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Rolls-Royce sets the standard for the future

Permanent Magnet technology for thrusters

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After five decades of development, Rolls-Royce has propelled the azimuth thruster to new levels of efficiency with its permanent magnet technology and advanced hydrodynamics. Sea trials have exceeded expectations

the future is here and now

words: richard goslan

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In the 50 years since Rolls-Royce delivered the first azimuth thrusters, the company has led their development through

many stages. Today’s permanent magnet azimuth thruster (AZ-PM) builds on those five decades of research and operating experience, to deliver a design that combines advanced hydrodynamics with our latest permanent magnet technology.

The AZ-PM combines a ring-type permanent magnet electric motor, propeller and nozzle in a tightly integrated propulsion unit. It offers improved efficiency, simplicity and ease of installation compared with conventional geared azimuth thrusters with separate electric motors in the thruster room.

The AZ-PM thruster has fewer components subject to wear and tear compared with mechanical azimuthing thrusters, meets strict quality requirements and is environmentally friendly, with oil volume reduced by more than 50 per cent.

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Tried and testedRolls-Royce delivered its first permanent magnet tunnel thruster (TT-PM) in 2012,

to Norwegian company Olympic Shipping, for operation on its vessel Olympic Octopus, a multifunctional anchorhandling vessel (AHTS) of Rolls-Royce UT 712 L design.

The TT-PM brought numerous advantages over traditional tunnel thrusters, including a highly efficient motor, a significant reduction in noise and vibration, and the ability to be removed underwater, eliminating the need for dry docking.

The subsequent launch of the AZ-PM follows a rigorous year-long evaluation programme of sea trials on the 31m long RV Gunnerus, the Norwegian University of Science and Technology’s (NTNU) research vessel, based in Trondheim.

Gunnerus was designed by Polarconsult and delivered by the Larsnes Mek. Verksted shipyard in Norway in 2006. It is equipped to carry out research in the fields of biology, technology, geology, archeology, oceanography and fisheries, supporting many of the courses offered by NTNU. When the

ship was conceived, the aim was that it should not only be a platform for scientific research, but also a tool for the development of innovative marine technology. That made it the ideal vessel on which to carry out the thruster development research.

As built, Gunnerus has diesel electric propulsion in a traditional twin-screw arrangement. For the new thruster fit, Moen Marin won the contract for the conversion work, with Polarconsult designing the hull modifications to accept the azimuth thruster mountings and feed the propulsion forces into the structure.

On installation aboard the Gunnerus, the vessel’s shaftlines and rudders were converted to PM azimuth thrusters. The existing high-speed diesel generator sets were maintained and fitted with extremely flexible mountings, to stop any structural-borne noise potentially affecting research results. This allowed exact comparisons to be made of performance before and after installation. As part of the conversion work the propellers, shaftlines, rudders and steering gear were removed, and the sterntubes plugged, but the skeg remains in place.

benefits • Improved propulsion efficiency

• Improved fuel efficiency

• Reduced noise and vibration

• Reduced hydro acoustic noise

• Reduced or no cavitation

• Uses EAL (environmentaly acceptable oil) with reduced volume

• Reduced hull space demand

• No inboard cooling system

• Only a small oil circulation pump is required

• No hydraulic pumps or powerpacks

• Less maintenance on board

• Reduced maintenance at docking intervals

• Reduced steering gear torque

• Equipped for condition monitoring.

INSET: Gunnerus, owned and operated by NTNU, is equipped to carry out research in the fields of biology, technology, geology, archeology, oceanography and fisheries.

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HOW IT WORKSEach AZ-PM comprises three main assemblies – the PM underwater unit of motor, propeller and nozzle, the hull mounting system which includes the azimuth bearing and frequency-controlled electric steering gear, and the inboard power unit which feeds electric power to the thruster.

The permanent magnet motor consists of two main parts – a stator that carries a number of electrical coil windings, and a rotor fitted with a number of very strong permanent magnets.

A rotating magnetic field is created by the stator. It interacts with the fields of the permanent magnets on the rotor, which generates force to drag the rotor around, providing the mechanical power.

The AZ-PM has fewer moving parts than conventional thrusters, leading to reduced maintenance costs. It combines a ring-type permanent magnet electric motor,

propeller and nozzle in a tightly integrated propulsion unit. Simple bearings in the rotor hub carry all loads, and only a small oil circulation pump is required. This system carries less oil than conventional thrusters, reducing the risk of environmental damage if spilled. The system uses biodegradable oil, which has the Vessel General Permit (VGP) for Environmental Acceptable Lubrications (EAL) approval.

The AZ-PM is designed to be space saving and easy to install and maintain. The only components to be found in the thruster room are the slipring case that transfers power to the thruster, the compact electric steering motors and a small lubrication swivel. With a submerged motor, no cooling equipment is required which saves further space and reduces installation cost.

LEFT AND BELOW: The thruster and blade shapes

have been optimised by use of state-of-the-art CFD tools.

Watch our video of the AZ-PM in action athttp://bit.ly/2fURGkp

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Over the course of the trial, a pair of thrusters demonstrated efficiency savings of between seven

and 13 per cent depending on ship speed, and in comparison to azimuth thrusters powered by a conventional diesel-electric system.

Researchers found a very high motor efficiency – 97 per cent at nominal speed and very high over the whole load range. There was an increase in bollard pull of more than 20 per cent. Speed also increased by approximately one knot over the range for the same input power, with airborne and structural noise readings lower or similar to previous performance.

Course stability was good, and manoeuvrability greatly improved. Station keeping and propulsion thrust response were better, valuable

advantages in this type of vessel. Hydro acoustic noise readings were mostly reduced along the frequency range. The thrusters have each now run more than 3,000 hours trouble-free.

PM tunnel thruster performance and reliability has also been verified. The unit run in the quayside test stand for more than 1,300 hours was subjected to severe ice impact testing, with blocks of hard ice delivered by chute to hit the propeller blades under full load. Instrumentation and inspection revealed no damage.

Reflecting on the vessel’s performance, Gunnerus Captain Arve Knudsen said: “Station keeping and propulsion thrust response are better, manoeuvrability has improved and course stability is good. It’s a very impressive product.”

Gary Nutter, Rolls-Royce, Director Products, said: “The sea trials exceeded our expectations in terms of efficiency. Findings have significant implications for future ship sustainability in both the marine and offshore sectors, as PM thrusters become a valuable supplement to traditional thruster technology.”

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The testing showed an increase in bollard pull of more than 20 per cent

More information at www.rolls-royce.com/marine

Watch our video at YouTube.com http://bit.ly/2fURGkp

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RIGHT: The Rolls-Royce portfolio of PM thrusters now covers tunnel thrusters, azimuth thruster and, in 2017, an Azipull PM with an L-drive will be introduced to the market.

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Tunnel thrusters PMTT-PM 1600 - 0.5-1.1MW

TT-PM 2000 - 1-1.6MW

TT-PM 2600 - 2.6MW (from 2018)

Azimuth thrusters PMAZ-PM 1900 - 0.5-1.1MW

AZ-PM 2600 - 2.6MW (from 2018)

Azipull thrusters PMAZP PM 85-L 700 - 1800 kW (in development)

AZP PM 100-L 1800 - 2500 kW (in development)

AZP PM 120-L 2500 - 3500 kW (available)

Permanent magnet propulsion products

© 2016 Rolls-Royce plcThe information in this document is the property of Rolls-Royce plc and may not be copied or communicated to a third party, or used for any purpose other than that for which it is supplied without the express written consent of Rolls-Royce plc. This information is given in good faith based upon the latest information available to Rolls-Royce plc, no warranty or representation is given concerning such information, which must not be taken as establishing any contractual or other commitment binding upon Rolls-Royce plc or any of its subsidiary or associated companies.

www.rolls-royce.com/marine