Diesel-electric propulsion pushes ahead
Nov 20, 2007
Richard O. Aichele
First and Ten is one of 10 190-foot platform support vessels designed by Guido Perla & Associates and built by Bollinger for Rigdon Marine. (Courtesy Bollinger Shipyards Inc.)
The evolution of ship propulsion has brought greater use of diesel-electric technology to meet challenges of reliability and increasingly stringent environmental standards.
Because of its versatility and redundancy, diesel-electric is adaptable to vessels of almost all sizes. In diesel-electric systems, multiple diesel engines, each driving an electric generator, produce the electric power that energizes the electric motors connected to the propellers as well as other electrical loads on the ship. Depending on electrical demand, not all diesel generators have to be operating at all times.
Paul Murray, operations manager at Converteam, Inc. in Pittsburgh, explained the basic advantage offered by diesel-electric systems: "The old way of propulsion has a diesel engine's output usually going through a gear box, then to a shaft line going back to the propeller. With mechanical propulsion, if a ship loses one of two diesel engines, it loses fifty percent of its propulsion power. The beauty of diesel-electric propulsion is the diesel engines are turned on only as they are needed, and if an engine is lost, the remaining diesel-engine generators on the system can still provide power to both shaft lines."
Dan Koch, vice president of engineering at Guido Perla & Associates Inc. (GPA) in Seattle, noted some of the other benefits: "Diesel-electric propulsion provides many propulsion options and advantages. The first is efficiency by being able to run appropriately sized diesel-engine generators based on demand versus a large diesel engine often idling along and that leads to better control of emissions. Many of our ship diesel-electric designs use multiple engines of different sizes. With combinations of small, medium and large diesel generators, you are able to bring on power in small increments to keep the engines at appropriate loadings and operating at their highest efficiencies."
With diesel-electric systems, ship designers have been able to use below-deck space more efficiently, since the diesel generators, switchgear and propulsion motors can be located almost anywhere.
"The big low-speed diesel engines with the gear boxes and shaft lines to the propeller are located deep down in the V section of the hull," Murray said. "With diesel-electric, the diesel engines and generators can be put higher up, giving the operating benefits of less ducting for air intakes and exhausts that also takes up less space within the vessels. It also allows a better distribution of the equipment," said Murray.
Koch observed, "Noise and comfort are big design issues, but it's not solely related to diesel-electric or not. A traditional shaft line running to the engine, which is hard mounted to the ship, will produce more vibration and noise. A generator set is an isolated piece of machinery, and that benefit is it can be vibration-isolated."
|Delivered in June 2006, Lewis and Clark is the first in a class of dry cargo-ammunition ships being built by Nassco for the U.S. Navy. Its diesel-electric plant is rated at 35 megawatts. (Courtesy Nassco) |
MV Midnight Sun and MV North Star ro-ro ships built at Nassco in San Diego, Calif., for Totem Ocean Trailer Express, Inc. (TOTE) in 2002 and 2003 are examples of the operational benefits of diesel-electric propulsion. TOTE had decided to build two new ships to replace its fleet of three aging steam-propulsion ships used on the Tacoma, Wash.-Anchorage, Alaska runs. At the start of construction, a Nassco statement noted the innovative designs of the new 24-knot vessels would make them "the first ships in the U.S. powered by an integral diesel-electric system."
During the early design phase for the new ships, it was apparent that traditional low-speed diesel engines linked mechanically to the propellers would have been the least expensive capital investment option but costly in operation. John Boylston, a naval architect based on Dresden, Maine, explained, "One of the problems of low-speed diesels is, while they have fantastic fuel consumption rates running at the engine's optimum point, the fuel rates diminish very quickly above or below that point. In TOTE's case, it was determined the ships would be running 80 percent of the time suboptimally and that would be a horror in terms of operating costs."
The diesel-electric systems installed on the ships use four MAN Diesel 9L 58/64 medium-speed diesels rated at 11,700 kw at 400 rpm driving Alstom 11.4-MW generators. Two MAN Diesel 9L 27/38 medium-speed diesel rated at 2,700 kw at 720 rpm each drive Alstom 2.6-MW generators.
Each of the two propellers is driven by Alstom variable-speed reversible motors rated at 19.75 MW. Synchroconverters facilitate starting and speed control of the two propulsion motors.
Several other operating factors had also influenced TOTE's decision to install diesel-electric systems. During the TOTE ships' design phase, the naval architects determined that the use of diesel-electric with medium-speed low-height engines would permit reductions of the engine room height. Since the engine room would not extend through the second deck, movement of cargo at that level would be opened up, allowing free movement from one side of the ship to the other. The result was greater efficiency in the cargo operations.
TOTE "had been really concerned about reliability and redundancy," in part because there was no port of refuge on the Tacoma-Anchorage run. "The multiple diesel engines, generators and electric motors of a diesel-electric system offered an incredible amount of redundancy," Boylston explained.
With two ships instead of three, reducing or eliminating downtime for engine repairs was also important. "The normal voyages were to be 66 hours each way, with eight hours in port.
There was no time to spend 24 hours in port for replacing liners in a low-speed diesel engine. In the diesel-electric engine rooms, all the necessary gear was installed so the medium-speed diesels could be maintained underway. An engine can be taken offline, the heads removed and the liners replaced. They have done a whole set of engines on each ship now, so the concept has been proven," Boylston said.
The advantages of diesel-electric propulsion are increasingly being exploited in large ships where reliability, fuel economy and close control of emissions are important operating factors. Cruise ships have long recognized the advantages because of their large hotel electrical loads, varying speeds during cruising and lengthy port layovers, often in environmentally sensitive areas.
The U.S. Navy's acceptance of integrated electric-drive propulsion systems for surface ships became established with the 689-foot T-AKE class of dry cargo and ammunition supply ships being built by Nassco. These ships can carry 7,000 tons of dry cargo and ammunition and 23,500 barrels of marine diesel fuel.
BP Shipping's commitment to "environmentally friendly" tanker construction and operation included selecting diesel-electric propulsion for the 185,000-dwt Alaska class tankers built by Nassco. The ships have four MAN Diesel 6L48/60, 6.3 MW medium-speed diesels and Alstom 6.6 kv electrical systems with two synchronous, variable speed, reversible electric motors rated at 10.0 MW at 85 rpm. The power plants are located in two separate engine rooms. A BP Shipping statement explained: "Diesel-electric propulsion was chosen because it significantly increases reliability and reduces air emissions and maintenance downtime."
The designs for AHL Shipping Co.'s three new 617-foot, 330,000-barrel Jones Act tankers specify a diesel-electric propulsion system with state-of-the-art pulse-width modulation converter technology in medium voltage and two induction motors rated at 4.65 MW each. Construction of the vessels has not yet started.
Bob Salmon, general manager at Shell Shipping, which will operate the vessels, said diesel-electric propulsion would provide improved vessel efficiency and reduced emissions.
Improved PSV designs
Alaskan Frontier was delivered in 2004 by Nassco to BP Oil Shipping Co. USA. The 941-foot, 185,000-dwt tanker has four MAN Diesels rated at 6.3 MW each.(Courtesy Nassco)
In a diesel-electric design, the diesel engines and their associated generators can be positioned away from the electric motors they power. That means the diesels and generators can be located anyplace the naval architect chooses. This flexibility allows designers to devise much more efficient layouts compared with direct mechanical drives.
Koch, of Guido Perla & Associates, noted, "Traditional long shaft lines gave two choices: locate the engine room aft so as to not have long shafts running through the cargo and tank areas or sacrifice cargo space by running the shafts through those areas. Diesel-electric gives the options of locating the equipment almost anywhere.
"With diesel-electric propulsion, a number of our PSV (platform support vessel) designs have the engine room located above the main deck, which makes for very spacious cargo areas, and tankage definitely has its merits for the owners."
The 190-foot GPA 654 class of PSV vessels designed by Guido Perla & Associates for Rigdon Marine is being built by Bollinger Shipyards in Lockport, Miss. These vessels incorporate the latest features of diesel-electric technologies and applicable ship designs. With the engine room, classed as an automated machinery space, located just above the main deck, the vessels have a tremendous amount of tankage space below deck, Koch explained.
The GPA 654's diesel-electric propulsion system consists of two azimuthing and one fixed Steerprop SP 10 L-Drives using General Electric 752 traction motors and two 560-kw fixed-pitch bow thrusters for maneuvering. Electrical power is produced by two Cummins KTA50DM diesel generator sets, each rated at 1,235 kw, and one Cummins KTA19DM diesel generator set rated at 435 kw.
"That gives them great flexibility in terms of power management," Koch said. "When the vessels are on station and not primarily using the electrical power for propulsion, but for station keeping, considerable electrical power generation capacity is available for other purposes, such as cargo discharge. In the past, with traditional long-shaft-line propulsion, large pumps or large compressors required dedicated diesel generators. With diesel-electric, sufficient power is available when it is not used for propulsion. With the GPA 654's diesel-electric generation capacity, the PSV's larger pumps can move cargo at a faster rate and to a higher discharge head. With PSVs, vessel efficiency is not just how fast they get out there, but how fast they can do their job."
Modern ship designs and operations are increasingly focused on cutting operating costs with improved engine fuel efficiencies and reductions of stack emissions. Environmental standards continue to become stricter.
The ability of advanced diesel-electric control systems to match electrical power outputs to changing electrical demands is an important element of overall electrical system performance. Electrical equipment technologies are steadily advancing such as Converteam's evolving electric propulsion solutions with high-torque density induction motors, improved converters and advanced digital propulsion controls.
One basic diesel-electric operational challenge is closely matching actual electrical generation to varying electrical demands of ship systems while running any operating diesel engine at their level of peak performance. Diesel engine manufacturers such as MAN Diesel recognized that wide variations in electrical loads make it likely that "at least one generator set will always be operating under part load." The latest diesel engine technologies such as common-rail fuel injection, water injection methods, or turbo-charger advances are intended to provide optimized medium-speed diesel engine performance and reduced emissions over the complete load range.
What were once perceived as the disadvantages of diesel-electric propulsion -- higher costs and greater technical complexity -- are being steadily overcome.
"Like any newer technology, as it becomes more prevalent, lots of the downsides become minimized. We are reaching that point now," Koch said.