Permanent magnet rim-driven thrusters

An innovative option for more efficient propulsion.

As ship operators continue to face challenging market conditions and work to improve their economic and environmental performance, Kongsberg Maritime is innovating to help make operators and shipping more sustainable. In this K-magazine article we take a closer look at Kongsberg Maritime’s permanent magnet rim-driven thrusters and why they are an attractive option for more efficient propulsion.

Improving ship efficiency and reducing environmental impact are two key issues generating much attention within the shipping industry. The increased focus is intensified by the global get to zero movement, including the IMO’s (International Maritime Organisation) goals of reduced carbon emissions from shipping. Soaring energy prices and overall operating costs are also impacting on the operators bottom line.  

These market developments often leave stakeholders with more questions than answers. To help customers find the right answers for their unique needs Kongsberg Maritime has developed an extensive portfolio of products to harmonise handling, operations and energy systems on a wide range of vessel types. As an example, solutions for power unit optimisation include power regeneration using permanent magnetic motors for rotating equipment such as its permanent magnet rim-driven azimuth thrusters. Commonly known as the AZ-PM thruster, the unit provides propulsion, steering and positioning thrust for superior manoeuvrability.

Challenging times

“It is recognised that shipping continues to experience a very challenging market and the regulatory developments demand an advanced technology-driven approach to meet the increasingly stringent efficiency requirements. In addition, pollution is regulated and there are moves to reduce the underwater noise generated by ships,” explains Agathe Kalvatn, technical product manager, Kongsberg Maritime (KM).

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“Kongsberg Maritime’s AZ-PM thrusters highlight our future green propulsion technology and demonstrate how we are continuously investing time and resources into developing our products and solutions to help operators meet their challenges, and make them and the shipping industry more efficient, greener, safer and sustainable” – Agathe Kalvatn, technical product manager, Kongsberg Maritime.

She continues, “There are commercial drivers too for ship operators to have more efficient vessels. As well as reducing fuel consumption, they need to look at overall operating costs and maintenance. Another imperative is reducing the space needed for machinery so as to allow more revenue generating capacity in the ship.

According to Agathe this thruster range comprises both AZ-PM thrusters for main propulsion and tunnel thrusters (TT-PM) providing side force to the ship. The AZ-PM provides forward and reverse thrust and can azimuth 360 degrees, which eliminates the need for rudders and stern tunnel thrusters.

New generation of thrusters

Typically, thrusters would be powered by an induction electric motor, but this new generation developed by KM is making use of permanent magnet (PM) technology that is proving to be more efficient, space saving and with many other operational and environmental benefits.

In a permanent magnet rim-driven thruster, the propeller blades are not driven by a traditional gear line with separate electric motor. Instead, an integrated permanent magnet motor is fitted directly to the periphery of the propeller providing a gearless direct electric drive. The nozzle includes a stator with copper windings inside and permanent magnets are integrated into the outer propeller ring forming the rotor. The ends of the rotor shaft are supported by hydrodynamically optimised stays providing a robust load carrying solution with excellent hydrodynamic performance.

Crucially, a permanent magnet rim-driven thruster has fewer moving parts than conventional thrusters thus maintenance is reduced. The propeller spins on bearings supported by a static shaft, and only a small oil circulation pump is required. This system carries less oil than conventional thrusters reducing the risk of environmental damage. It has also been developed to use biodegradable oils that have been approved as Environmentally Accepted Lubricants under the US vessel general permit (VGP) regulations.

Multiple benefits

An azimuthing permanent magnet rim-driven thruster is also space saving and easy to install and maintain. The only components to be found in the thruster room are the slipring unit, 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.

The system permits direct drive to the propeller which is the only moving part. With no gears required this provides a rapid response time with very low energy use. Few moving parts also reduces the potential of system failure and thus potentially reduces the risk of operational down-time for customers. The thrusters are also quiet compared with traditional thrusters. A bonus for setting environmental standards but also a desirable feature for specific vessels requiring low Underwater Radiated Noise (URN).

Also, a frequency converter allows for softer starts and accurate control of propeller speed. The whole system can be monitored for performance and is designed to be integrated into the Kongsberg  systems as shown below.

Another efficiency measure incorporated into the design is the twin hydrofoils which has replaced the single thick vertical stay connecting the propeller unit to the vessel. This reduces both weight and drag.

“Any propulsion system needs to be adapted to the ship hull to optimize efficiency. This is particularly true for AZ-PM thrusters where specially designed head boxes are required to minimize the propulsor – hull interaction. The head boxes are tailor made to fit individual hull lines and are designed by our hydrodynamic experts using computational fluid dynamics (CFD) to optimize the shape of them,” says Rune Garen, senior vice president business concepts, Kongsberg Maritime.

Fuel and emission savings

KM has analysed operation data from an existing vessel and can see that by replacing the currently installed KM propulsion system with a fully electric AZ-PM 2600 system the owner can save fuel oil consumption close to 10 % and corresponding annual CO2 emission savings of around 1000 tons.

The analysis performed by KM has used historical operational data from the existing thrusters to document the real operation profile of the thrusters and to simulate the equivalent power input demand for AZ-PM for the given vessel speed and thrust.

KM has incorporated the permanent magnet motor technology across its range of thrusters where the AZ-PM rim driven thrusters and Elegance pod systems are the fully electrical thruster products. The AZ-PM range comprises 1100kW and 2600kW with additional sizes in the pipeline.

Orders for new buildings

An initial retrofit installation of the technology on the research vessel Gunnerus has been followed by numerous orders for newbuildings. These include the OV Ryvingen and OV Hekkingen utility vessels operated by Norwegian Coastal Administration (Kystverket) which have the AZ-PM installed. The new highly environmentally friendly coastal ferries Havilla Capella and her three sisters operated by Havila Kystruten, and Hurtigruten’s expedition cruise ships Fridtjof Nansen and Roald Amundsen have the TT-PM tunnel thrusters installed.

“The Norwegian Coastal Administration has so far had good experiences with the AZ-PM thrusters installed on OV Ryvingen and OV Hekkingen. The thrusters are quiet, energy efficient and have good efficiency, which is important to us”, says head of section for operations, Alf Arne Borgund.

KM has also secured orders for four CSOV vessels to VARD’s 4 19 design that are being built in Norway and Vietnam for Norwind Offshore and Rem Offshore. In addition, there are four CSOV vessels to Vard’s design with TT-PM in the bow for North Star Renewables.

“KM has invested significant resources in the development of the permanent magnet motor and rim thruster technology over many years. This has positioned KM in the front seat of modern and efficient fully electric propulsion which, in turn, benefits our customers who are seeking innovative ways to achieve sustainable operations,” concludes Agathe.