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Pictures of the Future
The Magazine for Research and Innovation

Electric Mobility

Silence at Sea

From pipe-laying ships to research vessels, many marine activities benefit from quiet, energy-efficient, and adaptable diesel-electric drives.

Norway has a long tradition of shipbuilding. Engineers there are developing drive concepts for ship propulsion that are energy efficient and nearly as silent as those of submarines. Hybrid drives play a key role in this effort.

The Vikings loved their ships. These seafarers sailed and rowed their “dragon boats” all the way to North America. More than a thousand years later, one of their homelands, Norway, is still one of the world’s leading shipbuilders. In fact, each year the country’s shipbuilding industry earns around eight billion euros in revenue. Siemens is among the many companies that count on the skills of Scandinavian ship designers. One of them is Odd Moen. From Trondheim, where he works in sales for Siemens Marine & Shipbuilding, Moen oversees projects around the world. But his goal is always the same: to optimally adapt drive system to anything afloat.

Moen’s department has already outfitted more than 200 ships with complete power-trains, thus doing much for maritime energy efficiency in the process. “We use diesel-electric propulsion systems for many ships,” he says. “In these installations, the ships’ propellers, or ‘screws,’ are turned by inverter-fed electric motors that get their energy from diesel-powered generators. This arrangement gives us much finer control of the screws, which results in fuel savings.”

Precision Maneuvers

Many ships must either be able to maneuver with precision on the open seas or, alternately, maintain fixed positions, for example to drill or service installations on the ocean floor. In such situations, the drive usually only needs to deliver minimal power that turns the screws slightly in order to maintain the ship’s position. Compared to faster power-transfer systems, the output of the engine in a purely diesel-powered system fluctuates greatly. As a result, the engine often does not operate at optimal efficiency. In addition, such ships have not made efficient use of the energy content of their fuel. “In order to maintain a vessel’s position at sea or to move at very slow speeds, the amount of propulsion needed is sometimes so minimal that it need not be more than the power to adjust the pitch of the propeller blades,” explains Moen. “The propeller turns at a constant speed. As a result, the diesel engine also does so.”

Today, the frequency converter, by way of the electric motor, controls the propeller rotation speed directly. As a result, the propeller can turn much more slowly. This configuration saves fuel because electric motors operate at high efficiency even at low speeds. A frequency converter controls the speed of the electric motors and propellers. Depending on how much thrust is required, the converter adjusts the frequency and amplitude of the alternating current from the diesel generator.

How to Cut Fuel Demand by One Third

Depending on the type of ship, a diesel-electric propulsion system consists of four to six diesel generators. The ship’s power demand determines how many of the generators are running. As a result, since they produce only the energy that is needed and are not directly coupled to the speed of the screws, diesel engines can operate at a very high level of efficiency. In ships where the load on the propulsion system changes frequently, the savings provided by this type of hybrid system more than compensate for the loss in efficiency due to converting the mechanical energy produced by the diesel engine into electrical energy. Compared to purely diesel-powered vessels, diesel-electric ships are quieter and use up to a third less fuel.

Siemens has been using this technology since 1996. The Skandi Marstein, a supply boat for drilling platforms, was the first vessel with a diesel-electric drive in the North Sea. “That ship was a milestone for us,” says Moen. On a three-day cruise, the Skandi Marstein used 35 percent less energy than a diesel vessel.”

Over the years, the diesel-electric principle has changed very little. However, the use of state-of-the-art components can still improve vessel efficiency. “The complexity of a project grows as the complexity of the individual components increases,” explains Moen. Starting in 2016, exactly 20 years after the Skandi Marstein entered service, four pipeline-laying ships with diesel-electric drive technology will be launched  — the largest contract in the history of Siemens Marine & Shipbuilding in Norway. 

"On a three-day cruise, the Skandi Marstein used 35 percent less energy than a diesel vessel."

“Although the Skandi Marstein was advanced for its time, it is actually no more than a floating truck,” observes Moen. “It delivers supplies to a drilling platform and hauls away trash. But the new pipeline ships will have to operate under much more extreme conditions, hold their exact positions in deep water, and provide plenty of energy for welding, insulating, and laying pipelines.” Up to 150 meters long and with enough buoyancy to carry a 650-ton pipe-laying tower, these ships are true leviathans — and they will have to maintain their positions even in harsh conditions. Each ship is equipped with six diesel engines, which provide the electrical supply for six propellers and the pipe-laying equipment. At any given time, only those diesel engines that are actually needed will be in operation. This strategy, together with the use of electric motors, will reduce fuel use and maintenance costs.  

How Hybrid Vessels are Connecting Germany and Denmark

Looking ahead, Moen sees a lot of potential for drives similar to those found in hybrid cars that use batteries to compensate for fluctuations in propulsion power. An example of this type of system is found in the hybrid-powered Prinsesse Benedikte and Schleswig-Holstein ferries, which are operated by Scandlines. The ferries connect Denmark and Germany via the shortest possible route. Each vessel carries more than 300 cars and over 1,000 passengers per trip. The ships, each of which is 142 meters in length, have a propulsion system similar to those in the pipeline ships. Generators powered by 17.4 kW diesel engines produce sufficient electricity to drive the ships’ electric motors with the help of frequency converters.

In this case, however,Siemens integrated a battery with a storage capacity of around 2,900 kilowatt-hours into the drive system. The battery compensates for the motors’ varying energy requirements depending on whether the ferry is moving or docked. As a result, the diesel engines can operate more evenly and at close to their optimal efficiency. The addition of a battery has allowed the ferry to save up to 15 percent on fuel, while helping to reduce wear on the diesel engines. For short trips with long port calls, all-electric propulsion systems are feasible. Here, power is drawn from batteries, which can be recharged while the ferry is docked. The first purely electric ferry entered ferry is set to enter service in 2015, transporting cars and people across a Norwegian fjord.

Many other kinds of vessels have also been equipped with Siemens drive technology. For instance, research ships benefit because they require particularly quiet drives (almost as quiet as those used for submarines). And in the case of fishing boats, ship designers at Siemens have been able to increase onboard storage space by 40 percent. Since the diesel engines no longer have to be directly connected to the propellers, the drive system can be installed with more flexibility. As a result, available space can be used more effectively. “We always find the right solution for each ship,” says Moen. Nevertheless, despite the vast range of design possibilities, he admits that a Viking ship would present a formidable challenge. Of course, it would be efficient and save fuel, but who wants to row when there’s no wind? 

Andreas Wenleder