne bollard pull, while producing approximately 20% fewer emissions than a comparable conventional vessel.
“This vessel is the most powerful tug operating with hybrid propulsion and it can be considered as a benchmark in modern shipping,” says Wärtsilä Marine director of sales and business intelligence Giulio Tirelli.
The tug was built at Gondan Shipbuilders in Asturias, Spain and delivered at the end of June 2019 after successfully completing commissioning and sea trials. The Wärtsilä HY module installed has been specially developed to meet the needs of the tug market. Operating modes include ‘green mode’ with zero emissions and no noise; a ‘power boost’ that delivers a higher bollard pull than any other conventional solution of comparable size; and ‘smokeless operation’ whereby no smoke is produced even during start-up of the main engines. All have been successfully tested and demonstrated at Wärtsilä’s hybrid centre in Trieste, Italy.
The energy management system is designed to efficiently cope with drastically changing operating conditions and has been tuned based on real at-sea data acquired during one year of field operations. Its functionalities have been fully tested at the Wärtsilä Hybrid Centre. In Luleå, the vessel will alternate between normal summertime operations and working in 1-m thick ice during the winter months. Having an adaptable ‘brain’ ensures the tug’s ability to maintain the highest efficiency performance in both situations.
Whether with conventional-fuelled engines or electric solutions, there are means for scaling down particulate matter emissions in advance of anticipated regulation. Given their small size and high visibility, tugboats have an opportunity to become early adopters as shipping grapples with a new emissions challenge.