Hybrid tug designs aim for efficiencies during stop-and-go harbor operations

Ten years ago hybrid cars like the Toyota Prius seemed a bit far-fetched to most of us, but today, despite the higher cost, their popularity continues to grow with more than 1 million vehicles sold. At the recent International Tug & Salvage Convention in Singapore, four companies presented their versions of hybrid technology for harbor tugs.

Foss is building a hybrid Dolphin-class tug at its Rainier yard. The tug is due for completion this fall.  (Photo courtesy Foss Maritime)

Harbor tugs experience the maritime equivalent of stop-and-go city driving with a lot more “stop” than “go.” In some harbors, especially those with minimal run-in from the sea buoy, ship-handling tugs spend less than 10 percent of their time under full power. Yet marine diesels are designed to function at their greatest efficiency at maximum rpm. For the more than 80 percent of the time that some harbor tugs spend idling on standby or between jobs, designers are coming up with a variety of non-diesel options.

In Finland, Wartsila has designed a hybrid tug that employs a combination of engines and batteries powered by diesel and liquefied natural gas (LNG). An oft-cited problem with LNG in boats is the great volume required for storing it. In their paper on this concept tug, Wartsila’s Noel Dunstan and Oskar Levander explained:

“Compared to marine diesel oil (MDO), an equal energy content of LNG requires about 1.9 times more volume. When adding the tank insulation and bearing in mind the maximum filling ration of 95 percent, the required volume is increased to about 2.3 times. The practical space required in the ship becomes about four times higher when also taking into account the squared space around the cylindrical LNG tank.”

But these designers do not see this as an insurmountable problem in a 115-foot harbor tug with 100 tonnes of bollard pull and 6,080 kW of power provided by two generators driven by a Wartsila 6L34DF and an 8L34DF. Electric motors would power the tug’s z-drive so that the battery bank could meet power needs at the lowest levels. The engines could be brought on as a single or double power source. For normal harbor and ship docking, the mechanical engines would run on LNG, although the option of MDO will be retained for movement between ports or emergency tows of a longer duration.

Crowley Maritime Corp. is looking to LNG as the most effective way to reduce harbor tug emissions. Working with naval architects from Seattle-based Glosten Associates, they have designed a DNV-classed 90-foot tug with 72.5 tonnes of bollard pull.

Seattle-based Glosten Associates has designed a 90-foot hybrid with 72.5 tonnes of bollard pull for Crowley Maritime.  (Photo courtesy Crowley Maritime)

Designed to meet the requirements of the Los Angeles/Long Beach ports, the tug will utilize a pair of medium-speed 2,600-kW Rolls-Royce KVMS-12G4 engines mechanically coupled to Rolls-Royce US255 CP z-drives.

The natural gas will be stored as a liquid in two 5,500-gallon double-walled tanks in the forward third of the hull. Emission reductions using natural gas instead of low-sulfur diesel are calculated at 89 percent for nitrogen oxides; 90 percent for particulates; 94 percent for sulfur dioxide, and 24 percent for carbon dioxide.  The boat also will have tanks for 2,000 gallons of diesel to run the hotel generator. Under normal operating conditions, the natural gas refueling cycle is predicted at five days. Fuel can be delivered by tank truck, the current method for a variety of port machinery that is already powered by natural gas.

Michiel Wijsmuller is from the family of the well-known Netherlands tug company of the same name that was sold to Svitzer. Now, with his firm WorldWise Marine, he is working with Dutch tug operators Iskes from the port of IJmuiden and Smit from Rotterdam. Pointing out that the “greenest” tug would be an all-electric vessel, he acknowledged the impracticality of total reliance on a shore-side power source.

The next best thing, Wijsmuller said, is a hydrogen hybrid harbor tug (HHHT). He explained that this tug is similar to the battery/diesel hybrid developed by Foss Maritime Co., but with the addition of fuel cells and tanks in which gaseous hydrogen is stored under a pressure of 430 bar. The fuel cells and batteries would be able to provide sufficient power to operate the tug for at least 70 percent of the time; the diesel electric generators would only need to be brought on line for the 15 percent to 30 percent of the time that the tug is performing berthing and unberthing operations. This would mean that for 70 percent to 85 percent of the time the tug would have zero emissions.

The Wijsmuller tug will be a 99-foot azimuthing stern drive vessel with a single winch and 50 tonnes of bollard pull. It also will be fitted with a 300-kilowatt inline Voith retractable bow propulsion unit to be used with hydrogen power for moving around harbors and getting to and from jobs. The electric motors driving the stern units will be mounted directly on top of the units to reduce mechanical loss and provide more room for the batteries and hydrogen cells.

“Green tug” technology was a hot topic at the International Tug & Salvage Convention, but for all the talk, Foss Maritime is the only company with a boat under construction. Despite some delays, the company expects to have its Dolphin-class hybrid working in Los Angeles/Long Beach by the end of the year, said Gary Faber, Foss president and chief operating officer. As reported in earlier issues of this magazine, the Dolphin tug is a Robert Allan-designed 78-foot vessel with 5,080 horsepower and 60 tons of bollard pull.

After a brief introduction at the convention in Singapore, Faber left the technical part of the presentation to Jason Aspin of the Canadian company Aspin Kemp & Associates. As they did for the tugs mentioned earlier, the designers here started with the particular operation parameters of their harbor vessels — short periods requiring full power and extended periods of reduced power.

While a line-haul tug has two operating modes, full power and shutdown, a harbor tug has a variety of modes in between. Working with the computerized memories of the engines on the two conventionally powered Dolphin-class tugs working in Los Angeles/Long Beach, the designers identified a range of power demands. These have been translated into several combinations of power options and operating modes for the hybrid tug.

While the Foss tug was designed with a dramatic reduction in pollutants as the primary motivation, it is also expected to provide significant savings in fuel and lube oil costs. As was pointed out by the other designers, this is an important factor in a time of ever-escalating fuel prices. All of the designers also stressed that while this first generation of “green tugs’ will not be the definitive solution, each will make a contribution to understanding alternatives to conventional diesel/mechanical propulsion systems so that in 10 years such technologies will be more widely accepted in the ports of the world.

By Professional Mariner Staff