From tugs to tankers, diesel-electric propulsion enjoys such a high profile these days that it’s easy to forget that most new ships are much more conventionally powered. But architects and engineers who gathered near Washington, D.C., in February for a symposium on electric ships heard a reminder from the keynote speaker that the proportion of diesel-electric vessels built each year varies between just 2 and 4.5 percent.
“Electric propulsion is a niche market,” said Jukka Kuuskoski, vice president for ABB Marine in Finland.
That said, diesel-electric dominates some vessel types. Looking at the current world order backlog, ABB puts the market share at 100 percent for cruise ships, icebreakers and drilling vessels, and 45 percent for LNG carriers. Even orders for large OSVs — defined by Kuuskoski as those with more than 4 to 6 MW of installed power — are currently 50 percent diesel-electric. For example, New Orleans-based Tidewater, which operates the world’s largest fleet of offshore vessels, recently announced that it is building 12 new diesel-electric 286-footers in China.
The advantages of electric — usually diesel-electric — for commercial vessels, especially when combined with podded propulsion, were spelled out in an article last year (PM #110). They include a more efficient propulsion system, less pollution, increased operational efficiency, especially at slow speeds (important for applications such as dynamic positioning), and flexibility in locating machinery compartments.
The chief disadvantage is high up-front cost. “Commercial shipowners are very hesitant to take risks on the shipbuilding side. They take enough risks on the market side,” said Keith Michel, president of the Society of Naval Architects and Marine Engineers (SNAME). The symposium was a joint brainstorming session with ASNE, the American Society of Naval Engineers.
The cost factor has held back more widespread adoption of the technology, although some U.S. shipyards have extended experience with diesel-electric propulsion, notably General Dynamics Nassco with its Orca-class ro-ro vessels, BP Alaska-class tankers and underway replenishment ships for the Military Sealift Command. Among brownwater operators, Larry Rigdon was an early convert to diesel-electric; Bender Shipbuilding delivered the first of a series of 210-foot DP-2 OSVs to Rigdon in 2004.
One of the broadest perspectives at the conference came from Peter G. Noble, chief naval architect for Houston-based ConocoPhillips, which sees diesel-electric as the answer for many types of vessels involved in offshore work and operating in depths of 10,000 to 12,000 feet — seismic vessels, drillships, semisubmersibles and others.
Diesel-electric has significant application for tugs, and Glosten Associates of Seattle has designed a 90-foot hybrid tug with 72.5 tonnes of bollard pull for Crowley Maritime (PM #118). Glosten has also been evaluating the possibility of a battery-electric tug for Southern California that would not use any fossil fuels.
In addition, Glosten has completed the design for a 242-foot research vessel for the University of Alaska Fairbanks that demonstrates why diesel-electric is the first choice for vessels capable of operating in ice. The design calls for two ice-strengthened azimuth thrusters to power the vessel.
“With the diesel-electric podded propulsion we can have full torque on the props at zero speed,” said Jeff Hawxhurst, senior electrical engineering technician at Glosten. “We don’t want to get stuck out there.”