Eco-Friendly Tugs

By Mike Brezonick22 September 2017

By Bo Svensson

Since tugs typically operate in or close to harbors and populated areas, they are particularly affected by environmental considerations and the need for regulatory compliance is an increasing concern for tug owners and operators worldwide.

Wärtsilä has already introduced tug designs featuring liquefied natural gas (LNG) fuel technology as an efficient means for complying with environmental regulations. Now the company’s new HYTug series highlights Wärtsilä’s competencies in hybrid propulsion technology, along with extensive experience from more than 1000 tugs based on Wärtsilä’s ship designs.

“We believe that new tugs will in future rely more and more on battery and hybrid propulsion, which very well complements today’s operational requirements,” said Riku-Pekka Hägg, vice president, Ship Design, Wärtsilä Marine Solutions. “As a total solutions provider with unmatched in-house capabilities combining ship design, engine technology, propulsion, and electrical and automation systems, Wärtsilä is uniquely qualified to produce tug designs that fully meet the needs of the industry; and this we have now done.”

The newly launched Wärtsilä HYTug designs come in three different hull sizes — a 28 m harbor tug with a 50 tonne bollard pull, a 29.5 m harbor tug with a 75 tonne bollard pull and, eventually, a 35 m escort tug with a 75 tonne bollard pull.

The Wärtsilä HYTugs are designed for a variety of duties, as assisting in the berthing and launching of vessels calling at ports and terminals, along with towing and shiphandling operations in ports and terminals and eventually for high-speed escort service. They could also be used for fire-fighting, the company said.

Several propulsion configurations have been studied and the results indicated that the best efficiency was achieved by the Wärtsilä HYTug diesel-electric with a fixed pitch propeller arrangement and by the HYTug diesel-mechanical with controllable pitch propeller configuration.

The propulsion configuration is such that each design option includes the option of a diesel mechanical hybrid or diesel-electric hybrid propulsion. The designs are able to cover the 40 to 90 bollard pull range with the appropriate Wärtsilä equipment modules, the company said.

Enhanced efficiency enables the total installed main engine power to be less than with conventional designs, Wärtsilä said. Relying less on engine power also reduces exhaust emissions levels and the vessels’ environmental impact as well as saving on fuel and maintenance costs, the company said.

The comparison between the conventional Wärtsilä diesel/mechanical (DM) configuration and the new Wärtsilä HYTug diesel-electric (DE) design shows that the HY DE configuration offers lower emissions of CO2, NOx, Sox and particulates.

The HYTugs incorporate a distinctive look and have been optimized for low hull resistance, high towing/escort performance, seakeeping, crew safety and comfort, along with easy maintenance, Wärtsilä said.

“The new tugs are designed with thrusters from the Wärtsilä WST series featuring large propeller diameters for efficient and high performance propulsion,” Hägg said. “In addition to their excellent hydrodynamics and reliability, these thrusters are also easy to install, operate and maintain. The efficiency and low operational costs enabled by the WST thrusters enhance the environmental friendliness and overall economics of the new tugs.”

The development work has been carried out in close cooperation with the ABS, BV and Lloyds Register classification societies and the designs have received Approval-in-Principle (AiP) certification. This certification means that the design complies with class safety regulations and will be class approved in actual projects. The Wärtsilä HYTug solution has received AiP certification from the Lloyds Register classification society.

The newly launched Wärtsilä HYTug designs come in three different hull sizes, including a diesel-electric hybrid configuration with a 75 tonne bollard pull.

Wärtsilä HYTug’s hybrid configuration provides the simultaneous optimization of both low and high load performance as well as reducing emissions significantly, the company said.

Several propulsion configurations or alternative system, such as high-speed versus medium-speed engines, fixed pitch propeller (FPP) versus controllable pitch propeller (CPP) have been studied and compared with HYTug solutions with battery support.  Wärtsilä said results show that the best efficiency is achieved by the Wärtsilä HYTug diesel-mechanical configuration with CPP and diesel-electric with FPP configuration. The efficiency comparison has been performed based on fuel consumption per kW delivered to the thrusters with all mechanical and electrical losses taken into account.

“The control system (Power & Energy Management System) and the operator interface will secure an intuitive operation of the complete system,” Hägg said. “To enable the crew to keep full focus on the tug operation, the system has been developed to simply select mode of operation, then the control system will ensure that needed power for this mode. The control system will also secure that any sudden load step cannot overload the system.”

The graph shows a 75 tonne bollard pull Wärtsilä HYTug equipped with a diesel-mechanical hybrid configuration. An ac motor/generator is located in each of the propeller shafts.
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