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Are rim-driven propulsors the future?

The Naval Architect - July/Aug 2017Hubless Rim Drive Design

Rim-driven propulsors could come to play an important role in the propulsion of future electric ships. Steven Fletcher and Robert Hayes, engineers at Frazer-Nash Consultancy, present the technology's pros and cons and discuss the form future adoption may take for large commercial ships.


The shipping industry has experienced a significant technology shift in recent years with increasing use of electrical propulsion systems. Electrical propulsion, supported by a substantial change in the ship electrical system, has allowed individual ship operators to experience benefits such as reduced maintenance and reduced fuel consumption, something that is helping to meet emission reduction targets.


As part of this response, it has been noted that advanced system design and technology implementation can play a key role in managing these emissions, with reductions in the order of 75% achievable according to the European Commission. This presents an opportunity for the further exploitation of low-carbon electrical propulsion designs and technologies, such as the rim-driven propulsors (RDPs).


RDPs are a novel and emerging electrical propulsion system that integrate an electric motor within a ducted propeller, resulting in a compact, electrically-driven propulsion package. RDPs have huge potential benefits for a range of applications because they remove the need for conventional mechanical drivetrains and open up a wide range of alternative platform arrangements. In our role as a systems engineering consultancy, Frazer-Nash has been involved in a number of studies seeking to assess the viability and benefits of RDPs for different application areas. Key areas of interest include the impact of such systems on ship architecture and electrical integration challenges, as will be discussed.


The following sections discuss why electrical propulsion can be advantageous and the types of power levels required to help illustrate what RDP systems must be capable of, before offering further detail on RDP systems and discussion of the challenges associated with their future application on large commercial ships.

Electrical propulsion
Electrical propulsion, where electrical motors are used to provide mechanical power to a propulsor, is increasingly utilised on a range of ship types and sizes. Examples of these include:

• Naval vessels (Type 45, Type 26, Queen Elizabeth Carrier, Zumwalt);
• LNG supply vessels;
• Cruiseships;
• Offshore supply vessels;
• Ferries.


There are three general ship configurations within which electrical propulsion may play a role. This can be in a parallel hybrid, series hybrid or fully-electric propulsion system. Using a parallel hybrid configuration, both an electric propulsor and an engine can provide propulsive power by connecting to the same shaft. This hybrid propulsion arrangement can provide operational efficiency benefits while retaining elements of traditional propulsion system operation. This configuration is not applicable to an RDP system, given the need for a mechanical shaft.


Fully electric systems can use energy storage (or potentially more novel electrical power generation technologies) to provide power to the propulsor. These are limited to short duration, lower power applications, such as ferries, given the limitations of available power sources (e.g. battery storage).


The majority of platforms described as having electrical propulsion feature a series hybrid configuration, which is where an engine (usually a gas turbine and/or diesel generator for ship applications) is used to generate electrical power. Power is then distributed and used to supply both ship service loads and the propulsion system. These ships therefore contain complex and highly integrated electrical systems. This can bring its own challenges (e.g. the well publicised? issues experienced on the Type 45) as well as many opportunities for optimisation.


Why is electrical propulsion advantageous?
Electrical propulsion (in the series hybrid system configuration) has a range of potential advantages which has supported the increased uptake in its implementation. 


In terms of power generation, by breaking the mechanical connection between the prime mover (e.g. a diesel engine) and the propulsor, it provides the ability to distribute prime movers, and associated electrical generators, around the ship. Removing prime movers from the base of the ship can reduce noise and vibration and the location of propulsors is also made more flexible.


Electrical propulsion provides the ability to use smaller distributed prime movers, enabling optimal use across the operating profile by matching the number of operating prime movers with demand. This configuration can also support greater system redundancy
and availability.


These and other benefits combine to offer a more flexible ship design and propulsion system which reduces emissions, fuel consumption, mass, volume, noise and vibration levels and maintenance costs.
As a future potential benefit, the flexibility offered by electrical propulsion may facilitate the integration of contra-rotating propellers, which have been demonstrated to provide improvements in propulsion efficiency.


How much power is required?
The level of propulsive power required on a given platform will have a significant impact on the suitability and availability of rim-drive technology. Propulsion power requirements vary widely, based on ship size and top speed requirements, and clearly ship architecture will determine the power rating of individual propulsive units. To provide some examples:

  • Offshore vessel propulsive power is in the order of 4MW, with main propulsors around 2MW
    (there is a larger variability given the number of platform configurations);
  • Cruiseships may use propulsors rated between 7.5MW and 15MW;
  • The Yamal LNG tanker uses three 15MW propulsors;
  • The Type 45 ship and the QEC both use 20MW propulsion units (two on the Type 45 and four on the QEC);
  • The Zumwalt ship is designed with 70MW of propulsive power, provided by two 34.6MW motors.


These existing applications help to judge the potential scale of RDPs if they are to replace current electrical propulsion systems.


Rim-driven propulsion
While electrical powered propulsion is now well established, opportunities exist to further improve these platforms in areas such as controllability, efficiency and reliability. RDPs are one such emerging technology under investigation for this purpose.


An RDP is a propeller type which does not need a hub for transmission of the driving torque. In these thrusters, the stator of the electrical motor is housed within the thruster duct, and the machine rotor forms a ring around the tips of the propeller, onto which the propulsor blades are rigidly attached.


A rim-driven motor can technically be driven by any electrical machine type (e.g. DC or induction motors), but there is a general preference for permanent magnet machine types due to the greater power density and efficiency that they offer. This is reflected in the commercial offering available. In this configuration, a ring of permanent magnets are mounted on a rotating rim.


There are a number of potential advantages in the use of RDP systems over more conventional electrical propulsion. At the individual propulsor level, RDP systems can reduce acoustic noise signatures due to a reduced propeller tip speed (note however that there are a range of sources of noise and calculation of this can be complex. RDP electrical machines are generally high torque and low rpm to deliver this performance. Weight can be removed from the propeller due to the removal of the shaft, enabling more rapid control of the rotational speed. However, the significance of this depends on the size of the propulsion (i.e. this will have proportionately more impact in smaller systems). Certain designs can be cooled by the surrounding water and so no separate cooling system is required for the RDP. However, this may not be the case for high rated RDP units, due to high current carrying requirements, and hence increased heating, of machine windings.


The full benefits of RDP technology are seen when considering the potential impact on whole ship architecture. The removal of the shaft allows for units to be positioned with more freedom, which opens up the opportunity for novel configurations to provide enhanced platform capability (e.g. multiple unit primary propulsion, secondary propulsion, dynamic positioning or hover control).


One key area of interest is the potential impact of RDPs on hydrodynamic efficiency. Initial published research, based on a scaled cruiseship model, estimated that an efficiency improvement in the order of 7% was achievable. This level of benefit is also reflected in the information published by Rolls-Royce on its rim-driven thruster technology. This is significant as it not only reduces the total energy consumed from ship propulsion, but also reduces the total installed generation power on the ship, associated infrastructure and fuel usage, saving cost and space. Such an improvement would facilitate a reduction in ship size or an increase in payload. Alternatively, higher speed operation could be achieved for the same installed power.


To explain the potential hydrodynamic benefits in more detail, it is due to the interaction of the hull and the development of the boundary layer that the flow into and around the propeller is non-uniform. Aft-end design plays a significant factor in shaping this flow, and overall propulsive efficiency is a combination of an aft-end that trades off hull resistance for improved propeller inflow.


On a conventional single or twin screw ship the aft-end design is limited by shafting line, propeller clearances and rudder locations. With an RDP the propulsion units can be placed in a location that stops the flow from being impeded (e.g. under the hull). This decreases variations in blade loading and promotes cleaner flow which would be expected to increase the propeller efficiency. This can be matched with either a pram-type stern or bulbous stern, both of which promote buttock flow and decrease overall hull resistance. There is a potential risk that the flow may stall slightly due to the hull not accelerating flow into the propeller. However, the ducting of the RDP would seek to offset this. The duct would increase propulsive efficiency for a given propeller size for high thrust, slow speed applications (e.g. oil tankers), but higher speed vessels (e.g. cargo ships or naval vessels) may be negatively affected by the duct.


There are a few usability issues on vessels with RDP. First, most designs are confined by draught and therefore the design would need to place the RDPs in a stern tunnel configuration, offsetting many of the gains with regards to clean flow. Second, there still needs to be sufficient flow over a control surface to enable manoeuvrability even at slower speeds. The RDP could azimuth, but previous experience with podded propulsion has shown this to be unreliable (with pods being lost in transit). Finally, any type of unit attached to the vessel with a large projected area will have a significant drag component both from the struts and from the unit itself.


Which applications currently employ RDPs?
RDPs currently exist as COTS technology and are used as bow/stern thrusters and as propulsion units in azimuth pod and retractable configurations. RDP units are in-service across a range of commercial applications, including yachts, ferries, tug boats, offshore support vessels, research vessels and cruise liners. In total, it is estimated that there are between 100–200 rim drives in service within commercial applications. The majority of these units are below 0.5MW rated power.


To date, propulsion applications have been a smaller part of the market compared to tunnel thrusters. Frazer-Nash has recently undertaken an investigation into propulsion applications, which involved the testing of two 500kW propulsion units on a research vessel. This trial demonstrated improvements in energy consumption and vibration transmission, compared to conventional shaft-driven propellers and suggests that the technology offers an attractive solution for future electric ships.


A number of larger scale RDPs are also coming to market and will play a role in the propulsion of future platforms. Examples of these include RDPs of up to 1MW from Schottel and Brunvoll and between 1MW and 3MW from Rolls-Royce and Voith.


What are the challenges?
A key constraint in the wider utilisation of RDP systems is the availability of high power units. Clearly there is a disparity between the RDP ratings currently available compared to the size of the motors used in larger commercial and naval applications. While there are no fundamental issues that would prevent RDP systems being upgraded, Frazer-Nash research has found that significant design changes are likely to be required for the multi-megawatt units needed for application of RDPs on large commercial vessels. Issues may include the design of bearings and cooling. There will also be significant costs associated with the design, manufacture and testing of these higher power devices. Interestingly, research has even been conducted on highly novel superconducting electrical machine types at this power level.


The design of a power electronic drive to supply the RDP may also be an issue. The electrical machines in RDP are typically designed with a high number of pole pairs (these being pairs of machine windings and magnets) for optimal size and efficiency. This means that despite the low RDP rotor speed, the electrical frequency from the drive is much higher than typical industrial motors of similar power (which are designed around 60Hz operation compared with a possible range of 100-300Hz for RDPs). Ultimately, this requires the drive to be switched faster, increasing switching losses. Managing the associated thermal issues can be more challenging as power levels scale up.


There are also several challenges of scaling up this technology to larger ships from a hydrodynamic and structural design point of view. A single large RDP will experience large drag and thrust components and subsequently there will be high cyclic loading on the structure. Podded propulsion units have had issues with being lost in heavy seas and it is envisioned that RDPs could be even more susceptible to this occurring. To minimise the risk of this occurring the RDPs would either need to be anchored by multiple attachment points or a large single strut: in either case the likelihood is that azimuthing capabilities would be impacted. In addition, single propulsion units would be unsuitable due to risk of loss.


Given that single commercially-available units are unlikely to be sufficiently rated to provide all the propulsion power needed for the vessel, the RDPs would need to be stacked. However, this could be configured advantageously by stacking in a contra-rotating fashion, which is easier to achieve for RDP given the flexibility in location). While increasing the number of RDPs required, this could potentially provide increased recovery of rotative losses, leading to higher efficiency.


The development of RDP for higher power applications is achievable, but significant development and demonstration will be required in order to scale up this technology. On this basis, it may be a number of years before RDPs fully challenge conventional electrical technologies for large ship propulsion.

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